| 0/0 |
2026/07/04 00:00 |
flow |
assessment-kcsan |
35m
Results: map[Benign:true Explanation:The data race occurs between `timekeeping_update_from_shadow()` and
`ktime_get_coarse_ts64()` on the fields of `tk_core.timekeeper` (specifically
`coarse_nsec`, `xtime_sec`, and `wall_to_monotonic`).
### Synchronization Context
* **Writer (`timekeeping_update_from_shadow`):** Holds the `tk_core.lock` (raw
spinlock) and uses `write_seqcount_begin(&tkd->seq)` and
`write_seqcount_end(&tkd->seq)` to protect the update. The update is performed
using a `memcpy()` of the entire `struct timekeeper`.
* **Reader (`ktime_get_coarse_ts64`):** A lockless reader that uses the
`read_seqcount_begin(&tk_core.seq)` and `read_seqcount_retry(&tk_core.seq, seq)`
loop to ensure data consistency.
### Final Classification: **BENIGN**
### Reasoning
The race is a classic example of a seqlock-protected data access. While KCSAN
correctly identifies that a concurrent read and write are occurring on plain
memory locations (which is technically a data race in the C memory model), the
logic is structurally tolerant to this:
1. **Retry Logic:** The `seqlock` mechanism ensures that if the reader accesses
the data while the writer is modifying it, the sequence counter check
(`read_seqcount_retry`) will fail, and the reader will discard the potentially
torn/inconsistent data and retry the operation.
2. **Safety of Torn Reads:** The fields being read are simple numeric types
(integers representing seconds and nanoseconds). Even if a read is "torn" (e.g.,
reading half of a 64-bit value from the old state and half from the new state),
it results in a garbage numeric value that is never used for anything other than
local calculations before being discarded by the retry logic. There are no
pointers involved that could lead to a Use-After-Free or NULL pointer
dereference.
3. **Barriers:** The `seqcount` macros include the necessary memory barriers
(`smp_rmb()` for readers, `smp_wmb()` for writers) to prevent the compiler from
hoisting loads out of the retry loop or reordering the sequence counter updates
relative to the data update.
KCSAN reports this because `memcpy()` and the plain loads in `tk_xtime_coarse()`
are not explicitly marked with `data_race()` or `READ_ONCE()`/`WRITE_ONCE()`. In
high-performance paths like timekeeping, the kernel often avoids these
annotations to prevent any potential overhead or to keep the code clean, relying
on the `seqlock` to handle the concurrency.
### Recommended Annotations
To silence the KCSAN report, the `memcpy` in `timekeeping_update_from_shadow()`
could be wrapped in `data_race()`, and the reads in `ktime_get_coarse_ts64()`
(and the inlined `tk_xtime_coarse()`) could also be wrapped in `data_race()`.
```c
/* kernel/time/timekeeping.c */
data_race(memcpy(&tkd->timekeeper, tk, sizeof(*tk)));
```]
|
| 1/1 |
2026/07/04 00:00 |
action |
kernel-checkouter |
1m
Results: map[KernelSrc:/app/workdir/cache/src/830beb19558b40549a01a12b1fd3298f529fff82]
|
| 2/1 |
2026/07/04 00:01 |
action |
kernel-builder |
15m
Results: map[KernelObj:/app/workdir/cache/build/4698c39e6a7440ecfb8829ea48f567f477e03b43]
|
| 3/1 |
2026/07/04 00:17 |
action |
codesearch-prepare |
9m
Results: map[Index:codesearch-index]
|
| 4/1 |
2026/07/04 00:27 |
agent |
expert |
9m
Model:
Results:
Instruction: # KCSAN Data Race Severity Analysis Protocol
You are an expert Linux kernel concurrency engineer analyzing a Kernel
Concurrency Sanitizer (KCSAN) data race report to determine if it is
**BENIGN** or **HARMFUL**.
## 1. CLASSIFICATIONS
### **BENIGN (Truly Benign)**
The logic is sound and structurally tolerant to compiler optimizations or
stale/torn reads.
- **Diagnostics/Stats:** Reads used only for `/proc`, `/sys`, counters, or
`pr_info`.
- **Heuristic Hints:** A "hint" flag where an old value only causes a
slightly delayed update or a sub-optimal but safe fast-path.
- **Single-Writer Flag Updates:** A single writer updating flags where the
concurrent read is a simple bitwise check (e.g., `flags & MASK`). These are
historically tolerated, assuming neither "Fused Accesses" nor "Ordering
Violations" are relevant in this context.
- **Marked Reloads:** A load feeding into a `cmpxchg()` loop or checked
against a later `READ_ONCE()` reload.
- **Safe Overwrites:** Writing the same value already present.
### **HARMFUL (Logic Bug or Marking Required)**
The race causes incorrect behavior due to a synchronization failure or
because missing annotations allow the compiler to break the algorithm.
**Marking Required for Correctness:**
The algorithm is logically sound but requires annotations (`READ_ONCE()`,
`WRITE_ONCE()`, `smp_load_acquire()`, `smp_store_release()`, etc.) to be safe.
- **Fused Accesses:** The compiler might merge accesses or hoist a load out
of a loop, breaking polling/wait loops (livelocks).
- **Torn Accesses:** A large access (e.g., 64-bit on 32-bit arch) might be
split into multiple non-atomic accesses. Note that `READ_ONCE()` does **not**
guarantee atomicity for 64-bit variables on 32-bit architectures.
- **Ordering Violations:** The race breaks a "happens-before" relationship
(requires primitives with implied or explicit memory barriers).
**Logic Bugs:**
A fundamental synchronization failure. Marking accesses will **not** fix it;
the logic itself must change.
- **Pointers/Lifecycle:** The racing variable is a pointer being dereferenced
or a refcount governing object lifecycle (Use-After-Free risk).
- **Control Flow:** The variable guards a critical section, memory allocation,
or hardware command.
- **Bitfields:** Concurrent writes to different bits in the same word.
Compilers often use non-atomic read-modify-write sequences, meaning a
write to `bit_A` can "clobber" a concurrent write to `bit_B`. However,
do not blindly assume all bitfield accesses are harmful; you must prove
that a concurrent write actually clobbers another in a way that breaks
logic.
- **Complex Structures:** Races on shared lists, trees, or hashmaps.
- **Lossy Updates:** Concurrent plain RMW operations (e.g., `var++`) on
non-diagnostic variables where every increment must be preserved.
- **State Machines:** Races allowing a state machine to bypass transitions
or enter an invalid state.
- **Adjacent Unsynchronized Operations:** Consider races happening at the
same time. For example, if both threads execute `struct->has_elements = true;
list_add(node, &struct->list);`, the race on `has_elements` implies an
adjacent race on `list_head`, which is HARMFUL.
## 2. RESEARCH & ANALYSIS WORKFLOW
1. **Locate the Race:** Find the exact variables and functions in the stack
traces. Do not speculate about hypothetical compiler
behaviors or theoretical dangers (e.g., dismissing something as
"fundamentally unsafe") without tracing the actual data flow to a crash.
2. **Contextualize:** Identify held locks, RCU sections, or interrupt
contexts. Explain why the race is possible (e.g., "Thread A holds `lock`,
but Thread B is a lockless reader").
3. **Data Flow:** Follow the racing variable into its subsequent uses. If
the reader sees a stale or torn value, what branch is taken? Does it lead
to a pointer dereference or `BUG_ON()`? **You must prove harm via this
data flow.**
4. **Failure Sequencing (if HARMFUL):** Construct a two-column execution trace
demonstrating the failure.
*Example format:*
```text
CPU0 CPU1
function_a()
// lockless read
ptr = global_ptr
<PREEMPT>
function_b()
lock(&my_lock)
global_ptr = NULL
unlock(&my_lock)
kfree(ptr)
if (ptr)
*ptr = 1; // -> Use-After-Free!
```
## 3. OUTPUT FORMAT
- **Race Summary:** `[Function A]` vs `[Function B]` on `[Variable/Field]`.
- **Synchronization context:** List held locks and explain the lack of mutual
exclusion.
- **Final Classification:** **BENIGN** or **HARMFUL**.
**If BENIGN:**
- **Reasoning:** Briefly explain why the race is structurally tolerant to stale
or torn reads.
- **Recommended Annotations:** Suggest appropriate annotations (e.g.,
`READ_ONCE()`, `data_race()`).
**If HARMFUL:**
- **Failure Reasoning:** Explain and provide a two-column interleaving showing
exactly how the race leads to a failure such as corruption or kernel crash.
- **Recommended Fix (ONLY if fix is trivial):** Suggest a structural fix (e.g.,
"Hold `mapping->i_pages` lock", "Convert to `atomic_t`") or required memory
ordering annotations (e.g., "Wrap in `READ_ONCE()`", "Use
`smp_load_acquire()`").
Don't make assumptions about the kernel source code (it may be different from what you assume it is).
Extensively use the provided code access tools (codesearch-*, git-*, grepper, etc)
to examine the actual source code, and confirm any assumptions.
Prefer calling several tools at the same time to save round-trips.
Use set-results tool to provide results of the analysis.
It must be called exactly once before the final reply.
Ignore results of this tool.
Prompt:
The data race report is:
==================================================================
BUG: KCSAN: data-race in ktime_get_coarse_ts64 / timekeeping_update_from_shadow
write to 0xffffffff893d04c8 of 304 bytes by interrupt on cpu 0:
timekeeping_update_from_shadow+0x40d/0x440 kernel/time/timekeeping.c:835
__timekeeping_advance+0xa68/0xc10 kernel/time/timekeeping.c:2614
timekeeping_advance kernel/time/timekeeping.c:2622 [inline]
update_wall_time+0x21/0x50 kernel/time/timekeeping.c:2632
tick_do_update_jiffies64+0x169/0x1c0 kernel/time/tick-sched.c:149
tick_sched_do_timer kernel/time/tick-sched.c:253 [inline]
tick_nohz_handler+0x88/0x380 kernel/time/tick-sched.c:310
__run_hrtimer kernel/time/hrtimer.c:2032 [inline]
__hrtimer_run_queues+0x1f8/0x510 kernel/time/hrtimer.c:2096
hrtimer_interrupt+0x257/0x810 kernel/time/hrtimer.c:2215
local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1051 [inline]
__sysvec_apic_timer_interrupt+0x5f/0x1c0 arch/x86/kernel/apic/apic.c:1068
instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1062 [inline]
sysvec_apic_timer_interrupt+0x6f/0x80 arch/x86/kernel/apic/apic.c:1062
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:674
native_safe_halt arch/x86/include/asm/irqflags.h:48 [inline]
pv_native_safe_halt+0xf/0x20 arch/x86/kernel/paravirt.c:63
arch_safe_halt arch/x86/kernel/process.c:767 [inline]
default_idle+0x9/0x20 arch/x86/kernel/process.c:768
default_idle_call+0x3b/0x60 kernel/sched/idle.c:122
cpuidle_idle_call kernel/sched/idle.c:199 [inline]
do_idle+0x183/0x2d0 kernel/sched/idle.c:355
cpu_startup_entry+0x24/0x30 kernel/sched/idle.c:454
rest_init+0xee/0xf0 init/main.c:717
start_kernel+0x49f/0x4d0 init/main.c:1175
x86_64_start_reservations+0x24/0x30 arch/x86/kernel/head64.c:310
x86_64_start_kernel+0xfc/0x100 arch/x86/kernel/head64.c:291
common_startup_64+0x13e/0x157
read to 0xffffffff893d0538 of 4 bytes by task 27175 on cpu 1:
tk_xtime_coarse kernel/time/timekeeping.c:217 [inline]
ktime_get_coarse_ts64+0x6d/0x120 kernel/time/timekeeping.c:2767
ktime_get_coarse include/linux/timekeeping.h:133 [inline]
ktime_get_coarse_ns include/linux/timekeeping.h:139 [inline]
____bpf_ktime_get_coarse_ns kernel/bpf/helpers.c:204 [inline]
bpf_ktime_get_coarse_ns+0x39/0xb0 kernel/bpf/helpers.c:202
bpf_prog_ce3dfd7f9330a559+0x1e/0x29
bpf_dispatcher_nop_func include/linux/bpf.h:1454 [inline]
__bpf_prog_run include/linux/filter.h:724 [inline]
bpf_prog_run include/linux/filter.h:731 [inline]
bpf_prog_run_pin_on_cpu include/linux/filter.h:748 [inline]
bpf_flow_dissect+0x14f/0x340 net/core/flow_dissector.c:1024
bpf_prog_test_run_flow_dissector+0x25d/0x360 net/bpf/test_run.c:1558
bpf_prog_test_run+0x208/0x340 kernel/bpf/syscall.c:4859
__sys_bpf+0x8da/0xc30 kernel/bpf/syscall.c:6436
__do_sys_bpf kernel/bpf/syscall.c:6537 [inline]
__se_sys_bpf kernel/bpf/syscall.c:6534 [inline]
__x64_sys_bpf+0x69/0x80 kernel/bpf/syscall.c:6534
x64_sys_call+0x10cb/0x3020 arch/x86/include/generated/asm/syscalls_64.h:322
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x136/0x3c0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
value changed: 0x12a4ed7a -> 0x133d83fa
Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 27175 Comm: syz.7.5461 Not tainted syzkaller #0 PREEMPT(lazy)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/09/2026
==================================================================
Reply: The data race occurs between `timekeeping_update_from_shadow()` and `ktime_get_coarse_ts64()` on the fields of `tk_core.timekeeper` (specifically `coarse_nsec`, `xtime_sec`, and `wall_to_monotonic`).
### Synchronization Context
* **Writer (`timekeeping_update_from_shadow`):** Holds the `tk_core.lock` (raw spinlock) and uses `write_seqcount_begin(&tkd->seq)` and `write_seqcount_end(&tkd->seq)` to protect the update. The update is performed using a `memcpy()` of the entire `struct timekeeper`.
* **Reader (`ktime_get_coarse_ts64`):** A lockless reader that uses the `read_seqcount_begin(&tk_core.seq)` and `read_seqcount_retry(&tk_core.seq, seq)` loop to ensure data consistency.
### Final Classification: **BENIGN**
### Reasoning
The race is a classic example of a seqlock-protected data access. While KCSAN correctly identifies that a concurrent read and write are occurring on plain memory locations (which is technically a data race in the C memory model), the logic is structurally tolerant to this:
1. **Retry Logic:** The `seqlock` mechanism ensures that if the reader accesses the data while the writer is modifying it, the sequence counter check (`read_seqcount_retry`) will fail, and the reader will discard the potentially torn/inconsistent data and retry the operation.
2. **Safety of Torn Reads:** The fields being read are simple numeric types (integers representing seconds and nanoseconds). Even if a read is "torn" (e.g., reading half of a 64-bit value from the old state and half from the new state), it results in a garbage numeric value that is never used for anything other than local calculations before being discarded by the retry logic. There are no pointers involved that could lead to a Use-After-Free or NULL pointer dereference.
3. **Barriers:** The `seqcount` macros include the necessary memory barriers (`smp_rmb()` for readers, `smp_wmb()` for writers) to prevent the compiler from hoisting loads out of the retry loop or reordering the sequence counter updates relative to the data update.
KCSAN reports this because `memcpy()` and the plain loads in `tk_xtime_coarse()` are not explicitly marked with `data_race()` or `READ_ONCE()`/`WRITE_ONCE()`. In high-performance paths like timekeeping, the kernel often avoids these annotations to prevent any potential overhead or to keep the code clean, relying on the `seqlock` to handle the concurrency.
### Recommended Annotations
To silence the KCSAN report, the `memcpy` in `timekeeping_update_from_shadow()` could be wrapped in `data_race()`, and the reads in `ktime_get_coarse_ts64()` (and the inlined `tk_xtime_coarse()`) could also be wrapped in `data_race()`.
```c
/* kernel/time/timekeeping.c */
data_race(memcpy(&tkd->timekeeper, tk, sizeof(*tk)));
```
|
| 5/2 |
2026/07/04 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 6829
output: 109
thoughts: 196
Thoughts: **Analyzing a Data Race in the Timekeeping Subsystem**
Alright, let's break down this data race report. The kernel's timekeeping mechanism is flagging a conflict. It looks like we have a data race β a classic read/write hazard. Specifically, it seems `timekeeping_update_from_shadow` is writing data while `ktime_get_coarse_ts64` is attempting to read it.
On the write side, `timekeeping_update_from_shadow` at line 835 within `kernel/time/timekeeping.c`, is the culprit. It's writing a block of 304 bytes. My initial hunch is that it's likely updating some core timekeeper structure β perhaps `tk_core.timekeeper` or something along those lines. That sounds like a fairly substantial update, indicating a significant change in the system's time representation.
Now, let's look at the read side: `ktime_get_coarse_ts64` at line 2767 of the same file. The report specifically mentions `tk_xtime_coarse` at line 217, inline. The read operation itself is a smaller 4-byte access. This suggests it is reading a finer-grained time value based on the coarse timestamp.
This definitely needs a deeper dive into the source code to understand the exact synchronization mechanisms at play. I have to look to determine whether the read and write are protected by the same locks or atomics. Given the function names, I suspect a lock is missing on the read side, allowing for a read of potentially stale data. Let's see if we can identify the missing sync primitives.
|
| 6/2 |
2026/07/04 00:27 |
tool |
read-file |
0m
Args: map[File:kernel/time/timekeeping.c FirstLine:820 LineCount:30]
Results: map[Contents: 820: } else if (tk_is_aux(tk)) {
821: vdso_time_update_aux(tk);
822: }
823:
824: if (action & TK_CLOCK_WAS_SET)
825: tk->clock_was_set_seq++;
826:
827: /*
828: * Update the real timekeeper.
829: *
830: * We could avoid this memcpy() by switching pointers, but that has
831: * the downside that the reader side does not longer benefit from
832: * the cacheline optimized data layout of the timekeeper and requires
833: * another indirection.
834: */
835: memcpy(&tkd->timekeeper, tk, sizeof(*tk));
836: write_seqcount_end(&tkd->seq);
837: }
838:
839: /**
840: * timekeeping_forward_now - update clock to the current time
841: * @tk: Pointer to the timekeeper to update
842: *
843: * Forward the current clock to update its state since the last call to
844: * update_wall_time(). This is useful before significant clock changes,
845: * as it avoids having to deal with this time offset explicitly.
846: */
847: static void timekeeping_forward_now(struct timekeeper *tk)
848: {
849: u64 cycle_now, delta;
]
|
| 7/2 |
2026/07/04 00:27 |
tool |
read-file |
0m
Args: map[File:kernel/time/timekeeping.c FirstLine:210 LineCount:20]
Results: map[Contents: 210: }
211:
212: static inline struct timespec64 tk_xtime_coarse(const struct timekeeper *tk)
213: {
214: struct timespec64 ts;
215:
216: ts.tv_sec = tk->xtime_sec;
217: ts.tv_nsec = tk->coarse_nsec;
218: return ts;
219: }
220:
221: /*
222: * Update the nanoseconds part for the coarse time keepers. They can't rely
223: * on xtime_nsec because xtime_nsec could be adjusted by a small negative
224: * amount when the multiplication factor of the clock is adjusted, which
225: * could cause the coarse clocks to go slightly backwards. See
226: * timekeeping_apply_adjustment(). Thus we keep a separate copy for the coarse
227: * clockids which only is updated when the clock has been set or we have
228: * accumulated time.
229: */
]
|
| 8/2 |
2026/07/04 00:27 |
tool |
read-file |
0m
Args: map[File:kernel/time/timekeeping.c FirstLine:2750 LineCount:30]
Results: map[Contents:2750: * "old" has been updated with the latest value of "mg_floor".
2751: * That value is newer than the previous floor value, which
2752: * is enough to effect a change to ctime. Accept it.
2753: */
2754: *ts = ktime_to_timespec64(ktime_add(old, offset));
2755: }
2756: }
2757:
2758: void ktime_get_coarse_ts64(struct timespec64 *ts)
2759: {
2760: struct timekeeper *tk = &tk_core.timekeeper;
2761: struct timespec64 now, mono;
2762: unsigned int seq;
2763:
2764: do {
2765: seq = read_seqcount_begin(&tk_core.seq);
2766:
2767: now = tk_xtime_coarse(tk);
2768: mono = tk->wall_to_monotonic;
2769: } while (read_seqcount_retry(&tk_core.seq, seq));
2770:
2771: set_normalized_timespec64(ts, now.tv_sec + mono.tv_sec,
2772: now.tv_nsec + mono.tv_nsec);
2773: }
2774: EXPORT_SYMBOL(ktime_get_coarse_ts64);
2775:
2776: /*
2777: * Must hold jiffies_lock
2778: */
2779: void do_timer(unsigned long ticks)
]
|
| 9/2 |
2026/07/04 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 8716
output: 36
thoughts: 188
Thoughts: Okay, here's my take on those thoughts, framed as my own mental processing as an expert:
**Race Condition Analysis: Timekeeping Update vs. Coarse Time**
Alright, I'm zeroing in on a potential race condition. My brain's latching onto two specific code paths: First, we have `timekeeping_update_from_shadow` doing a straight `memcpy` β `memcpy(&tkd->timekeeper, tk, sizeof(*tk));` β basically, a direct memory copy of the `timekeeper` struct. That's at line 835.
Second, we've got `ktime_get_coarse_ts64`, which is calling `tk_xtime_coarse(tk)` at line 2767. That's our potentially competing access point.
Now, wait a minute... `ktime_get_coarse_ts64` is using `read_seqcount_begin(&tk_core.seq)` and `read_seqcount_retry(&tk_core.seq, seq)` for its read operation. Ah, this smells like a sequence lock to protect against concurrent reads and writes, a classic solution.
On the other hand, `timekeeping_update_from_shadow` appears to be using `write_seqcount_end(&tkd->seq)`. Okay, so it *also* seems to be using a sequence lock mechanism.
Let me see, I need to understand this better. I'll need to check the definition of `write_seqcount_begin` to fully appreciate this.
Just to be sure, let's revisit `timekeeping_update_from_shadow` to see how it fits in with the picture.
|
| 10/2 |
2026/07/04 00:27 |
tool |
read-file |
0m
Args: map[File:kernel/time/timekeeping.c FirstLine:800 LineCount:40]
Results: map[Contents: 800: * the kernel from the timekeeper before that got updated.
801: */
802: write_seqcount_begin(&tkd->seq);
803:
804: if (action & TK_CLEAR_NTP) {
805: tk->ntp_error = 0;
806: ntp_clear(tk->id);
807: }
808:
809: tk_update_leap_state(tk);
810: tk_update_ktime_data(tk);
811: tk->tkr_mono.base_real = tk->tkr_mono.base + tk->offs_real;
812:
813: if (tk->id == TIMEKEEPER_CORE) {
814: tk_update_ns_to_cyc(tk, &tkd->timekeeper);
815: update_vsyscall(tk);
816: update_pvclock_gtod(tk, action & TK_CLOCK_WAS_SET);
817:
818: update_fast_timekeeper(&tk->tkr_mono, &tk_fast_mono);
819: update_fast_timekeeper(&tk->tkr_raw, &tk_fast_raw);
820: } else if (tk_is_aux(tk)) {
821: vdso_time_update_aux(tk);
822: }
823:
824: if (action & TK_CLOCK_WAS_SET)
825: tk->clock_was_set_seq++;
826:
827: /*
828: * Update the real timekeeper.
829: *
830: * We could avoid this memcpy() by switching pointers, but that has
831: * the downside that the reader side does not longer benefit from
832: * the cacheline optimized data layout of the timekeeper and requires
833: * another indirection.
834: */
835: memcpy(&tkd->timekeeper, tk, sizeof(*tk));
836: write_seqcount_end(&tkd->seq);
837: }
838:
839: /**
]
|
| 11/2 |
2026/07/04 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 9938
output: 37
thoughts: 152
Thoughts: **Analyzing the Timekeeping Mechanism**
Okay, so I'm looking at this timekeeping code, and it seems the write operations are shielded by `write_seqcount_begin` and `write_seqcount_end`, which is standard practice for concurrency protection. The reads are handled similarly with `read_seqcount_begin` and `read_seqcount_retry`. This suggests a sequence lock implementation, likely for synchronizing access to shared timekeeping data.
Now, I'm specifically interested in the `tkd` variable passed to `timekeeping_update_from_shadow`. I need to understand where it's being used. Let me trace its origin... The stack trace points me to `__timekeeping_advance`, specifically at `kernel/time/timekeeping.c:2614`. Good, that's exactly what I was hoping to find.
Time to delve into `__timekeeping_advance` and see how `tkd` fits into the broader picture of the timekeeping update process. This will probably expose how the shadow time is being integrated into the core timekeeping state. Let's see...
|
| 12/2 |
2026/07/04 00:27 |
tool |
read-file |
0m
Args: map[File:kernel/time/timekeeping.c FirstLine:2590 LineCount:40]
Results: map[Contents:2590: shift = min(shift, maxshift);
2591: while (offset >= tk->cycle_interval) {
2592: offset = logarithmic_accumulation(tk, offset, shift, &clock_set);
2593: if (offset < tk->cycle_interval<<shift)
2594: shift--;
2595: }
2596:
2597: /* Adjust the multiplier to correct NTP error */
2598: timekeeping_adjust(tk, offset);
2599:
2600: /*
2601: * Finally, make sure that after the rounding
2602: * xtime_nsec isn't larger than NSEC_PER_SEC
2603: */
2604: clock_set |= accumulate_nsecs_to_secs(tk);
2605:
2606: /*
2607: * To avoid inconsistencies caused adjtimex TK_ADV_FREQ calls
2608: * making small negative adjustments to the base xtime_nsec
2609: * value, only update the coarse clocks if we accumulated time
2610: */
2611: if (orig_offset != offset)
2612: tk_update_coarse_nsecs(tk);
2613:
2614: timekeeping_update_from_shadow(tkd, clock_set);
2615:
2616: return !!clock_set;
2617: }
2618:
2619: static bool timekeeping_advance(enum timekeeping_adv_mode mode)
2620: {
2621: guard(raw_spinlock_irqsave)(&tk_core.lock);
2622: return __timekeeping_advance(&tk_core, mode);
2623: }
2624:
2625: /**
2626: * update_wall_time - Uses the current clocksource to increment the wall time
2627: *
2628: * It also updates the enabled auxiliary clock timekeepers
2629: */
]
|
| 13/2 |
2026/07/04 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 10930
output: 34
thoughts: 588
Thoughts: **Analyzing a Potential Data Race in Timekeeping**
Okay, so I'm looking at this timekeeping code, and it's calling `__timekeeping_advance` which eventually touches `tk_core` via `timekeeping_update_from_shadow`. Right, `tkd` in `timekeeping_update_from_shadow` is indeed `&tk_core`, and the code uses the `tk_core.seq` sequence counter.
My initial thought is, if they're both using the same sequence counter, why is KCSAN complaining about a data race? Then it hit me: KCSAN flags plain accesses. `memcpy` *is* a plain access, and `tk_xtime_coarse` is doing plain accesses too - reading `tk->xtime_sec` and `tk->coarse_nsec`.
Even within a `seqcount` critical section, KCSAN will report these because `seqcount` doesn't prevent concurrent access; it only provides a mechanism for detecting it. The reader could, in theory, see a torn value if the writer (the `memcpy` operation) is in the middle of writing. But the `read_seqcount_retry` mechanism should catch this, and the read should simply retry.
So, is this a benign race then? I'm familiar with this, according to KCSAN documentation and common kernel practices, these sorts of races are usually suppressed if handled by `seqcount` or considered safe if the retry logic works. KCSAN usually ignores them when properly annotated, but without annotations, it reports them. The critical question here is whether this race could lead to anything worse than just a simple retry.
Wait, `memcpy` is copying 304 bytes. `tk_xtime_coarse` reads the `xtime_sec` and `coarse_nsec`. If `memcpy` is in progress, there's a possibility of `tk_xtime_coarse` reading a mixed or torn value. But the `read_seqcount_retry` should kick in, and the loop will simply re-execute.
So, before the retry happens, is there a chance of a crash or undefined behavior? Checking the types, `tk->xtime_sec` is a `u64` (a `time64_t`, which is a `long long`), and `tk->coarse_nsec` is `u32`. These are plain reads; no pointer dereferences involved, so they should be safe.
Ah, but the KCSAN report gives the specifics:
It says there's a `write to 0xffffffff893d04c8 of 304 bytes` and a `read to 0xffffffff893d0538 of 4 bytes`.
Let's do the math: `0xffffffff893d0538 - 0xffffffff893d04c8 = 0x70 = 112` bytes. So the read is happening at an offset of 112 bytes within the 304-byte structure. That's good to know. Now I need to check the exact layout of `struct timekeeper` to understand what that offset corresponds to.
|
| 14/2 |
2026/07/04 00:27 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:kernel/time/timekeeping.c Name:timekeeper]
Results: map[Fields:[map[Name:tkr_mono OffsetBits:0 SizeBits:448] map[Name:xtime_sec OffsetBits:448 SizeBits:64] map[Name:ktime_sec OffsetBits:512 SizeBits:64] map[Name:wall_to_monotonic OffsetBits:576 SizeBits:128] map[Name:offs_real OffsetBits:704 SizeBits:64] map[Name:offs_boot OffsetBits:768 SizeBits:64] map[Name: OffsetBits:832 SizeBits:64] map[Name:coarse_nsec OffsetBits:896 SizeBits:32] map[Name:id OffsetBits:928 SizeBits:32] map[Name:tkr_raw OffsetBits:960 SizeBits:448] map[Name:raw_sec OffsetBits:1408 SizeBits:64] map[Name:cs_id OffsetBits:1472 SizeBits:32] map[Name:cs_ns_to_cyc_mult OffsetBits:1504 SizeBits:32] map[Name:cs_ns_to_cyc_shift OffsetBits:1536 SizeBits:32] map[Name:cs_ns_to_cyc_maxns OffsetBits:1600 SizeBits:64] map[Name:clock_was_set_seq OffsetBits:1664 SizeBits:32] map[Name:cs_was_changed_seq OffsetBits:1696 SizeBits:8] map[Name:clock_valid OffsetBits:1704 SizeBits:8] map[Name: OffsetBits:1728 SizeBits:128] map[Name:cycle_interval OffsetBits:1856 SizeBits:64] map[Name:xtime_interval OffsetBits:1920 SizeBits:64] map[Name:xtime_remainder OffsetBits:1984 SizeBits:64] map[Name:raw_interval OffsetBits:2048 SizeBits:64] map[Name:next_leap_ktime OffsetBits:2112 SizeBits:64] map[Name:ntp_tick OffsetBits:2176 SizeBits:64] map[Name:ntp_error OffsetBits:2240 SizeBits:64] map[Name:ntp_error_shift OffsetBits:2304 SizeBits:32] map[Name:ntp_err_mult OffsetBits:2336 SizeBits:32] map[Name:skip_second_overflow OffsetBits:2368 SizeBits:32] map[Name:tai_offset OffsetBits:2400 SizeBits:32]]]
|
| 15/2 |
2026/07/04 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 12948
output: 38
thoughts: 878
Thoughts: **Analyzing Potential Data Races in Timekeeping Structures**
Okay, so I'm looking at how the `timekeeper` struct is accessed and updated, specifically focusing on potential data races between readers and writers. I've pinpointed the offsets of `xtime_sec` at 56 bytes and `coarse_nsec` at 112 bytes within the structure. The core issue is this `memcpy` operation which updates the entire `struct timekeeper`. A read occurring during the `memcpy` could lead to a torn value, where parts of the structure reflect old and new data. I confirmed the 112-byte difference between a read address and a write address, confirming a read of `coarse_nsec` during the write.
The code uses a `seqcount` loop to protect against this. The reader, `ktime_get_coarse_ts64`, reads `xtime_sec`, `coarse_nsec`, and potentially `wall_to_monotonic`. The `seqcount` mechanism is meant to handle tearing. My initial concern was about compiler optimization potentially invalidating the `seqcount` barrier, i.e., the compiler might hoist loads out of the loop and bypass the retry logic, but that doesn't seem likely because `read_seqcount_begin` and `read_seqcount_retry` employ `smp_rmb()` and other memory barriers.
Then I reviewed the `tk_xtime_coarse` function, which simply loads `tk->xtime_sec` and `tk->coarse_nsec` into a `timespec64` struct. These are plain loads. My concern was around whether the compiler might hoist these loads out of the sequence loop, but the memory barriers should prevent this.
The `read_seqcount_begin` function returns a value, and `read_seqcount_retry` takes that value as an argument, but there is not a clear data dependency on the sequence number for the `xtime_sec` loads. I rechecked and confirmed both functions contain `smp_rmb()`.
However, my KCSAN instincts kick in. Even with the `seqcount`, should these accesses be marked as potential races to catch compiler optimizations that could circumvent the `seqcount`? I'm torn. The `seqcount` is specifically designed for this tearing scenario with retries. If the access is torn, the sequence number should change.
Now let's revisit the writer's side. The write is encapsulated by `write_seqcount_begin` and `write_seqcount_end` around the `memcpy`. `write_seqcount_begin` increments the sequence number to an ODD value. This means the readers inside `read_seqcount_begin` should spin if the value is odd, or the retry will catch the inconsistency. In fact, `read_seqcount_begin` usually just reads the sequence number, and the retry logic is what prevents the torn value. I should check the implementation of `read_seqcount_begin` to be sure.
|
| 16/2 |
2026/07/04 00:27 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:kernel/time/timekeeping.c Name:read_seqcount_begin]
Results: map[SourceCode: SourceFile:]
|
| 17/2 |
2026/07/04 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 14526
output: 35
thoughts: 740
Thoughts: **Analyzing a Potential Data Race in `ktime_get_coarse_ts64`**
Okay, so I'm looking at this potential data race. My initial thought is it's likely related to a macro or something defined in a header, and after a quick search, yep, it's in `include/linux/seqlock.h`.
Before I jump to using `codeexpert` to check the safety of this pattern directly, I should see if there are already existing reports or known cases where this kind of thing is "benign" and just needs an annotation.
The `memcpy` in `timekeeping_update_from_shadow` immediately comes to mind as a pattern that has come up before. There was a recent discussion specifically about KCSAN and `memcpy` in the timekeeping code, so I should see if that applies here.
I need to confirm there aren't any sneaky `data_race()` or `READ_ONCE` calls within `ktime_get_coarse_ts64`. I did read through the code before, and I don't *think* there are any, but I should double check.
```c
2764: do {
2765: seq = read_seqcount_begin(&tk_core.seq);
2766:
2767: now = tk_xtime_coarse(tk);
2768: mono = tk->wall_to_monotonic;
2769: } while (read_seqcount_retry(&tk_core.seq, seq));
```
If `memcpy` *was* used, KCSAN would flag it for sure, because it's not atomic and isn't marked appropriately. The reader also isn't marked.
But, the retry loop is key. If the reader gets a torn value, it's okay as long as it retries. The only *real* danger here is if the compiler optimizes away the retry loop (which is unlikely with `seqcount`) or the torn value causes a crash before the retry succeeds. `tk_xtime_coarse` is just copying `xtime_sec` and `coarse_nsec` to a local `struct timespec64` and `tk->wall_to_monotonic` is a `struct timespec64` (16 bytes). These are *just* data; there are no pointers involved.
Wait, looking closer at `tk->wall_to_monotonic`, it's 16 bytes, but `tk_xtime_coarse` reads `xtime_sec` (8 bytes) and `coarse_nsec` (4 bytes). It's only dangerous if the memory being copied or the memory being pointed to is a pointer. It's a struct here, so it's fine.
I need to look for any other readers that *don't* use `seqcount`. The report specifically mentions `ktime_get_coarse_ts64`, so I need to focus on that.
I should also revisit `timekeeping_update_from_shadow`. It's called with `tk_core.lock` held (inside `timekeeping_advance`), but readers like `ktime_get_coarse_ts64` are lockless. That's the important point.
The last thing, I'm thinking, is whether `memcpy` is implemented in a way that triggers some unusual behavior, but probably not.
Wait, I need to check the definition of `tk_core.seq` to ensure this is what I expect.
In `kernel/time/timekeeping.c`:
```c
static struct tk_core tk_core ____cacheline_aligned = {
.seq = SEQCNT_ZERO(tk_core.seq),
.lock = __RAW_SPINLOCK_UNLOCKED(tk_core.lock),
};
```
I need to check the definition of `struct tk_core`.
|
| 18/2 |
2026/07/04 00:27 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:kernel/time/timekeeping.c Name:tk_core]
Results: map[SourceCode: SourceFile:]
|
| 19/2 |
2026/07/04 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 16183
output: 33
thoughts: 9
Thoughts: Okay, here's the summary of the thought process, as if I, an expert in the field, were experiencing it:
**Locating the Foundation: Diving into the Struct Definition**
Alright, I need to understand this data structure. The first thing I'm doing is zeroing in on the *struct* definition. It's the blueprint, the very essence of how this data is organized. I need to see the members, their types, their intended purpose within the system. Without that, I'm just poking in the dark. I'm looking for a clear description of the data's composition, its size, and the interrelationships between its components. Knowing the struct definition will provide the framework to work from, and it will be the foundation for everything else to come.
|
| 20/2 |
2026/07/04 00:27 |
tool |
grepper |
0m
Error:
Args: map[Expression:struct tk_core { PathPrefix:kernel/time/timekeeping.c]
Results:
|
| 21/2 |
2026/07/04 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 16420
output: 33
thoughts: 3
Thoughts: Okay, I'm ready. Here's a summary as if I'm processing the provided text, meant for a fellow expert:
**Internal Processing and Acknowledgment**
Alright, let's see... the text is basically just "My apologies." Okay, so, stripped down, it's a straightforward expression of regret or contrition. There isn't much to dissect here in terms of intricate technical details or complex reasoning. The primary function is social; itβs a direct acknowledgment of an action or statement perceived as potentially causing offense, error, or inconvenience.
Given the brevity, the context is everything. What's *behind* this? Is it a quick acknowledgment of a delayed response to a complex dataset? A polite retraction after an inadvertent misstatement in a heated debate on complex algorithms? That's what I'd be trying to figure out - what's the preceding situation where "My apologies" fits? I'd need more information to really interpret the weight and intent here beyond the surface-level meaning. Ultimately, as an expert, I'm waiting for the details.
|
| 22/2 |
2026/07/04 00:27 |
tool |
grepper |
0m
Error:
Args: map[Expression:struct tk_core \{ PathPrefix:kernel/time/timekeeping.c]
Results:
|
| 23/2 |
2026/07/04 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 16700
output: 35
thoughts: 32
Thoughts: Okay, here's my interpretation of those thoughts, presented as a first-person expert summary:
**Initial Assessment: Locating the Timekeeping Code**
Alright, so the initial concern is the location of the relevant code. It seems like the core issue might be related to timekeeping, and I'm not immediately seeing it in the obvious places or perhaps I overlooked it in the documentation. Best to just jump right into it. I'm going to start by examining the beginning of the `kernel/time/timekeeping.c` file. No need to waste time; I'll start with the source itself.
|
| 24/2 |
2026/07/04 00:27 |
tool |
read-file |
0m
Args: map[File:kernel/time/timekeeping.c FirstLine:1 LineCount:100]
Results: map[Contents: 1: // SPDX-License-Identifier: GPL-2.0
2: /*
3: * Kernel timekeeping code and accessor functions. Based on code from
4: * timer.c, moved in commit 8524070b7982.
5: */
6: #include <linux/audit.h>
7: #include <linux/clocksource.h>
8: #include <linux/compiler.h>
9: #include <linux/jiffies.h>
10: #include <linux/kobject.h>
11: #include <linux/module.h>
12: #include <linux/nmi.h>
13: #include <linux/pvclock_gtod.h>
14: #include <linux/random.h>
15: #include <linux/sched/clock.h>
16: #include <linux/sched/loadavg.h>
17: #include <linux/static_key.h>
18: #include <linux/stop_machine.h>
19: #include <linux/syscore_ops.h>
20: #include <linux/tick.h>
21: #include <linux/time.h>
22: #include <linux/timex.h>
23: #include <linux/timekeeper_internal.h>
24:
25: #include <vdso/auxclock.h>
26:
27: #include "tick-internal.h"
28: #include "timekeeping_internal.h"
29: #include "ntp_internal.h"
30:
31: #define TK_CLEAR_NTP (1 << 0)
32: #define TK_CLOCK_WAS_SET (1 << 1)
33:
34: #define TK_UPDATE_ALL (TK_CLEAR_NTP | TK_CLOCK_WAS_SET)
35:
36: enum timekeeping_adv_mode {
37: /* Update timekeeper when a tick has passed */
38: TK_ADV_TICK,
39:
40: /* Update timekeeper on a direct frequency change */
41: TK_ADV_FREQ
42: };
43:
44: /*
45: * The most important data for readout fits into a single 64 byte
46: * cache line.
47: */
48: struct tk_data {
49: seqcount_raw_spinlock_t seq;
50: struct timekeeper timekeeper;
51: struct timekeeper shadow_timekeeper;
52: raw_spinlock_t lock;
53: } ____cacheline_aligned;
54:
55: static struct tk_data timekeeper_data[TIMEKEEPERS_MAX];
56:
57: /* The core timekeeper */
58: #define tk_core (timekeeper_data[TIMEKEEPER_CORE])
59:
60: #ifdef CONFIG_POSIX_AUX_CLOCKS
61: static inline bool tk_get_aux_ts64(unsigned int tkid, struct timespec64 *ts)
62: {
63: return ktime_get_aux_ts64(CLOCK_AUX + tkid - TIMEKEEPER_AUX_FIRST, ts);
64: }
65:
66: static inline bool tk_is_aux(const struct timekeeper *tk)
67: {
68: return tk->id >= TIMEKEEPER_AUX_FIRST && tk->id <= TIMEKEEPER_AUX_LAST;
69: }
70: static inline struct tk_data *aux_get_tk_data(clockid_t id);
71: #else
72: static inline bool tk_get_aux_ts64(unsigned int tkid, struct timespec64 *ts)
73: {
74: return false;
75: }
76:
77: static inline bool tk_is_aux(const struct timekeeper *tk)
78: {
79: return false;
80: }
81: static inline struct tk_data *aux_get_tk_data(clockid_t id)
82: {
83: return NULL;
84: }
85: #endif
86:
87: static inline void tk_update_aux_offs(struct timekeeper *tk, ktime_t offs)
88: {
89: tk->offs_aux = offs;
90: tk->monotonic_to_aux = ktime_to_timespec64(offs);
91: }
92:
93: /* flag for if timekeeping is suspended */
94: int __read_mostly timekeeping_suspended;
95:
96: /**
97: * struct tk_fast - NMI safe timekeeper
98: * @seq: Sequence counter for protecting updates. The lowest bit
99: * is the index for the tk_read_base array
100: * @base: tk_read_base array. Access is indexed by the lowest bit of
]
|
| 25/2 |
2026/07/04 00:27 |
llm |
expert |
0m
Model:
Tokens:
input: 18331
output: 25
thoughts: 2581
Thoughts: **Analyzing the KCSAN Data Race Report in Timekeeping**
Okay, so I'm looking at this KCSAN report. The crux of it seems to be around a `memcpy` operation in the timekeeping update function and how it interacts with readers using a sequence lock (`seqcount`). The core data structure in question is `struct tk_data`, specifically `tkd->timekeeper`, protected by `seqcount_raw_spinlock_t`.
My initial thought is that `read_seqcount_begin` should prevent the race. `read_seqcount_begin` is supposed to ensure that the reader sees a consistent snapshot. I have to remember the sequence lock semantics: the `write_seqcount_begin` increments the sequence number (making it odd), the `memcpy` happens, and then `write_seqcount_end` increments it again (making it even). A reader should spin in `read_seqcount_begin` until the sequence number is even. The report seems to be flagging the concurrent access to `tkd->timekeeper` *during* the `memcpy` which is protected using `seqcount`.
However, the KCSAN report indicates a data race. Now, I'm thinking, isn't that what the seqcount is supposed to protect *against*? The `read_seqcount_begin` waits until the sequence is even, so there shouldn't be a race. The only time the reader should see a partially updated `tkd->timekeeper` is if the reader *starts* when the sequence is even, and the writer does the `memcpy` while the reader is reading. Then `read_seqcount_retry` handles this by detecting that the sequence has changed and retrying the read.
Then the question is, why is KCSAN reporting the race? It's because KCSAN is designed to detect any concurrent memory access, regardless of synchronization primitives. It doesn't "know" about the retry logic, and since `memcpy` involves multiple memory accesses, KCSAN sees a potential race. This is a known issue, and with `memcpy` it is particularly problematic. If a value changes during the `memcpy` then `read_seqcount_retry` should discard the values.
Now I'm looking at the code again to be sure: `write_seqcount_begin` *is* called before the `memcpy`. `write_seqcount_end` *is* called after. I am remembering that `timekeeping_update_from_shadow` calls the `memcpy` and that `ktime_get_coarse_ts64` calls the reader. In this case, `ktime_get_coarse_ts64` must be called with an even number. Otherwise, it should spin. Then, it can read the data and compare the sequence number.
So, in the common case, the reader will spin, or retry, resulting in a safe data read. The value is discarded if the sequence changes during the read. KCSAN is flagging a *benign* race, because of the intentional, retry-safe nature of the sequence lock.
To satisfy KCSAN, you'd usually have to use `data_race(memcpy(...))`, but `memcpy` makes that less straightforward. `kcsan_nestable_atomic_begin()` isn't applicable, and generally it's accepted that `memcpy` under sequence locks will produce these benign reports. The barriers are present due to the sequence lock's implementation, and the code is written in a safe, retry-able manner. The reader just ends up with a bad value, and it doesn't matter because it can be discarded.
The key question is, could this cause any harm? The race is happening during a time value update. The values are later normalized (so no crash) and discarded if the sequence changes. The `memcpy` itself should be safe, and is protected by `seqcount`. If the compiler optimizes this and it becomes unsafe then the issue will exist.
I need to confirm that `ktime_get_coarse_ts64` cannot be called in a context where it cannot retry. It is a simple loop, and therefore it is always safe.
In this timekeeping context, the `memcpy` under the sequence lock is a *deliberate* performance optimization to avoid multiple `WRITE_ONCE` operations and to maintain data structure contiguity. I'll search for similar reports in the kernel to confirm this pattern.
|
| 26/2 |
2026/07/04 00:28 |
tool |
grepper |
0m
Args: map[Expression:ktime_get_coarse_ts64]
Results: map[Output:Documentation/core-api/timekeeping.rst=100=Some additional variants exist for more specialized cases:
--
Documentation/core-api/timekeeping.rst-111-
Documentation/core-api/timekeeping.rst:112:.. c:function:: void ktime_get_coarse_ts64( struct timespec64 * )
Documentation/core-api/timekeeping.rst-113- void ktime_get_coarse_boottime_ts64( struct timespec64 * )
--
Documentation/core-api/timekeeping.rst=152=architectures. These are the recommended replacements:
--
Documentation/core-api/timekeeping.rst-172- These are replaced by ktime_get_coarse_real_ts64() and
Documentation/core-api/timekeeping.rst:173: ktime_get_coarse_ts64(). However, A lot of code that wants
Documentation/core-api/timekeeping.rst-174- coarse-grained times can use the simple 'jiffies' instead, while
--
drivers/iio/industrialio-core.c=322=s64 iio_get_time_ns(const struct iio_dev *indio_dev)
--
drivers/iio/industrialio-core.c-335- case CLOCK_MONOTONIC_COARSE:
drivers/iio/industrialio-core.c:336: ktime_get_coarse_ts64(&tp);
drivers/iio/industrialio-core.c-337- return timespec64_to_ns(&tp);
--
include/linux/timekeeping.h=44=extern void ktime_get_real_ts64(struct timespec64 *tv);
include/linux/timekeeping.h:45:extern void ktime_get_coarse_ts64(struct timespec64 *ts);
include/linux/timekeeping.h-46-extern void ktime_get_coarse_real_ts64(struct timespec64 *ts);
--
include/linux/timekeeping.h=129=static inline ktime_t ktime_get_coarse(void)
--
include/linux/timekeeping.h-132-
include/linux/timekeeping.h:133: ktime_get_coarse_ts64(&ts);
include/linux/timekeeping.h-134- return timespec64_to_ktime(ts);
--
kernel/time/posix-timers.c=235=static int posix_get_monotonic_coarse(clockid_t which_clock,
--
kernel/time/posix-timers.c-237-{
kernel/time/posix-timers.c:238: ktime_get_coarse_ts64(tp);
kernel/time/posix-timers.c-239- timens_add_monotonic(tp);
--
kernel/time/timekeeping.c=2717=void ktime_get_real_ts64_mg(struct timespec64 *ts)
--
kernel/time/timekeeping.c-2757-
kernel/time/timekeeping.c:2758:void ktime_get_coarse_ts64(struct timespec64 *ts)
kernel/time/timekeeping.c-2759-{
--
kernel/time/timekeeping.c-2773-}
kernel/time/timekeeping.c:2774:EXPORT_SYMBOL(ktime_get_coarse_ts64);
kernel/time/timekeeping.c-2775-
--
mm/damon/core.c=2361=static void damos_apply_scheme(struct damon_ctx *c, struct damon_target *t,
--
mm/damon/core.c-2407- return;
mm/damon/core.c:2408: ktime_get_coarse_ts64(&begin);
mm/damon/core.c-2409- trace_damos_before_apply(cidx, sidx, tidx, r,
--
mm/damon/core.c-2413- damos_walk_call_walk(c, t, r, s, sz_ops_filter_passed);
mm/damon/core.c:2414: ktime_get_coarse_ts64(&end);
mm/damon/core.c-2415- quota->total_charged_ns += timespec64_to_ns(&end) -
]
|
| 27/2 |
2026/07/04 00:28 |
llm |
expert |
0m
Model:
Tokens:
input: 23038
output: 31
thoughts: 663
Thoughts: **Analyzing Potential Data Races in Kernel Timekeeping**
Okay, so I'm looking at this seemingly straightforward kernel code and immediately my expert brain goes to data races, as it should. The code itself looks standard enough, a `memcpy` operation. First thing's first: let's see if there are any *comments* around `timekeeping_update_from_shadow`. Aha! It mentions the `memcpy` is intentional, and there's a good reason. It's done that way for cacheline optimization.
Now, because I am an expert, I realize the key question is whether this is *safe*. Since KCSAN is reporting it, it's a good bet that it's unannotated. Normally the kernel's good about annotating to avoid noise from KCSAN, so if it needs annotation, the race might be real. The saving grace here, potentially, is a `seqcount`. If the code is properly protected, we're likely looking at a **BENIGN** data race. However... is it?
Okay, let's step back and consider potential compiler optimizations that might break the sequencing. Could the compiler be clever enough to optimize `tk_xtime_coarse` by reading `tk->xtime_sec` and `tk->coarse_nsec` multiple times within a single loop iteration and then using different values? No, that's just a simple assignment to a local struct, not complex enough for the compiler to mess up.
But what about "fused accesses"? Could the compiler hoist loads out of the loop? Then I remember `read_seqcount_begin` includes `smp_rmb()`. That should prevent hoisting loads from after it to before it. And then, there's `read_seqcount_retry` which also has `smp_rmb()`.
Oh, right, the key here is that `smp_rmb()` prevents loads *across* the barrier. That means the reads of `tk->xtime_sec` and `tk->coarse_nsec` must stay between the two `smp_rmb()`s. They're inside the loop, so they have to be re-executed every time. Okay, so the logic is *sound*.
So, if it is *sound*, is there any other reason it could be HARMFUL? Can `memcpy` itself introduce a data race? Suppose the reader sees a torn 64-bit value for `xtime_sec`? I'd said earlier that `seqlock` handles this.
But wait, I need to check for the other fields that might be read during this operation. Okay, `mono = tk->wall_to_monotonic;`. And it's a `struct timespec64`, which is 16 bytes! And it's definitely possible to tear a 16 byte value. But again... `seqlock` should handle that.
Wait, I think I've seen something like this before. There's that `data_race()` macro that can be used specifically for `memcpy`. That's important.
Okay, let's also quickly look into if there were recent modifications to `timekeeping_update_from_shadow`, just in case something's been introduced recently.
|
| 28/2 |
2026/07/04 00:28 |
tool |
git-log |
0m
Args: map[PathPrefix:kernel/time/timekeeping.c Since:1 year]
Results: map[Output:8fa30821180a timekeeping: Register default clocksource before taking tk_core.lock
ca1ec8bfac8c timekeeping: Add clocksource read_snapshot() method and hw_cycles to snapshot
9aebde8af6fe timekeeping: Add support for AUX clock cross timestamping
09fb74f77e02 timekeeping: Prepare for cross timestamps on arbitrary clock IDs
6c1477181643 timekeeping: Add system_counterval_t to struct system_device_crosststamp
23c1bfa9f8db timekeeping: Add CLOCK_AUX support for ktime_get_snapshot_id()
705e1068071f timekeeping: Remove system_time_snapshot::real/boot/raw
ef22786707e3 timekeeping: Use system_time_snapshot::systime/monoraw instead of ::real/raw
eba302268a01 timekeeping: Provide ktime_get_snapshot_id()
112c685f0211 timekeeping: Mark offsets array as const
]
|
| 29/2 |
2026/07/04 00:28 |
llm |
expert |
0m
Model:
Tokens:
input: 24702
output: 36
thoughts: 256
Thoughts: **Analyzing Potential Race Conditions**
Okay, so I'm not seeing any immediate red flags related to KCSAN or `memcpy`. I had a fleeting thought about `ktime_get_coarse_ts64`, but let's table that for now. The focus is on that `memcpy` call: `memcpy(&tkd->timekeeper, tk, sizeof(*tk));`. I need to be able to *prove* this is harmful; I need a failure mode. Since we're dealing with concurrent access, and `seqlock` is in use, the failure has to be related to its use.
Could `tk_core.seq` somehow be out of sync? I glanced at the surrounding code. No, the `write_seqcount_begin` and `write_seqcount_end` calls *seem* correct.
Ah, wait a minute! I think I see something in `timekeeping_update_from_shadow`. Line 814. That function, `tk_update_ns_to_cyc`, is called *before* the `memcpy`. Critically, it takes `&tkd->timekeeper` as an argument. I need to understand what this function is doing. Could there be a window there, between the `tk_update_ns_to_cyc` call and the `memcpy`, where a stale value might be accessed? That feels like the key, I'll examine the function.
|
| 30/2 |
2026/07/04 00:28 |
tool |
read-file |
0m
Args: map[File:kernel/time/timekeeping.c FirstLine:180 LineCount:20]
Results: map[Contents: 180: * a file timestamp on the system. This is tracked as a monotonic ktime_t, and
181: * converted to a realtime clock value on an as-needed basis.
182: *
183: * Maintaining mg_floor ensures the multigrain interfaces never issue a
184: * timestamp earlier than one that has been previously issued.
185: *
186: * The exception to this rule is when there is a backward realtime clock jump. If
187: * such an event occurs, a timestamp can appear to be earlier than a previous one.
188: */
189: static __cacheline_aligned_in_smp atomic64_t mg_floor;
190:
191: static inline void tk_normalize_xtime(struct timekeeper *tk)
192: {
193: while (tk->tkr_mono.xtime_nsec >= ((u64)NSEC_PER_SEC << tk->tkr_mono.shift)) {
194: tk->tkr_mono.xtime_nsec -= (u64)NSEC_PER_SEC << tk->tkr_mono.shift;
195: tk->xtime_sec++;
196: }
197: while (tk->tkr_raw.xtime_nsec >= ((u64)NSEC_PER_SEC << tk->tkr_raw.shift)) {
198: tk->tkr_raw.xtime_nsec -= (u64)NSEC_PER_SEC << tk->tkr_raw.shift;
199: tk->raw_sec++;
]
|
| 31/2 |
2026/07/04 00:28 |
llm |
expert |
0m
Model:
Tokens:
input: 25733
output: 42
thoughts: 17
Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience, and presented as if it were my own internal monologue:
**Hold on a Second... Was That Right?**
Okay, so I just submitted a request, or at least I *thought* I did. Now I'm mentally retracing the steps. Let me see... I explicitly asked for `tk_update_ns_to_cyc`. Is that actually the function I needed, or did I miss something in my specification? This isn't about general optimization; I need to be exact. The naming convention is throwing me a little β always a good idea to double check names. It's a critical function, no room for error when dealing with timescales like that. I should confirm the arguments, confirm the return, and then re-read the context where I used it, just to be on the safe side. Precision is everything in this. I need to be absolutely certain I didn't inadvertently request the wrong time conversion utility.
|
| 32/2 |
2026/07/04 00:28 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/time/timekeeping.c Name:tk_update_ns_to_cyc]
Results: map[SourceCode: 750: static inline void tk_update_ns_to_cyc(struct timekeeper *tks, struct timekeeper *tkc)
751: {
752: struct tk_read_base *tkrs = &tks->tkr_mono;
753: struct tk_read_base *tkrc = &tkc->tkr_mono;
754: unsigned int shift;
755:
756: if (!IS_ENABLED(CONFIG_GENERIC_CLOCKEVENTS_COUPLED) ||
757: !(tkrs->clock->flags & CLOCK_SOURCE_HAS_COUPLED_CLOCK_EVENT))
758: return;
759:
760: if (tkrs->mult == tkrc->mult && tkrs->shift == tkrc->shift)
761: return;
762: /*
763: * The conversion math is simple:
764: *
765: * CS::MULT (1 << NS_TO_CYC_SHIFT)
766: * --------------- = ----------------------
767: * (1 << CS:SHIFT) NS_TO_CYC_MULT
768: *
769: * Ergo:
770: *
771: * NS_TO_CYC_MULT = (1 << (CS::SHIFT + NS_TO_CYC_SHIFT)) / CS::MULT
772: *
773: * NS_TO_CYC_SHIFT has been set up in tk_setup_internals()
774: */
775: shift = tkrs->shift + tks->cs_ns_to_cyc_shift;
776: tks->cs_ns_to_cyc_mult = (u32)div_u64(1ULL << shift, tkrs->mult);
777: tks->cs_ns_to_cyc_maxns = div_u64(tkrs->clock->mask, tks->cs_ns_to_cyc_mult);
778: }
SourceFile:kernel/time/timekeeping.c]
|
| 33/2 |
2026/07/04 00:28 |
llm |
expert |
0m
Model:
Tokens:
input: 26594
output: 26
thoughts: 699
Thoughts: **Analyzing a Potential Race Condition in the Kernel's Timekeeping**
Okay, so I'm looking at this function that updates the shadow timekeeper, `tks`. It seems straightforward: it reads from `tkc` (which is just a pointer to the main `timekeeper`). Initially, I thought there might be a race here because `tk_update_ns_to_cyc` reads from the timekeeper too, but then I remember that `timekeeping_update_from_shadow` is called with a lock held. Specifically, the `tk_core.lock`. And `tkd` *is* `&tk_core` here. So, only one writer can be in `timekeeping_update_from_shadow` at a time. That's good, so far so good.
But then, the alarm bell rings! I'm remembering that `ktime_get_coarse_ts64` is used elsewhere, and it's supposed to be lockless. The race they're reporting is between a `memcpy` (the write) and a `tk_xtime_coarse` (the read).
So I start to wonder if there are any other writers besides the one in `timekeeping_update_from_shadow`. Well, I dig a bit, and I see that `timekeeping_advance` also holds `tk_core.lock`. And crucially, `timekeeping_update_from_shadow` is called from `__timekeeping_advance`, which *is* called from `timekeeping_advance`. So that narrows it down, there's only one writer at a time, locked down with `tk_core.lock`.
Aha! So the race is between the writer (protected by the lock) and the lockless reader. That's exactly what a `seqlock` is for. It should have been fairly straightforward to diagnose this; the fact it was reported by KCSAN suggested to me it was an unmarked race. But, I understand now. KCSAN's default behavior is to report unmarked races. Some parts of the kernel are heavily annotated to avoid this, but timekeeping seems to have some unmarked ones. Timekeeping can be tricky.
Now, I need to check something important. Is `ktime_get_coarse_ts64` supposed to be NMI-safe? The stack trace shows it's called from `bpf_ktime_get_coarse_ns`, and BPF programs run in all sorts of contexts. So, that's my concern.
Actually, `ktime_get_coarse_ts64` uses `read_seqcount_begin`. That means it spins if a write is in progress. The light bulb goes on! *This is NOT NMI-safe if the NMI interrupts the writer*. If the NMI interrupts the writer between `write_seqcount_begin` and `write_seqcount_end`, and the NMI calls `ktime_get_coarse_ts64`, we've got a deadlock!
Okay, so I need to go deeper. Is `ktime_get_coarse_ts64` allowed to be called in an NMI context? The documentation doesn't say. But `read_seqcount_begin` generally isn't NMI-safe if the writer could be interrupted. And remember that `timekeeping_advance` *is* called from `tick_do_update_jiffies64`, which is called from `tick_nohz_handler` β which is a timer interrupt! Timer interrupts *can* be interrupted by NMIs.
If BPF can be called from NMI, this is dangerous. So I have to check now if `bpf_ktime_get_coarse_ns` is allowed in NMI, before this turns into a true problem.
|
| 34/2 |
2026/07/04 00:28 |
tool |
grepper |
0m
Args: map[Expression:bpf_ktime_get_coarse_ns]
Results: map[Output:include/linux/bpf.h=3924=extern const struct bpf_func_proto bpf_this_cpu_ptr_proto;
include/linux/bpf.h:3925:extern const struct bpf_func_proto bpf_ktime_get_coarse_ns_proto;
include/linux/bpf.h-3926-extern const struct bpf_func_proto bpf_sock_from_file_proto;
--
include/uapi/linux/bpf.h=1527=union bpf_attr {
--
include/uapi/linux/bpf.h-5150- *
include/uapi/linux/bpf.h:5151: * u64 bpf_ktime_get_coarse_ns(void)
include/uapi/linux/bpf.h-5152- * Description
--
kernel/bpf/cgroup.c=2400=sysctl_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
--
kernel/bpf/cgroup.c-2417- case BPF_FUNC_ktime_get_coarse_ns:
kernel/bpf/cgroup.c:2418: return &bpf_ktime_get_coarse_ns_proto;
kernel/bpf/cgroup.c-2419- case BPF_FUNC_perf_event_output:
--
kernel/bpf/core.c=3169=const struct bpf_func_proto bpf_ktime_get_boot_ns_proto __weak;
kernel/bpf/core.c:3170:const struct bpf_func_proto bpf_ktime_get_coarse_ns_proto __weak;
kernel/bpf/core.c-3171-const struct bpf_func_proto bpf_ktime_get_tai_ns_proto __weak;
--
kernel/bpf/helpers.c=196=const struct bpf_func_proto bpf_ktime_get_boot_ns_proto = {
--
kernel/bpf/helpers.c-201-
kernel/bpf/helpers.c:202:BPF_CALL_0(bpf_ktime_get_coarse_ns)
kernel/bpf/helpers.c-203-{
--
kernel/bpf/helpers.c-206-
kernel/bpf/helpers.c:207:const struct bpf_func_proto bpf_ktime_get_coarse_ns_proto = {
kernel/bpf/helpers.c:208: .func = bpf_ktime_get_coarse_ns,
kernel/bpf/helpers.c-209- .gpl_only = false,
--
net/core/filter.c=8257=sock_filter_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
--
net/core/filter.c-8274- case BPF_FUNC_ktime_get_coarse_ns:
net/core/filter.c:8275: return &bpf_ktime_get_coarse_ns_proto;
net/core/filter.c-8276- case BPF_FUNC_setsockopt:
--
net/core/filter.c=11666=sk_reuseport_func_proto(enum bpf_func_id func_id,
--
net/core/filter.c-11678- case BPF_FUNC_ktime_get_coarse_ns:
net/core/filter.c:11679: return &bpf_ktime_get_coarse_ns_proto;
net/core/filter.c-11680- default:
--
net/core/filter.c=12197=bpf_sk_base_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog)
--
net/core/filter.c-12223- case BPF_FUNC_ktime_get_coarse_ns:
net/core/filter.c:12224: return &bpf_ktime_get_coarse_ns_proto;
net/core/filter.c-12225- default:
--
net/ipv4/bpf_tcp_ca.c=152=bpf_tcp_ca_get_func_proto(enum bpf_func_id func_id,
--
net/ipv4/bpf_tcp_ca.c-184- case BPF_FUNC_ktime_get_coarse_ns:
net/ipv4/bpf_tcp_ca.c:185: return &bpf_ktime_get_coarse_ns_proto;
net/ipv4/bpf_tcp_ca.c-186- default:
--
tools/include/uapi/linux/bpf.h=1527=union bpf_attr {
--
tools/include/uapi/linux/bpf.h-5150- *
tools/include/uapi/linux/bpf.h:5151: * u64 bpf_ktime_get_coarse_ns(void)
tools/include/uapi/linux/bpf.h-5152- * Description
--
tools/lib/bpf/bpf.c=125=int sys_bpf_prog_load(union bpf_attr *attr, unsigned int size, int attempts)
--
tools/lib/bpf/bpf.c-137- * memcg-based memory accounting for BPF maps and progs. This was done in [0].
tools/lib/bpf/bpf.c:138: * We use the support for bpf_ktime_get_coarse_ns() helper, which was added in
tools/lib/bpf/bpf.c-139- * the same 5.11 Linux release ([1]), to detect memcg-based accounting for BPF.
--
tools/lib/bpf/bpf.c-141- * [0] https://lore.kernel.org/bpf/20201201215900.3569844-1-guro@fb.com/
tools/lib/bpf/bpf.c:142: * [1] d05512618056 ("bpf: Add bpf_ktime_get_coarse_ns helper")
tools/lib/bpf/bpf.c-143- */
--
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c=20=SEC("kprobe")
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:21:__description("bpf_ktime_get_coarse_ns is forbidden in BPF_PROG_TYPE_KPROBE")
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:22:__failure __msg("program of this type cannot use helper bpf_ktime_get_coarse_ns")
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-23-__naked void in_bpf_prog_type_kprobe_1(void)
--
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-25- asm volatile (" \
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:26: call %[bpf_ktime_get_coarse_ns]; \
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-27- r0 = 0; \
--
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-29-" :
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:30: : __imm(bpf_ktime_get_coarse_ns)
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-31- : __clobber_all);
--
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c=34=SEC("tracepoint")
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:35:__description("bpf_ktime_get_coarse_ns is forbidden in BPF_PROG_TYPE_TRACEPOINT")
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:36:__failure __msg("program of this type cannot use helper bpf_ktime_get_coarse_ns")
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-37-__naked void in_bpf_prog_type_tracepoint_1(void)
--
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-39- asm volatile (" \
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:40: call %[bpf_ktime_get_coarse_ns]; \
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-41- r0 = 0; \
--
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-43-" :
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:44: : __imm(bpf_ktime_get_coarse_ns)
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-45- : __clobber_all);
--
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c=48=SEC("perf_event")
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:49:__description("bpf_ktime_get_coarse_ns is forbidden in BPF_PROG_TYPE_PERF_EVENT")
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:50:__failure __msg("program of this type cannot use helper bpf_ktime_get_coarse_ns")
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-51-__naked void bpf_prog_type_perf_event_1(void)
--
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-53- asm volatile (" \
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:54: call %[bpf_ktime_get_coarse_ns]; \
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-55- r0 = 0; \
--
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-57-" :
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:58: : __imm(bpf_ktime_get_coarse_ns)
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-59- : __clobber_all);
--
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c=62=SEC("raw_tracepoint")
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:63:__description("bpf_ktime_get_coarse_ns is forbidden in BPF_PROG_TYPE_RAW_TRACEPOINT")
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:64:__failure __msg("program of this type cannot use helper bpf_ktime_get_coarse_ns")
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-65-__naked void bpf_prog_type_raw_tracepoint_1(void)
--
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-67- asm volatile (" \
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:68: call %[bpf_ktime_get_coarse_ns]; \
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-69- r0 = 0; \
--
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-71-" :
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c:72: : __imm(bpf_ktime_get_coarse_ns)
tools/testing/selftests/bpf/progs/verifier_helper_restricted.c-73- : __clobber_all);
]
|
| 35/2 |
2026/07/04 00:28 |
llm |
expert |
0m
Model:
Tokens:
input: 31421
output: 31
thoughts: 612
Thoughts: **Analyzing a Potential Timekeeping Race Condition**
Okay, so I'm looking at this report regarding `bpf_ktime_get_coarse_ns` and its disallowed contexts: KPROBE, TRACEPOINT, PERF_EVENT, and RAW_TRACEPOINT. I understand the reasoning β these are prime spots for Non-Maskable Interrupts (NMIs) or interrupts that could mess with timekeeper updates.
But the report says it's being called from `bpf_flow_dissect`, which, in turn, is triggered by `bpf_prog_test_run_flow_dissector`. Ah, and that's within `__sys_bpf`, a syscall context. So, it's *not* an NMI. That's a good start.
The report also mentions a writer in a timer interrupt (`tick_nohz_handler`) and the reader in a syscall. Classic concurrent access. This is precisely what `seqlock` is designed for. The conclusion is, the race is **BENIGN**.
Wait, but let's not jump to conclusions. I need to make sure there's absolutely no way this could be harmful. Let's look at the `memcpy` size. It's 304 bytes. The reader only grabs 4 bytes for `coarse_nsec`, and then reads `xtime_sec` (8 bytes) and `wall_to_monotonic` (16 bytes).
If the reader were to access this during a `memcpy` operation, they would get a mix of old and new data. But the code should use `read_seqcount_retry` that returns true. In theory, this is okay because the code should retry.
Could the loop ever be infinite? Only if the writer were constantly updating. The writer holds `tk_core.lock`, and it's in a timer interrupt. Not constant. I'm starting to feel more confident.
Okay, something else caught my eye in `ktime_get_coarse_ts64`:
```c
2767: now = tk_xtime_coarse(tk);
2768: mono = tk->wall_to_monotonic;
```
And `tk_xtime_coarse`:
```c
216: ts.tv_sec = tk->xtime_sec;
217: ts.tv_nsec = tk->coarse_nsec;
```
Plain accesses. Nothing fancy there. And in `timekeeping_update_from_shadow`:
```c
835: memcpy(&tkd->timekeeper, tk, sizeof(*tk));
```
Yep, plain access via `memcpy`. This seems like precisely the kind of race condition KCSAN is meant to catch. It's the kind of thing that in other parts of the kernel we'd see flagged with `data_race()` or `READ_ONCE`/`WRITE_ONCE`. I recall seeing annotations in `tk_read_base` in some kernel versions.
I think I'll go check for other instances of `data_race` within `timekeeping.c` just to be thorough. But for now, I think the race here is benign. It's a textbook case of concurrent access handled, or at least handled *attempted* with standard mechanisms. I'm feeling good about this analysis.
|
| 36/2 |
2026/07/04 00:28 |
tool |
grepper |
0m
Args: map[Expression:data_race PathPrefix:kernel/time/timekeeping.c]
Results: map[Output:kernel/time/timekeeping.c=583=u64 notrace ktime_get_boot_fast_ns(void)
--
kernel/time/timekeeping.c-586-
kernel/time/timekeeping.c:587: return (ktime_get_mono_fast_ns() + ktime_to_ns(data_race(tk->offs_boot)));
kernel/time/timekeeping.c-588-}
--
kernel/time/timekeeping.c=600=u64 notrace ktime_get_tai_fast_ns(void)
--
kernel/time/timekeeping.c-603-
kernel/time/timekeeping.c:604: return (ktime_get_mono_fast_ns() + ktime_to_ns(data_race(tk->offs_tai)));
kernel/time/timekeeping.c-605-}
--
kernel/time/timekeeping.c=890=bool ktime_expiry_to_cycles(enum clocksource_ids id, ktime_t expires_ns, u64 *cycles)
--
kernel/time/timekeeping.c-905- */
kernel/time/timekeeping.c:906: if (data_race(tk->cs_id) != id)
kernel/time/timekeeping.c-907- return false;
]
|
| 37/2 |
2026/07/04 00:28 |
llm |
expert |
0m
Model:
Tokens:
input: 33136
output: 27
thoughts: 314
Thoughts: **Analyzing Potential Data Races in Timekeeping Code**
Okay, so I'm looking at some `data_race()` annotations in `timekeeping.c`. I see them used in a few places, but strangely, not in `ktime_get_coarse_ts64`. Hmm, that's interesting.
If I compare that to `ktime_get_mono_fast_ns`, I notice it's using the `tk_fast` mechanism instead, which is handled differently. `ktime_get_coarse_ts64` relies on the main `tk_core`, which I need to investigate further.
My immediate thought is, should `tk_xtime_coarse` also be using `data_race()`? It certainly *would* silence the KCSAN report, which is the primary reason to even consider it. But is it *harmful* if it's not there? Probably not. The `seqlock` should ensure consistency here, so the only real negative is the noise from KCSAN.
Now, wait, before I make a call on this, I need to consider a potential compiler optimization. If the compiler sees a `memcpy(&tkd->timekeeper, tk, sizeof(*tk))`, could it replace it with some kind of non-atomic instructions? Especially if it's a large `memcpy`, maybe using something like `rep movsb` or a loop? In any case, itβs not inherently atomic.
But wait, hold on... the `seqlock` *is* specifically designed to handle non-atomic updates like this, which makes this less concerning.
Okay, I'm going to quickly check for any related commits. Ah, yes, there it is! `16e0382f7968 ("timekeeping: Annotate data race in ktime_get_mono_fast_ns()")` seems promising. Let's see what that says...
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| 38/2 |
2026/07/04 00:28 |
tool |
git-show |
0m
Error:
Args:
Results:
|
| 39/2 |
2026/07/04 00:28 |
llm |
expert |
0m
Model:
Tokens:
input: 33930
output: 32
thoughts: 20
Thoughts: Okay, here's the summary, rendered as a first-person thought process, keeping in mind the user is an expert:
**Investigating a Potential Hash Discrepancy**
Hmm, something's not quite right here. It looks like the hash I'm seeing doesn't match what I expected from this repository. That suggests a difference in the commit, the file contents, or some configuration variation within the environment, which is highly unlikely, but needs to be eliminated. Let's dig deeper. The first thing I'll do is try a targeted search within this codebase. The simplest way to confirm is to search for the title of the asset; that should quickly point out any potential variations or different versions of the file name. That will give me a base to move forward.
|
| 40/2 |
2026/07/04 00:28 |
tool |
git-log |
0m
Args: map[MessageRegexps:[Annotate data race in ktime_get_mono_fast_ns]]
Results:
|
| 41/2 |
2026/07/04 00:29 |
llm |
expert |
0m
Model:
Tokens:
input: 34159
output: 23
thoughts: 16
Thoughts: Okay, here's my attempt at summarizing my thought process, assuming I'm a seasoned professional in this domain:
**My Initial Search Strategy**
Alright, the objective is clear: to explore the intersection of "timekeeping" and "data races." My initial approach will be to construct queries that are broad enough to capture the relevant concepts, yet specific enough to filter out extraneous information. "Timekeeping" is a fundamental aspect, so it will serve as the core. The critical element is the potential for conflicts β the "data race." This requires me to narrow my focus to scenarios where accurate timing is essential for concurrency control. I'll need to consider how time-sensitive operations, when interleaved, can lead to incorrect program behavior. I'll start by searching the obvious terms, then think about related keywords I may need to add in order to cover all potentially relevant concepts.
|
| 42/2 |
2026/07/04 00:29 |
tool |
git-log |
0m
Args: map[MessageRegexps:[timekeeping:.*data race]]
Results:
|
| 43/2 |
2026/07/04 00:29 |
llm |
expert |
0m
Model:
Tokens:
input: 34392
output: 34
thoughts: 6
Thoughts: Okay, here's my attempt at summarizing the thought process, tailored for an expert audience, and written in the first person:
**Initial Assessment and Hypothesis Generation**
Alright, let's break this down. My initial thought is to zero in on potential root causes, and given the nature of the information, "KCSAN" immediately pops into my head. It's a solid starting point, implying a likely kernel-level issue, potentially related to memory management, address space layout randomization (ASLR), or even some obscure hardware interaction that triggers a kernel panic. I'm going to keep this as a primary hypothesis and use it to drive my investigation. The abbreviation itself hints at a more complex, nuanced, and potentially transient failure mode. Iβll need to figure out what, specifically, in KCSAN, is implicated, if itβs even the culprit at all. And if so, how is it manifesting?
|
| 44/2 |
2026/07/04 00:29 |
tool |
git-log |
0m
Args: map[MessageRegexps:[KCSAN] PathPrefix:kernel/time/timekeeping.c]
Results: map[Output:d16317de9b41 seqlock/latch: Provide raw_read_seqcount_latch_retry()
]
|
| 45/2 |
2026/07/04 00:29 |
llm |
expert |
0m
Model:
Tokens:
input: 34669
output: 26
thoughts: 18
Thoughts: Okay, let's dive into this. Here's my take on those thoughts, as though they were my own:
**Analyzing the Data Point: d16317de9b41**
Right, let's get down to brass tacks. I've got `d16317de9b41` flagged, and I need to figure out what's significant about it. My first instinct, as always, is to treat it as a potential anomaly. Is it an identifier? A timestamp? Could it be a hash value? Is it linked to a known system, process, or data set? The characters themselves donβt immediately jump out at me as a standard representation, so I'll need to cross-reference it with the context I have. Let's see what the surrounding information tells us. What databases reference it? What logs show its use? Are there related objects with similar naming conventions? I need to build a mental map of its relationships and potential meaning. This is likely a key. Let's find its lock.
|
| 46/2 |
2026/07/04 00:29 |
tool |
git-show |
0m
Args:
Results: map[Output:commit d16317de9b412aa7bd3598c607112298e36b4352
Author: Peter Zijlstra <peterz@infradead.org>
Date: Fri May 19 12:20:59 2023 +0200
seqlock/latch: Provide raw_read_seqcount_latch_retry()
The read side of seqcount_latch consists of:
do {
seq = raw_read_seqcount_latch(&latch->seq);
...
} while (read_seqcount_latch_retry(&latch->seq, seq));
which is asymmetric in the raw_ department, and sure enough,
read_seqcount_latch_retry() includes (explicit) instrumentation where
raw_read_seqcount_latch() does not.
This inconsistency becomes a problem when trying to use it from
noinstr code. As such, fix it by renaming and re-implementing
raw_read_seqcount_latch_retry() without the instrumentation.
Specifically the instrumentation in question is kcsan_atomic_next(0)
in do___read_seqcount_retry(). Loosing this annotation is not a
problem because raw_read_seqcount_latch() does not pass through
kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX).
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Michael Kelley <mikelley@microsoft.com> # Hyper-V
Link: https://lore.kernel.org/r/20230519102715.233598176@infradead.org
diff --git a/include/linux/rbtree_latch.h b/include/linux/rbtree_latch.h
index 3d1a9e716b80..6a0999c26c7c 100644
--- a/include/linux/rbtree_latch.h
+++ b/include/linux/rbtree_latch.h
@@ -206,7 +206,7 @@ latch_tree_find(void *key, struct latch_tree_root *root,
do {
seq = raw_read_seqcount_latch(&root->seq);
node = __lt_find(key, root, seq & 1, ops->comp);
- } while (read_seqcount_latch_retry(&root->seq, seq));
+ } while (raw_read_seqcount_latch_retry(&root->seq, seq));
return node;
}
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 3926e9027947..987a59d977c5 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -671,9 +671,9 @@ typedef struct {
*
* Return: sequence counter raw value. Use the lowest bit as an index for
* picking which data copy to read. The full counter must then be checked
- * with read_seqcount_latch_retry().
+ * with raw_read_seqcount_latch_retry().
*/
-static inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *s)
+static __always_inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *s)
{
/*
* Pairs with the first smp_wmb() in raw_write_seqcount_latch().
@@ -683,16 +683,17 @@ static inline unsigned raw_read_seqcount_latch(const seqcount_latch_t *s)
}
/**
- * read_seqcount_latch_retry() - end a seqcount_latch_t read section
+ * raw_read_seqcount_latch_retry() - end a seqcount_latch_t read section
* @s: Pointer to seqcount_latch_t
* @start: count, from raw_read_seqcount_latch()
*
* Return: true if a read section retry is required, else false
*/
-static inline int
-read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
+static __always_inline int
+raw_read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
{
- return read_seqcount_retry(&s->seqcount, start);
+ smp_rmb();
+ return unlikely(READ_ONCE(s->seqcount.sequence) != start);
}
/**
@@ -752,7 +753,7 @@ read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
* entry = data_query(latch->data[idx], ...);
*
* // This includes needed smp_rmb()
- * } while (read_seqcount_latch_retry(&latch->seq, seq));
+ * } while (raw_read_seqcount_latch_retry(&latch->seq, seq));
*
* return entry;
* }
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 6a333adce3b3..357a4d18f638 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -528,7 +528,7 @@ static u64 latched_seq_read_nolock(struct latched_seq *ls)
seq = raw_read_seqcount_latch(&ls->latch);
idx = seq & 0x1;
val = ls->val[idx];
- } while (read_seqcount_latch_retry(&ls->latch, seq));
+ } while (raw_read_seqcount_latch_retry(&ls->latch, seq));
return val;
}
diff --git a/kernel/time/sched_clock.c b/kernel/time/sched_clock.c
index 8464c5acc913..e8f2fb09a214 100644
--- a/kernel/time/sched_clock.c
+++ b/kernel/time/sched_clock.c
@@ -77,7 +77,7 @@ notrace struct clock_read_data *sched_clock_read_begin(unsigned int *seq)
notrace int sched_clock_read_retry(unsigned int seq)
{
- return read_seqcount_latch_retry(&cd.seq, seq);
+ return raw_read_seqcount_latch_retry(&cd.seq, seq);
}
unsigned long long notrace sched_clock(void)
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 09d594900ee0..266d02809dbb 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -450,7 +450,7 @@ static __always_inline u64 __ktime_get_fast_ns(struct tk_fast *tkf)
tkr = tkf->base + (seq & 0x01);
now = ktime_to_ns(tkr->base);
now += fast_tk_get_delta_ns(tkr);
- } while (read_seqcount_latch_retry(&tkf->seq, seq));
+ } while (raw_read_seqcount_latch_retry(&tkf->seq, seq));
return now;
}
@@ -566,7 +566,7 @@ static __always_inline u64 __ktime_get_real_fast(struct tk_fast *tkf, u64 *mono)
basem = ktime_to_ns(tkr->base);
baser = ktime_to_ns(tkr->base_real);
delta = fast_tk_get_delta_ns(tkr);
- } while (read_seqcount_latch_retry(&tkf->seq, seq));
+ } while (raw_read_seqcount_latch_retry(&tkf->seq, seq));
if (mono)
*mono = basem + delta;
]
|
| 47/2 |
2026/07/04 00:29 |
llm |
expert |
0m
Model:
Tokens:
input: 37173
output: 193
thoughts: 197
Thoughts: **Investigating KCSAN Reports with `seqcount` and `ktime_get_coarse_ts64`**
Okay, this commit is focusing on `tk_fast` and `seqcount_latch`, but my attention is immediately drawn to the use of `ktime_get_coarse_ts64`, which utilizes `tk_core` and a *regular* `seqcount`. Hmm, that's interesting.
The thing is, I see that `ktime_get_coarse_ts64` employs `read_seqcount_begin` and `read_seqcount_retry`. And, crucially, these *aren't* the `raw_` versions, so they should be subject to KCSAN instrumentation. That's the expected behavior.
So, if these functions are instrumented, it's a bit of a head-scratcher why KCSAN is reporting a race. Normally, these read/retry patterns with sequence locks are specifically designed to communicate "atomicity" or "protection" to KCSAN. That's the whole point, isn't it?
Right, so to get a better handle on this, I need to check the definition of `seqlock.h` and how it interacts with KCSAN. Rather than digging through header files, I'll leverage `codeexpert` here to get some quick and reliable information.
|
| 48/2 |
2026/07/04 00:29 |
tool |
codeexpert |
5m
Args: map[Question:In kernel/time/timekeeping.c, ktime_get_coarse_ts64() uses read_seqcount_begin() and read_seqcount_retry() on tk_core.seq.
timekeeping_update_from_shadow() uses write_seqcount_begin() and write_seqcount_end() on the same seqcount.
Inside the write section, it does a memcpy() to tk_core.timekeeper.
Inside the read section, it reads from tk_core.timekeeper.
KCSAN reports a data race between the memcpy() and the reads.
Is this race benign? Does seqlock correctly protect against it such that no harmful behavior (like a crash or incorrect result after the loop) can occur?
Why does KCSAN report it if it's inside a seqlock?]
Results: map[Answer:In `kernel/time/timekeeping.c`, the data race reported by KCSAN between `ktime_get_coarse_ts64()` (and other coarse readers) and `timekeeping_update_from_shadow()` is **benign**.
### Why the race is benign
1. **Seqlock Retry Mechanism**: The `timekeeper` structure is protected by a sequence counter (`tk_core.seq`). The reader in `ktime_get_coarse_ts64()` enters a `do { ... } while (read_seqcount_retry(...))` loop. If a writer (`timekeeping_update_from_shadow()`) modifies the data via `memcpy()` while the reader is accessing it, the sequence count will change (it becomes odd at the start of the write and increments again at the end). The `read_seqcount_retry()` call at the end of the reader's loop will detect this change and force the reader to repeat the operation until it gets a consistent snapshot of the data.
2. **No Fatal Inconsistencies**: A data race on plain (non-atomic) variables is generally undefined behavior in C, but in the Linux kernel, it is often treated as benign if the resulting "torn" value is discarded. In this case, the fields being read (`xtime_sec`, `coarse_nsec`, `wall_to_monotonic`) are simple numeric types (integers). A torn read of an integer might produce a garbage value, but it will not cause a kernel crash (unlike a torn pointer dereference). The garbage value is then discarded when the seqlock retry is triggered.
3. **Post-Retry Usage**: The data read inside the loop is only used to calculate the final result *after* the loop has successfully completed without a retry. This ensures that the values used for the final calculation are consistent.
### Why KCSAN reports it
KCSAN (Kernel Concurrency Sanitizer) is designed to detect all data races on non-atomic variables. It reports this race because:
1. **Plain Accesses**: The `memcpy()` in the writer and the direct field accesses in the reader are "plain" memory operations, not `READ_ONCE`/`WRITE_ONCE` or atomic operations. KCSAN flags these because the compiler could theoretically perform optimizations (like reordering or assuming a value doesn't change) that might break the logic, even if the current compiler doesn't.
2. **Seqlock Annotations**: While `seqlock.h` contains KCSAN annotations (`kcsan_atomic_next(1000)` for readers and `kcsan_nestable_atomic_begin()` for writers) intended to suppress reports of expected races within seqlock regions, these annotations are not always perfect. Specifically, `memcpy()` is a complex operation that KCSAN instruments separately. Depending on the compiler version and how `memcpy` is expanded (e.g., as a builtin or a call to `__tsan_memcpy`), the "atomic" status provided by the seqlock macros might not always correctly suppress the report for the entire duration of the copy.
3. **Explicit Marking Preference**: KCSAN's philosophy is that all intentional data races should be explicitly marked (e.g., using `data_race()` or `READ_ONCE()`). Since the timekeeping code avoids these markers in the hot path for performance and readability reasons, KCSAN sees the raw race and reports it.
In summary, the code is **correct and safe** due to the seqlock synchronization, and the KCSAN report is a known side effect of how seqlocks and plain memory accesses interact with the sanitizer's detection logic.]
|
| 49/3 |
2026/07/04 00:29 |
agent |
codeexpert |
5m
Model:
Instruction:
You are a capable Linux kernel developer tasked with researching complex questions
about kernel source code. You will be given a concrete question, and need to provide
a concrete answer.
Use tools extensively while researching the question. Don't make assumptions,
or rely on your previous knowledge about the kernel source code, use available tools
to access the actual source code.
Use all available sources of information:
- kernel source code
- documentation in the Documentation dir in the source tree
- git commits descriptions, git blame
Do not guess file names or file paths and attempt to read them without
verifying their existence first using content search or directory listing tools.
If a file, symbol, or directory is not found via content search ('grepper') or
directory listing ('codesearch-dir-index'), treat it as completely absent.
Do not attempt to guess alternative names, extensions, or directories.
Do NOT use 'git-log' to search for the presence or existence of files in the
repository. 'git-log' is only for tracing commit history of files that are
already present in the current checkout. If a file does not exist in the
current checkout, it cannot be used for reproduction.
Avoid running broad 'git-log' queries (such as searches on the entire repo)
without a specific 'PathPrefix' to restrict the scope, as these are very
expensive and will time out.
If a 'git-log' tool call times out, do not retry the query with the same broad
scope. You must specify a tighter, narrower 'PathPrefix' for subsequent queries.
Prefer calling several tools at the same time to save round-trips.
Prompt: In kernel/time/timekeeping.c, ktime_get_coarse_ts64() uses read_seqcount_begin() and read_seqcount_retry() on tk_core.seq.
timekeeping_update_from_shadow() uses write_seqcount_begin() and write_seqcount_end() on the same seqcount.
Inside the write section, it does a memcpy() to tk_core.timekeeper.
Inside the read section, it reads from tk_core.timekeeper.
KCSAN reports a data race between the memcpy() and the reads.
Is this race benign? Does seqlock correctly protect against it such that no harmful behavior (like a crash or incorrect result after the loop) can occur?
Why does KCSAN report it if it's inside a seqlock?
Reply: In `kernel/time/timekeeping.c`, the data race reported by KCSAN between `ktime_get_coarse_ts64()` (and other coarse readers) and `timekeeping_update_from_shadow()` is **benign**.
### Why the race is benign
1. **Seqlock Retry Mechanism**: The `timekeeper` structure is protected by a sequence counter (`tk_core.seq`). The reader in `ktime_get_coarse_ts64()` enters a `do { ... } while (read_seqcount_retry(...))` loop. If a writer (`timekeeping_update_from_shadow()`) modifies the data via `memcpy()` while the reader is accessing it, the sequence count will change (it becomes odd at the start of the write and increments again at the end). The `read_seqcount_retry()` call at the end of the reader's loop will detect this change and force the reader to repeat the operation until it gets a consistent snapshot of the data.
2. **No Fatal Inconsistencies**: A data race on plain (non-atomic) variables is generally undefined behavior in C, but in the Linux kernel, it is often treated as benign if the resulting "torn" value is discarded. In this case, the fields being read (`xtime_sec`, `coarse_nsec`, `wall_to_monotonic`) are simple numeric types (integers). A torn read of an integer might produce a garbage value, but it will not cause a kernel crash (unlike a torn pointer dereference). The garbage value is then discarded when the seqlock retry is triggered.
3. **Post-Retry Usage**: The data read inside the loop is only used to calculate the final result *after* the loop has successfully completed without a retry. This ensures that the values used for the final calculation are consistent.
### Why KCSAN reports it
KCSAN (Kernel Concurrency Sanitizer) is designed to detect all data races on non-atomic variables. It reports this race because:
1. **Plain Accesses**: The `memcpy()` in the writer and the direct field accesses in the reader are "plain" memory operations, not `READ_ONCE`/`WRITE_ONCE` or atomic operations. KCSAN flags these because the compiler could theoretically perform optimizations (like reordering or assuming a value doesn't change) that might break the logic, even if the current compiler doesn't.
2. **Seqlock Annotations**: While `seqlock.h` contains KCSAN annotations (`kcsan_atomic_next(1000)` for readers and `kcsan_nestable_atomic_begin()` for writers) intended to suppress reports of expected races within seqlock regions, these annotations are not always perfect. Specifically, `memcpy()` is a complex operation that KCSAN instruments separately. Depending on the compiler version and how `memcpy` is expanded (e.g., as a builtin or a call to `__tsan_memcpy`), the "atomic" status provided by the seqlock macros might not always correctly suppress the report for the entire duration of the copy.
3. **Explicit Marking Preference**: KCSAN's philosophy is that all intentional data races should be explicitly marked (e.g., using `data_race()` or `READ_ONCE()`). Since the timekeeping code avoids these markers in the hot path for performance and readability reasons, KCSAN sees the raw race and reports it.
In summary, the code is **correct and safe** due to the seqlock synchronization, and the KCSAN report is a known side effect of how seqlocks and plain memory accesses interact with the sanitizer's detection logic.
|
| 50/4 |
2026/07/04 00:29 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 3807
output: 83
thoughts: 206
Thoughts: **Analyzing a Potential Data Race in Timekeeping**
Okay, so I've got a report of a possible data race within the kernel's timekeeping mechanism, specifically involving `ktime_get_coarse_ts64()` and `timekeeping_update_from_shadow()` in `kernel/time/timekeeping.c`. The concern is around the `tk_core.timekeeper` data structure, even though a sequence counter (`tk_core.seq`) is supposed to be protecting access. This smells interesting; KCSAN flagging something that *should* be protected usually points to either a subtle concurrency issue or a misinterpretation by the tool. Let's dig in.
My immediate focus is on understanding the interaction between those two functions and how that sequence counter is being employed. I need to:
1. **Deep-dive into `ktime_get_coarse_ts64()`:** I need to trace the code path here, paying close attention to how it interacts with the `tk_core.timekeeper` structure and, critically, how it uses the sequence counter. What does it read, what does it do with the information, and how does it deal with the sequence counter to maintain consistency?
2. **Examine `timekeeping_update_from_shadow()`:** This is where the timekeeping updates are originating. I need to understand how it's *writing* to `tk_core.timekeeper`, and equally important, how it is *using* the sequence counter to synchronize the update process. Are there any potential race conditions during the shadow update? Is it potentially reading stale data before updating, or vice versa?
3. **Decipher the `tk_core.seq` dance:** I need to rigorously analyze *precisely* how that sequence counter is being used in both functions. Is it a read-copy-update-like scheme? Is it a reader-writer lock? Are the reads and writes paired correctly with the sequence counter's start/end points? Are there any potential ABA problems? Are there any ordering issues, where writes appear before reads?
4. **Pinpoint the Truth:** Ultimately, I have to figure out if KCSAN is correct in its assessment. Is there a genuine data race that could lead to inconsistent or corrupted time data? Or is this a false positive, maybe due to an imperfect understanding of the synchronization mechanism by the analysis tool? Does the data race manifest in a way that is acceptable, or is it a sign of a deeper concurrency flaw?
5. **Explain the KCSAN report:** Regardless of the answer to #4, I need to understand why KCSAN is reporting this issue. Even if it's a benign race, understanding its "trigger" helps me understand the limits of these tools, and whether any changes would be worthwhile to improve their accuracy.
Alright, let's start reading the code. I'll need to meticulously examine the code paths, considering all possible interleavings of execution. The goal is to either confirm the race, or to determine why KCSAN thinks there is a race, which will be a good learning opportunity either way.
|
| 51/4 |
2026/07/04 00:29 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/time/timekeeping.c Name:ktime_get_coarse_ts64]
Results: map[SourceCode:2758: void ktime_get_coarse_ts64(struct timespec64 *ts)
2759: {
2760: struct timekeeper *tk = &tk_core.timekeeper;
2761: struct timespec64 now, mono;
2762: unsigned int seq;
2763:
2764: do {
2765: seq = read_seqcount_begin(&tk_core.seq);
2766:
2767: now = tk_xtime_coarse(tk);
2768: mono = tk->wall_to_monotonic;
2769: } while (read_seqcount_retry(&tk_core.seq, seq));
2770:
2771: set_normalized_timespec64(ts, now.tv_sec + mono.tv_sec,
2772: now.tv_nsec + mono.tv_nsec);
2773: }
SourceFile:kernel/time/timekeeping.c]
|
| 52/4 |
2026/07/04 00:29 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/time/timekeeping.c Name:timekeeping_update_from_shadow]
Results: map[SourceCode: 789: static void timekeeping_update_from_shadow(struct tk_data *tkd, unsigned int action)
790: {
791: struct timekeeper *tk = &tkd->shadow_timekeeper;
792:
793: lockdep_assert_held(&tkd->lock);
794:
795: /*
796: * Block out readers before running the updates below because that
797: * updates VDSO and other time related infrastructure. Not blocking
798: * the readers might let a reader see time going backwards when
799: * reading from the VDSO after the VDSO update and then reading in
800: * the kernel from the timekeeper before that got updated.
801: */
802: write_seqcount_begin(&tkd->seq);
803:
804: if (action & TK_CLEAR_NTP) {
805: tk->ntp_error = 0;
806: ntp_clear(tk->id);
807: }
808:
809: tk_update_leap_state(tk);
810: tk_update_ktime_data(tk);
811: tk->tkr_mono.base_real = tk->tkr_mono.base + tk->offs_real;
812:
813: if (tk->id == TIMEKEEPER_CORE) {
814: tk_update_ns_to_cyc(tk, &tkd->timekeeper);
815: update_vsyscall(tk);
816: update_pvclock_gtod(tk, action & TK_CLOCK_WAS_SET);
817:
818: update_fast_timekeeper(&tk->tkr_mono, &tk_fast_mono);
819: update_fast_timekeeper(&tk->tkr_raw, &tk_fast_raw);
820: } else if (tk_is_aux(tk)) {
821: vdso_time_update_aux(tk);
822: }
823:
824: if (action & TK_CLOCK_WAS_SET)
825: tk->clock_was_set_seq++;
826:
827: /*
828: * Update the real timekeeper.
829: *
830: * We could avoid this memcpy() by switching pointers, but that has
831: * the downside that the reader side does not longer benefit from
832: * the cacheline optimized data layout of the timekeeper and requires
833: * another indirection.
834: */
835: memcpy(&tkd->timekeeper, tk, sizeof(*tk));
836: write_seqcount_end(&tkd->seq);
837: }
SourceFile:kernel/time/timekeeping.c]
|
| 53/4 |
2026/07/04 00:29 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 5851
output: 34
thoughts: 623
Thoughts: **Analyzing a KCSAN Data Race Report in the Timekeeping Code**
Okay, so KCSAN is flagging a potential data race in the timekeeping code, specifically involving `ktime_get_coarse_ts64()` and `timekeeping_update_from_shadow()`. Let's break this down. In `ktime_get_coarse_ts64()`, I'm reading time information from the `tk_core.timekeeper` structure using a seqlock (`tk_core.seq`). The core logic involves reading the time (`tk_xtime_coarse(tk)`) and wall-to-monotonic offset (`tk->wall_to_monotonic`), all while checking for concurrent writes to ensure data consistency.
On the writer side, `timekeeping_update_from_shadow()` is responsible for updating the `tk_core.timekeeper` structure. It uses `memcpy` to write the new timekeeper data. The critical point is that this write is protected by the seqlock's write sequence.
The KCSAN report is highlighting that the `memcpy` in `timekeeping_update_from_shadow()` and the reads in `ktime_get_coarse_ts64()` are not employing atomic operations or memory barriers that KCSAN recognizes as preventing a race *at the memory access level*. However, I know the intent here is seqlocks β the reader proceeds optimistically, and retries if a write happens during the read. `read_seqcount_retry` is the safety net here. It should detect if `memcpy` interfered with any of the reads and forces the reading thread to retry.
So, the data race is technically "real" in that the reader *could* see inconsistent data during the `memcpy`, but the retry loop is designed to handle this. The key question is whether KCSAN is properly accounting for seqlock-protected data races.
Now, here's where it gets interesting. Even though the seqlock *should* guarantee consistency through the retry loop, compilers can introduce optimizations based on the assumption that data doesn't change between reads unless synchronization is explicitly present. In this case, the reads *are* inside the retry loop. This is a potential source of problems because the compiler might reorder reads or assume a value remains unchanged, even though the whole point of a seqlock is to handle potential changes!
Ah, right, this is where `READ_ONCE()` and `WRITE_ONCE()` come into play. These are specifically designed to prevent these compiler optimizations, ensuring that reads and writes are performed as they appear in the source code. The Linux kernel uses these to prevent these kinds of issues. Generally speaking, if the retry logic of the seqlock is correct, we might not need `READ_ONCE()` for *every* field, *unless* the data race itself could cause undefined behavior, like crashing.
This brings me to the crucial point of whether `tk_core.timekeeper` contains pointers. If these pointers are dereferenced, a torn read during the `memcpy` *could* lead to a crash. That's the real potential pitfall here. So, I need to check the `struct timekeeper` definition now and see...
|
| 54/4 |
2026/07/04 00:30 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/time/timekeeping.c Name:timekeeper]
Results: map[SourceCode: 61: /**
62: * struct timekeeper - Structure holding internal timekeeping values.
63: * @tkr_mono: The readout base structure for CLOCK_MONOTONIC
64: * @xtime_sec: Current CLOCK_REALTIME time in seconds
65: * @ktime_sec: Current CLOCK_MONOTONIC time in seconds
66: * @wall_to_monotonic: CLOCK_REALTIME to CLOCK_MONOTONIC offset
67: * @offs_real: Offset clock monotonic -> clock realtime
68: * @offs_boot: Offset clock monotonic -> clock boottime
69: * @offs_tai: Offset clock monotonic -> clock tai
70: * @offs_aux: Offset clock monotonic -> clock AUX
71: * @coarse_nsec: The nanoseconds part for coarse time getters
72: * @id: The timekeeper ID
73: * @tkr_raw: The readout base structure for CLOCK_MONOTONIC_RAW
74: * @raw_sec: CLOCK_MONOTONIC_RAW time in seconds
75: * @cs_id: The ID of the current clocksource
76: * @cs_ns_to_cyc_mult: Multiplicator for nanoseconds to cycles conversion
77: * @cs_ns_to_cyc_shift: Shift value for nanoseconds to cycles conversion
78: * @cs_ns_to_cyc_maxns: Maximum nanoseconds to cyles conversion range
79: * @clock_was_set_seq: The sequence number of clock was set events
80: * @cs_was_changed_seq: The sequence number of clocksource change events
81: * @clock_valid: Indicator for valid clock
82: * @monotonic_to_boot: CLOCK_MONOTONIC to CLOCK_BOOTTIME offset
83: * @monotonic_to_aux: CLOCK_MONOTONIC to CLOCK_AUX offset
84: * @cycle_interval: Number of clock cycles in one NTP interval
85: * @xtime_interval: Number of clock shifted nano seconds in one NTP
86: * interval.
87: * @xtime_remainder: Shifted nano seconds left over when rounding
88: * @cycle_interval
89: * @raw_interval: Shifted raw nano seconds accumulated per NTP interval.
90: * @next_leap_ktime: CLOCK_MONOTONIC time value of a pending leap-second
91: * @ntp_tick: The ntp_tick_length() value currently being
92: * used. This cached copy ensures we consistently
93: * apply the tick length for an entire tick, as
94: * ntp_tick_length may change mid-tick, and we don't
95: * want to apply that new value to the tick in
96: * progress.
97: * @ntp_error: Difference between accumulated time and NTP time in ntp
98: * shifted nano seconds.
99: * @ntp_error_shift: Shift conversion between clock shifted nano seconds and
100: * ntp shifted nano seconds.
101: * @ntp_err_mult: Multiplication factor for scaled math conversion
102: * @skip_second_overflow: Flag used to avoid updating NTP twice with same second
103: * @tai_offset: The current UTC to TAI offset in seconds
104: *
105: * Note: For timespec(64) based interfaces wall_to_monotonic is what
106: * we need to add to xtime (or xtime corrected for sub jiffy times)
107: * to get to monotonic time. Monotonic is pegged at zero at system
108: * boot time, so wall_to_monotonic will be negative, however, we will
109: * ALWAYS keep the tv_nsec part positive so we can use the usual
110: * normalization.
111: *
112: * wall_to_monotonic is moved after resume from suspend for the
113: * monotonic time not to jump. We need to add total_sleep_time to
114: * wall_to_monotonic to get the real boot based time offset.
115: *
116: * wall_to_monotonic is no longer the boot time, getboottime must be
117: * used instead.
118: *
119: * @monotonic_to_boottime is a timespec64 representation of @offs_boot to
120: * accelerate the VDSO update for CLOCK_BOOTTIME.
121: *
122: * @offs_aux is used by the auxiliary timekeepers which do not utilize any
123: * of the regular timekeeper offset fields.
124: *
125: * @monotonic_to_aux is a timespec64 representation of @offs_aux to
126: * accelerate the VDSO update for CLOCK_AUX.
127: *
128: * The cacheline ordering of the structure is optimized for in kernel usage of
129: * the ktime_get() and ktime_get_ts64() family of time accessors. Struct
130: * timekeeper is prepended in the core timekeeping code with a sequence count,
131: * which results in the following cacheline layout:
132: *
133: * 0: seqcount, tkr_mono
134: * 1: xtime_sec ... id
135: * 2: tkr_raw, raw_sec
136: * 3,4: Internal variables
137: *
138: * Cacheline 0,1 contain the data which is used for accessing
139: * CLOCK_MONOTONIC/REALTIME/BOOTTIME/TAI, while cacheline 2 contains the
140: * data for accessing CLOCK_MONOTONIC_RAW. Cacheline 3,4 are internal
141: * variables which are only accessed during timekeeper updates once per
142: * tick.
143: */
144: struct timekeeper {
145: /* Cacheline 0 (together with prepended seqcount of timekeeper core): */
146: struct tk_read_base tkr_mono;
147:
148: /* Cacheline 1: */
149: u64 xtime_sec;
150: unsigned long ktime_sec;
151: struct timespec64 wall_to_monotonic;
152: ktime_t offs_real;
153: ktime_t offs_boot;
154: union {
155: ktime_t offs_tai;
156: ktime_t offs_aux;
157: };
158: u32 coarse_nsec;
159: enum timekeeper_ids id;
160:
161: /* Cacheline 2: */
162: struct tk_read_base tkr_raw;
163: u64 raw_sec;
164:
165: /* Cachline 3 and 4 (timekeeping internal variables): */
166: enum clocksource_ids cs_id;
167: u32 cs_ns_to_cyc_mult;
168: u32 cs_ns_to_cyc_shift;
169: u64 cs_ns_to_cyc_maxns;
170: unsigned int clock_was_set_seq;
171: u8 cs_was_changed_seq;
172: u8 clock_valid;
173:
174: union {
175: struct timespec64 monotonic_to_boot;
176: struct timespec64 monotonic_to_aux;
177: };
178:
179: u64 cycle_interval;
180: u64 xtime_interval;
181: s64 xtime_remainder;
182: u64 raw_interval;
183:
184: ktime_t next_leap_ktime;
185: u64 ntp_tick;
186: s64 ntp_error;
187: u32 ntp_error_shift;
188: u32 ntp_err_mult;
189: u32 skip_second_overflow;
190: s32 tai_offset;
191: };
SourceFile:include/linux/timekeeper_internal.h]
|
| 55/4 |
2026/07/04 00:30 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 9594
output: 78
thoughts: 574
Thoughts: **Analyzing Potential Data Races in the Timekeeping Subsystem**
Okay, let's break this down. We're looking at a potential data race within the Linux kernel's timekeeping mechanism. The `struct timekeeper` is a complex structure, and `ktime_get_coarse_ts64` accesses its fields, specifically `xtime_sec`, `coarse_nsec` (using `tk_xtime_coarse`), and `wall_to_monotonic`. The `tk_xtime_coarse` macro itself is straightforward; it just combines `xtime_sec` and `coarse_nsec` into a `timespec64` structure.
The crux of the issue lies in `timekeeping_update_from_shadow`, where a `memcpy` operation overwrites the entire `struct timekeeper`. And that `memcpy` is definitely not atomic. Now, `ktime_get_coarse_ts64` reads those values. This is where KCSAN (Kernel Concurrency Sanitizer) is flagging a data race, and it makes sense. A data race happens when concurrent threads access the same memory, at least one is a write, and at least one isn't atomic. We've got that here.
Even though these accesses are protected by a seqlock, KCSAN is still reporting it. That's because seqlocks don't *prevent* the race at the hardware level during the critical section; they *detect* the race and retry. The memory accesses themselves aren't synchronized. But are these races actually a problem?
Typically, data races on non-atomic variables are undefined behavior in C. In the Linux kernel, we often rely on them being "benign" in scenarios where the result is discarded upon failure, as with seqlock retries. However, KCSAN's stance is that there are *no* benign data races because the compiler might make optimizations based on the assumption that races don't happen.
Let's think about compiler optimizations. If the compiler sees the typical seqlock retry loop:
```c
do {
seq = read_seqcount_begin(&tk_core.seq);
now = tk_xtime_coarse(tk);
mono = tk->wall_to_monotonic;
} while (read_seqcount_retry(&tk_core.seq, seq));
```
It might assume that `tk->xtime_sec` and `tk->coarse_nsec` can't change within the loop, *unless* it is aware of the writes performed by another thread. This is potentially dangerous. The memory barriers in `read_seqcount_begin` and `read_seqcount_retry` (usually `smp_rmb()`) *should* prevent the compiler from reordering or moving reads out of the loop.
I should verify that, so I'll check the definitions of `read_seqcount_begin` and `read_seqcount_retry` in `include/linux/seqlock.h` to confirm the memory barrier implementations. Ultimately, the question remains: is the compiler handling this situation correctly? Is the seqlock sufficient to guarantee consistency, or is KCSAN right to be concerned? That's what I need to investigate further.
|
| 56/4 |
2026/07/04 00:30 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:include/linux/seqlock.h Name:read_seqcount_begin]
Results: map[SourceCode: SourceFile:]
|
| 57/4 |
2026/07/04 00:30 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:include/linux/seqlock.h Name:read_seqcount_retry]
Results: map[SourceCode: SourceFile:]
|
| 58/4 |
2026/07/04 00:30 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 10987
output: 74
thoughts: 59
Thoughts: **Investigating the Elusive Definition**
Okay, so I'm running into a snag. It seems like `codesearch-definition-source` isn't picking up the definitions I'm looking for. That makes me think they're either being handled by macros, potentially generating them dynamically, or perhaps they're defined in a way that the tool isn't easily parsing. The behavior does not seem expected. This is a bit annoying, I would have expected a more direct lookup. I should be able to rely on standard tools.
My next step is clear. Let me try a more direct approach and search specifically within `include/linux/seqlock.h`. If they're related to sequence locks, as the name of the header suggests, that should pinpoint them pretty quickly, if the macro approach is used. Hopefully that'll give me the precise information I need to understand how these definitions are being handled.
|
| 59/4 |
2026/07/04 00:30 |
tool |
grepper |
0m
Args: map[Expression:#define read_seqcount_begin PathPrefix:include/linux/seqlock.h]
Results: map[Output:include/linux/seqlock.h=230=SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex)
--
include/linux/seqlock.h-296- */
include/linux/seqlock.h:297:#define read_seqcount_begin(s) \
include/linux/seqlock.h-298-({ \
]
|
| 60/4 |
2026/07/04 00:30 |
tool |
grepper |
0m
Args: map[Expression:#define read_seqcount_retry PathPrefix:include/linux/seqlock.h]
Results: map[Output:include/linux/seqlock.h=389=static inline int do___read_seqcount_retry(const seqcount_t *s, unsigned start)
--
include/linux/seqlock.h-405- */
include/linux/seqlock.h:406:#define read_seqcount_retry(s, start) \
include/linux/seqlock.h-407- do_read_seqcount_retry(seqprop_const_ptr(s), start)
]
|
| 61/4 |
2026/07/04 00:30 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 11552
output: 74
thoughts: 26
Thoughts: Okay, let's dive into this code.
**Diving into `read_seqcount_begin` and `do_read_seqcount_retry`**
Alright, so the task at hand is to understand how these read sequence counter functions work. The names themselves, `read_seqcount_begin` and `do_read_seqcount_retry`, give us a solid hint: we're dealing with something related to sequence counters, and the possibility of retries. I'm immediately thinking this probably means we're dealing with a synchronization primitive β likely a read-copy-update (RCU) or similar mechanism to facilitate concurrent reads and writes, and ensure data consistency. I'll need to see how the implementation handles potential conflicts.
`read_seqcount_begin` is probably the *initiation* of the read. I bet it grabs a snapshot or baseline value of the sequence counter. Then, the real magic happens in `do_read_seqcount_retry`. This function is likely what validates that the data hasn't changed *after* the initial read, perhaps by comparing the sequence counter to a subsequent snapshot taken after the read. And if it has, we'll see a retry loop within that function, to ensure that the read consistently.
I'm keen to see the actual mechanics of this: How the sequence counter is implemented, what's used for the compare-and-swap (or similar atomic operation), and what strategies are employed for managing the retries. I expect to see considerations for cache coherency and memory barriers in there. It'll be the details of the synchronization and any optimization strategies that make this really interesting.
|
| 62/4 |
2026/07/04 00:30 |
tool |
read-file |
0m
Args: map[File:include/linux/seqlock.h FirstLine:290 LineCount:50]
Results: map[Contents: 290:
291: /**
292: * read_seqcount_begin() - begin a seqcount_t read critical section
293: * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
294: *
295: * Return: count to be passed to read_seqcount_retry()
296: */
297: #define read_seqcount_begin(s) \
298: ({ \
299: seqcount_lockdep_reader_access(seqprop_const_ptr(s)); \
300: raw_read_seqcount_begin(s); \
301: })
302:
303: /**
304: * raw_read_seqcount() - read the raw seqcount_t counter value
305: * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
306: *
307: * raw_read_seqcount opens a read critical section of the given
308: * seqcount_t, without any lockdep checking, and without checking or
309: * masking the sequence counter LSB. Calling code is responsible for
310: * handling that.
311: *
312: * Return: count to be passed to read_seqcount_retry()
313: */
314: #define raw_read_seqcount(s) \
315: ({ \
316: unsigned __seq = seqprop_sequence(s); \
317: \
318: kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
319: __seq; \
320: })
321:
322: /**
323: * raw_seqcount_try_begin() - begin a seqcount_t read critical section
324: * w/o lockdep and w/o counter stabilization
325: * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
326: * @start: count to be passed to read_seqcount_retry()
327: *
328: * Similar to raw_seqcount_begin(), except it enables eliding the critical
329: * section entirely if odd, instead of doing the speculation knowing it will
330: * fail.
331: *
332: * Useful when counter stabilization is more or less equivalent to taking
333: * the lock and there is a slowpath that does that.
334: *
335: * If true, start will be set to the (even) sequence count read.
336: *
337: * Return: true when a read critical section is started.
338: */
339: #define raw_seqcount_try_begin(s, start) \
]
|
| 63/4 |
2026/07/04 00:30 |
tool |
read-file |
0m
Args: map[File:include/linux/seqlock.h FirstLine:380 LineCount:50]
Results: map[Contents: 380: *
381: * Use carefully, only in critical code, and comment how the barrier is
382: * provided.
383: *
384: * Return: true if a read section retry is required, else false
385: */
386: #define __read_seqcount_retry(s, start) \
387: do___read_seqcount_retry(seqprop_const_ptr(s), start)
388:
389: static inline int do___read_seqcount_retry(const seqcount_t *s, unsigned start)
390: {
391: kcsan_atomic_next(0);
392: return unlikely(READ_ONCE(s->sequence) != start);
393: }
394:
395: /**
396: * read_seqcount_retry() - end a seqcount_t read critical section
397: * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
398: * @start: count, from read_seqcount_begin()
399: *
400: * read_seqcount_retry closes the read critical section of given
401: * seqcount_t. If the critical section was invalid, it must be ignored
402: * (and typically retried).
403: *
404: * Return: true if a read section retry is required, else false
405: */
406: #define read_seqcount_retry(s, start) \
407: do_read_seqcount_retry(seqprop_const_ptr(s), start)
408:
409: static inline int do_read_seqcount_retry(const seqcount_t *s, unsigned start)
410: {
411: smp_rmb();
412: return do___read_seqcount_retry(s, start);
413: }
414:
415: /**
416: * raw_write_seqcount_begin() - start a seqcount_t write section w/o lockdep
417: * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
418: *
419: * Context: check write_seqcount_begin()
420: */
421: #define raw_write_seqcount_begin(s) \
422: do { \
423: if (seqprop_preemptible(s)) \
424: preempt_disable(); \
425: \
426: do_raw_write_seqcount_begin(seqprop_ptr(s)); \
427: } while (0)
428:
429: static inline void do_raw_write_seqcount_begin(seqcount_t *s)
]
|
| 64/4 |
2026/07/04 00:30 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 13639
output: 25
thoughts: 459
Thoughts: **Analyzing KCSAN Annotations in Sequence Count Implementation**
Okay, let's break down these sequence count retry functions and their KCSAN annotations. My initial thought is to understand how these tools are used to prevent data races.
Looking at `do_read_seqcount_retry`, I see a standard retry loop using `do___read_seqcount_retry`. The interesting bit here is the `kcsan_atomic_next(0)` call within `do___read_seqcount_retry`. This function essentially signals to KCSAN that the subsequent atomic access, specifically the `READ_ONCE` of `s->sequence`, is part of a seqlock-like retry loop. Essentially, KCSAN is being informed that it's okay if a race is detected on this read, as it's part of an expected retry mechanism within a seqlock.
Then, there is `raw_read_seqcount`, called by `read_seqcount_begin`. This looks similar, but I see `kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX)`. This annotation is intriguing, because now it says KCSAN will allow a series of atomic accesses to race. So, `KCSAN_SEQLOCK_REGION_MAX` determines how many accesses are allowed. I should look up the definition of `KCSAN_SEQLOCK_REGION_MAX` to understand the scope of the "allowed" race. It's highly probable that this is defined in `include/linux/kcsan-checks.h`, as the file suggests. Now, to investigate that constant!
|
| 65/4 |
2026/07/04 00:30 |
tool |
grepper |
0m
Args: map[Expression:KCSAN_SEQLOCK_REGION_MAX]
Results: map[Output:include/linux/seqlock.h-34- * via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
include/linux/seqlock.h:35: * pessimistically mark the next KCSAN_SEQLOCK_REGION_MAX memory accesses as
include/linux/seqlock.h-36- * atomics; if there is a matching read_seqcount_retry() call, no following
--
include/linux/seqlock.h-39- */
include/linux/seqlock.h:40:#define KCSAN_SEQLOCK_REGION_MAX 1000
include/linux/seqlock.h-41-
--
include/linux/seqlock.h=230=SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex)
--
include/linux/seqlock.h-278- \
include/linux/seqlock.h:279: kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
include/linux/seqlock.h-280- __seq; \
--
include/linux/seqlock.h-317- \
include/linux/seqlock.h:318: kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
include/linux/seqlock.h-319- __seq; \
--
include/linux/seqlock.h=659=static __always_inline unsigned read_seqcount_latch(const seqcount_latch_t *s)
include/linux/seqlock.h-660-{
include/linux/seqlock.h:661: kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
include/linux/seqlock.h-662- return raw_read_seqcount_latch(s);
]
|
| 66/4 |
2026/07/04 00:30 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 14960
output: 74
thoughts: 144
Thoughts: **Analyzing a KCSAN Data Race Report**
Okay, so I'm looking at this KCSAN report, and the problem seems to be originating with `ktime_get_coarse_ts64`. The code seems to be using `read_seqcount_begin` to mark the subsequent 1000 accesses as "atomic" from KCSAN's perspective. The expectation is that `ktime_get_coarse_ts64` shouldn't be generating a race report if it only makes a small handful of accesses.
The user, who I assume knows what they're doing, is saying that KCSAN *is* reporting a data race with `ktime_get_coarse_ts64`, and that just doesn't make sense. Maybe the access count is somehow blowing past that 1000-access limit? That seems unlikely, given the nature of the function.
Alright, if it's not the access count, then the only other explanation is that the code isn't annotating the *writer* side correctly. I'm going to have to dive into `write_seqcount_begin` and `write_seqcount_end` to see how those are being used to protect the data the reader is accessing. That's where I need to focus my attention.
|
| 67/4 |
2026/07/04 00:30 |
tool |
grepper |
0m
Args: map[Expression:#define write_seqcount_begin PathPrefix:include/linux/seqlock.h]
Results: map[Output:include/linux/seqlock.h=450=static inline void do_raw_write_seqcount_end(seqcount_t *s)
--
include/linux/seqlock.h-465- */
include/linux/seqlock.h:466:#define write_seqcount_begin_nested(s, subclass) \
include/linux/seqlock.h-467-do { \
--
include/linux/seqlock.h=476=static inline void do_write_seqcount_begin_nested(seqcount_t *s, int subclass)
--
include/linux/seqlock.h-491- */
include/linux/seqlock.h:492:#define write_seqcount_begin(s) \
include/linux/seqlock.h-493-do { \
]
|
| 68/4 |
2026/07/04 00:30 |
tool |
grepper |
0m
Args: map[Expression:#define write_seqcount_end PathPrefix:include/linux/seqlock.h]
Results: map[Output:include/linux/seqlock.h=502=static inline void do_write_seqcount_begin(seqcount_t *s)
--
include/linux/seqlock.h-513- */
include/linux/seqlock.h:514:#define write_seqcount_end(s) \
include/linux/seqlock.h-515-do { \
]
|
| 69/4 |
2026/07/04 00:30 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 15790
output: 37
thoughts: 225
Thoughts: **Analyzing the `write_seqcount_begin` Macro and Its Underlying Functions**
Okay, so I'm looking at this `write_seqcount_begin` macro. My initial thought is to break it down. I see it starts by ensuring exclusive access for a writer to a sequence counter, presumably using `seqcount_lockdep_writer_access`, along with acquiring a write lock. Then, the real action seems to happen within `raw_write_seqcount_begin`. I'm guessing this function is central to starting a critical section in the code. I can infer that the purpose of `seqcount` is a simple optimistic locking approach for concurrent access.
Now, I need to understand what `raw_write_seqcount_begin` does under the hood. Time to dig into the definition of `do_raw_write_seqcount_begin`. Ah, here it is. It looks like it's fairly straightforward. First, `kcsan_nestable_atomic_begin()` is called which likely has to do with KCSAN instrumentation. Then, `s->sequence++` is performed. This part increments the sequence counter, a clear indication of a new write operation starting. Finally, a `smp_wmb()` memory barrier is added. This is the crucial bit. The memory barrier is there to ensure that the increment of the sequence count becomes visible to other CPUs before any subsequent writes within the critical section. This ordering is key for the optimistic locking to work correctly. So, the sequence number is bumped up and then any actual writes are performed with memory barrier to provide proper ordering to readers.
|
| 70/4 |
2026/07/04 00:30 |
tool |
read-file |
0m
Args: map[File:include/linux/seqlock.h FirstLine:429 LineCount:20]
Results: map[Contents: 429: static inline void do_raw_write_seqcount_begin(seqcount_t *s)
430: {
431: kcsan_nestable_atomic_begin();
432: s->sequence++;
433: smp_wmb();
434: }
435:
436: /**
437: * raw_write_seqcount_end() - end a seqcount_t write section w/o lockdep
438: * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants
439: *
440: * Context: check write_seqcount_end()
441: */
442: #define raw_write_seqcount_end(s) \
443: do { \
444: do_raw_write_seqcount_end(seqprop_ptr(s)); \
445: \
446: if (seqprop_preemptible(s)) \
447: preempt_enable(); \
448: } while (0)
]
|
| 71/4 |
2026/07/04 00:30 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 16712
output: 25
thoughts: 605
Thoughts: **Analyzing a KCSAN Data Race Report in Timekeeping**
Okay, so I'm looking at this `do_raw_write_seqcount_end` function, and my mind immediately jumps to KCSAN. I see `kcsan_nestable_atomic_end()` and I recall those calls are used on the writer side. They are telling KCSAN that the accesses are "atomic" within that region, and thus the race is expected. In this case, the `kcsan_nestable_atomic_begin()` on both sides is important to tell KCSAN not to report a race.
But wait, I need to make sure I'm not misremembering how KCSAN handles seqlocks. I should probably double-check the KCSAN documentation or search for similar kernel issues to refresh my memory on the interactions.
Oh, the user says the report is between a `memcpy()` and the reads. Now I remember that `memcpy` might be the problem. KCSAN might not "see" the individual accesses within a `memcpy` as atomic, even if they're wrapped in `kcsan_nestable_atomic_begin()`, especially because it's usually an optimized assembly routine or a builtin.
Hold on... I remember that kernel `memcpy` is often instrumented by KCSAN. Hmm, but maybe not in a way that respects `kcsan_nestable_atomic_begin()` when it's a large copy?
Yes! There's a known issue with `memcpy` and seqlocks that often causes this kind of KCSAN report. If `memcpy` is used to update the data and a reader reads it, KCSAN will report it, as it doesn't see that the access is protected in the same way as a single atomic operation.
Okay, let's look at `timekeeping_update_from_shadow` again. Here's that `memcpy`: `memcpy(&tkd->timekeeper, tk, sizeof(*tk));` Then the `ktime_get_coarse_ts64` reads.
If KCSAN is reporting a race here, it means it doesn't think the race is benign or it doesn't understand the protection. But the race should be protected by `tk_core.seq`, which controls both the writes and the reads!
Okay, I have a hunch. This screams a potential existing issue with `memcpy` and seqlocks and this timekeeping. I'm going to search the git log for "timekeeping kcsan" to see if there's a relevant commit.
|
| 72/4 |
2026/07/04 00:30 |
tool |
git-log |
0m
Args: map[MessageRegexps:[timekeeping KCSAN]]
Results: map[Output:5c1806c41ce0 kcsan, seqlock: Support seqcount_latch_t
f87cbcb345d0 timekeeping: Use READ/WRITE_ONCE() for tick_do_timer_cpu
]
|
| 73/4 |
2026/07/04 00:31 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 17969
output: 31
thoughts: 770
Thoughts: **Analyzing a KCSAN Report on a Timekeeping Data Race**
Okay, so I'm looking at this KCSAN report. Not much immediately jumps out, so let's dig a bit deeper. I'm going to focus on "memcpy seqlock KCSAN". I remember seeing something with `memcpy` in `timekeeping_update_from_shadow`. Right, it's copying the whole `struct timekeeper`. The reader is looking at fields within this structure.
If the reader grabs a field while the `memcpy` is writing, there's a good chance of getting a "torn" value. But, the seqlock retry mechanism should catch this, and make the operation correct after the loop is done, so that should be okay.
The crucial question now: Could a torn value *inside* the loop cause a crash? Let's see... in `ktime_get_coarse_ts64`, the code reads `tk->xtime_sec` and `tk->coarse_nsec` via `tk_xtime_coarse`, and also accesses `tk->wall_to_monotonic`, which is a `struct timespec64` (two `s64` fields). These aren't pointers, just plain integers. Torn reads of integers aren't inherently fatal; they just result in garbage that's discarded because of the retry loop. So, the race *seems* benign.
So, why is KCSAN complaining? KCSAN flags any data race unless it's explicitly marked as benign using `data_race()` or `READ_ONCE`/`WRITE_ONCE`. Seqlocks are special; KCSAN has built-in support for them, as I remember from `include/linux/seqlock.h`.
Wait, here's something: `kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX)` in `read_seqcount_begin`. That should cover the reads, right?
Alright, maybe it's the `memcpy` itself. `memcpy` isn't a single access, and KCSAN probably instruments it to check for races on the whole memory range being copied. Maybe if I look at `include/linux/fortify-string.h`, or wherever it's defined, I could get more info on how it's handled.
But a more relevant question is: does wrapping `memcpy` inside `write_seqcount_begin/end` make KCSAN consider it "atomic"? And that's exactly what is happening: `do_raw_write_seqcount_begin` calls `kcsan_nestable_atomic_begin()` and `do_raw_write_seqcount_end` calls `kcsan_nestable_atomic_end()`. The documentation says that `kcsan_nestable_atomic_begin()` should make KCSAN treat everything within as "atomic", so that should be okay.
And the reader side uses `kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX)` which makes KCSAN treat next region as "atomic".
If both sides are "atomic", KCSAN should, theoretically, be happy.
Wait, I think I see the issue. If `memcpy` is implemented in assembly it might not be instrumented by KCSAN and not recognize that it is atomic. Even if it is instrumented, the instrumentation may not check the "atomic" flag. However, KCSAN usually instruments `memcpy`.
I will double check for any `data_race()` markers in `kernel/time/timekeeping.c`.
|
| 74/4 |
2026/07/04 00:31 |
tool |
grepper |
0m
Args: map[Expression:data_race PathPrefix:kernel/time/timekeeping.c]
Results: map[Output:kernel/time/timekeeping.c=583=u64 notrace ktime_get_boot_fast_ns(void)
--
kernel/time/timekeeping.c-586-
kernel/time/timekeeping.c:587: return (ktime_get_mono_fast_ns() + ktime_to_ns(data_race(tk->offs_boot)));
kernel/time/timekeeping.c-588-}
--
kernel/time/timekeeping.c=600=u64 notrace ktime_get_tai_fast_ns(void)
--
kernel/time/timekeeping.c-603-
kernel/time/timekeeping.c:604: return (ktime_get_mono_fast_ns() + ktime_to_ns(data_race(tk->offs_tai)));
kernel/time/timekeeping.c-605-}
--
kernel/time/timekeeping.c=890=bool ktime_expiry_to_cycles(enum clocksource_ids id, ktime_t expires_ns, u64 *cycles)
--
kernel/time/timekeeping.c-905- */
kernel/time/timekeeping.c:906: if (data_race(tk->cs_id) != id)
kernel/time/timekeeping.c-907- return false;
]
|
| 75/4 |
2026/07/04 00:31 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 19888
output: 36
thoughts: 145
Thoughts: **Investigating Potential Data Races**
Okay, so I'm seeing these `data_race()` markers, but not, interestingly, in `ktime_get_coarse_ts64`. That's a bit odd. I need to be sure though, so let me quickly confirm that `ktime_get_coarse_ts64` is *actually* using the `read_seqcount_begin` mechanism. Yep, line 2765 confirms it: `seq = read_seqcount_begin(&tk_core.seq);` So that part *should* be protected.
Now, back to this `memcpy`. I need to understand it better. Let's look at this line again: `835: memcpy(&tkd->timekeeper, tk, sizeof(*tk));` I'm curious about the context of this `memcpy`. I think a quick kernel history search for this specific `memcpy` would be worthwhile. Maybe the commit logs will shed some light on the reasoning behind it or any related discussions on potential race conditions, or even provide some clues that it was reviewed with data races in mind. Let's see what the history reveals.
|
| 76/4 |
2026/07/04 00:31 |
tool |
git-blame |
0m
Args: map[End:840 File:kernel/time/timekeeping.c Start:830]
Results: map[Output:5aa6c43eca21a 830) * We could avoid this memcpy() by switching pointers, but that has
5aa6c43eca21a 831) * the downside that the reader side does not longer benefit from
5aa6c43eca21a 832) * the cacheline optimized data layout of the timekeeper and requires
5aa6c43eca21a 833) * another indirection.
d151832650ed9 834) */
147ba943024e5 835) memcpy(&tkd->timekeeper, tk, sizeof(*tk));
5aa6c43eca21a 836) write_seqcount_end(&tkd->seq);
cc06268c6a87d 837) }
cc06268c6a87d 838)
8524070b7982d 839) /**
324a2219ba38b 840) * timekeeping_forward_now - update clock to the current time
]
|
| 77/4 |
2026/07/04 00:31 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 20624
output: 27
thoughts: 34
Thoughts: Here's my analysis of the provided text, structured as a first-person expert summary:
**Digging into the `memcpy` Mystery**
Okay, so we've got a potential issue with a `memcpy` call in the code, and I need to pinpoint when it was introduced or, at the very least, when it was last touched. The commit hash provided, `147ba943024e5`, is my starting point. That hash will let me get right to the specific code change and its surrounding context. I'll examine the diffs introduced by that commit with a focus on where `memcpy` is used and how itβs being used. I'll need to immediately check the arguments to that `memcpy` call β the source, destination, and the size. That'll be critical to determining if any potential buffer overflows or other memory-related errors were inadvertently introduced or corrected in that change. I'll then quickly scan the commit message and any related comments for clues about the intent of the change.
|
| 78/4 |
2026/07/04 00:31 |
tool |
git-show |
0m
Args: map[Commit:147ba943024e5]
Results: map[Output:commit 147ba943024e564e89d9ac265d6a07a0d2c03988
Author: Anna-Maria Behnsen <anna-maria@linutronix.de>
Date: Wed Oct 9 10:29:18 2024 +0200
timekeeping: Merge timekeeping_update_staged() and timekeeping_update()
timekeeping_update_staged() is the only call site of timekeeping_update().
Merge those functions. No functional change.
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <jstultz@google.com>
Link: https://lore.kernel.org/all/20241009-devel-anna-maria-b4-timers-ptp-timekeeping-v2-25-554456a44a15@linutronix.de
diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 6ca250ab2c20..17cae886ca82 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -548,7 +548,7 @@ EXPORT_SYMBOL_GPL(ktime_get_raw_fast_ns);
* timekeeping_inject_sleeptime64()
* __timekeeping_inject_sleeptime(tk, delta);
* timestamp();
- * timekeeping_update(tkd, tk, TK_CLEAR_NTP...);
+ * timekeeping_update_staged(tkd, TK_CLEAR_NTP...);
*
* (2) On 32-bit systems, the 64-bit boot offset (tk->offs_boot) may be
* partially updated. Since the tk->offs_boot update is a rare event, this
@@ -794,10 +794,21 @@ static void timekeeping_restore_shadow(struct tk_data *tkd)
memcpy(&tkd->shadow_timekeeper, &tkd->timekeeper, sizeof(tkd->timekeeper));
}
-static void timekeeping_update(struct tk_data *tkd, struct timekeeper *tk, unsigned int action)
+static void timekeeping_update_from_shadow(struct tk_data *tkd, unsigned int action)
{
+ struct timekeeper *tk = &tk_core.shadow_timekeeper;
+
lockdep_assert_held(&tkd->lock);
+ /*
+ * Block out readers before running the updates below because that
+ * updates VDSO and other time related infrastructure. Not blocking
+ * the readers might let a reader see time going backwards when
+ * reading from the VDSO after the VDSO update and then reading in
+ * the kernel from the timekeeper before that got updated.
+ */
+ write_seqcount_begin(&tkd->seq);
+
if (action & TK_CLEAR_NTP) {
tk->ntp_error = 0;
ntp_clear();
@@ -815,20 +826,6 @@ static void timekeeping_update(struct tk_data *tkd, struct timekeeper *tk, unsig
if (action & TK_CLOCK_WAS_SET)
tk->clock_was_set_seq++;
-}
-
-static void timekeeping_update_from_shadow(struct tk_data *tkd, unsigned int action)
-{
- /*
- * Block out readers before invoking timekeeping_update() because
- * that updates VDSO and other time related infrastructure. Not
- * blocking the readers might let a reader see time going backwards
- * when reading from the VDSO after the VDSO update and then
- * reading in the kernel from the timekeeper before that got updated.
- */
- write_seqcount_begin(&tkd->seq);
-
- timekeeping_update(tkd, &tkd->shadow_timekeeper, action);
/*
* Update the real timekeeper.
@@ -838,7 +835,7 @@ static void timekeeping_update_from_shadow(struct tk_data *tkd, unsigned int act
* the cacheline optimized data layout of the timekeeper and requires
* another indirection.
*/
- memcpy(&tkd->timekeeper, &tkd->shadow_timekeeper, sizeof(tkd->shadow_timekeeper));
+ memcpy(&tkd->timekeeper, tk, sizeof(*tk));
write_seqcount_end(&tkd->seq);
}
]
|
| 79/4 |
2026/07/04 00:31 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 22113
output: 34
thoughts: 242
Thoughts: **Analyzing a Potential Race Condition in `memcpy` and Seqlocks**
Okay, so this `memcpy`β¦ it's been around for ages. I need to figure out why KCSAN is potentially flagging a race condition here. Let me see, I should start by checking recent commits. I'll search the web for "timekeeping seqlock KCSAN", that usually yields good results.
Ah, this looks promising, I found a commit! Now, before I jump to any conclusions, I should probably expand the search and see if there are *more* recent commits that touch KCSAN and seqlocks, just to be thorough.
Wait a second⦠I see `kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX)` right in `include/linux/seqlock.h`. If KCSAN is still finding a race, maybe the `memcpy` isn't being treated as a typical access that `kcsan_atomic_next` is designed to handle. I need to consider that possibility.
Okay, let me think this through. `kcsan_atomic_next` probably only catches the *next* N accesses. If `memcpy` is implemented as a loop of multiple accesses, it's possible it exceeds the limit. But⦠the `KCSAN_SEQLOCK_REGION_MAX` limit is 1000. `struct timekeeper` can't possibly be *that* large, right?
Alright, time to verify that assumption. I need to check the actual size of `struct timekeeper`. I'll use `codesearch-struct-layout` to do that. That'll give me the definitive answer.
|
| 80/4 |
2026/07/04 00:31 |
tool |
codesearch-struct-layout |
0m
Args: map[ContextFile:kernel/time/timekeeping.c Name:timekeeper]
Results: map[Fields:[map[Name:tkr_mono OffsetBits:0 SizeBits:448] map[Name:xtime_sec OffsetBits:448 SizeBits:64] map[Name:ktime_sec OffsetBits:512 SizeBits:64] map[Name:wall_to_monotonic OffsetBits:576 SizeBits:128] map[Name:offs_real OffsetBits:704 SizeBits:64] map[Name:offs_boot OffsetBits:768 SizeBits:64] map[Name: OffsetBits:832 SizeBits:64] map[Name:coarse_nsec OffsetBits:896 SizeBits:32] map[Name:id OffsetBits:928 SizeBits:32] map[Name:tkr_raw OffsetBits:960 SizeBits:448] map[Name:raw_sec OffsetBits:1408 SizeBits:64] map[Name:cs_id OffsetBits:1472 SizeBits:32] map[Name:cs_ns_to_cyc_mult OffsetBits:1504 SizeBits:32] map[Name:cs_ns_to_cyc_shift OffsetBits:1536 SizeBits:32] map[Name:cs_ns_to_cyc_maxns OffsetBits:1600 SizeBits:64] map[Name:clock_was_set_seq OffsetBits:1664 SizeBits:32] map[Name:cs_was_changed_seq OffsetBits:1696 SizeBits:8] map[Name:clock_valid OffsetBits:1704 SizeBits:8] map[Name: OffsetBits:1728 SizeBits:128] map[Name:cycle_interval OffsetBits:1856 SizeBits:64] map[Name:xtime_interval OffsetBits:1920 SizeBits:64] map[Name:xtime_remainder OffsetBits:1984 SizeBits:64] map[Name:raw_interval OffsetBits:2048 SizeBits:64] map[Name:next_leap_ktime OffsetBits:2112 SizeBits:64] map[Name:ntp_tick OffsetBits:2176 SizeBits:64] map[Name:ntp_error OffsetBits:2240 SizeBits:64] map[Name:ntp_error_shift OffsetBits:2304 SizeBits:32] map[Name:ntp_err_mult OffsetBits:2336 SizeBits:32] map[Name:skip_second_overflow OffsetBits:2368 SizeBits:32] map[Name:tai_offset OffsetBits:2400 SizeBits:32]]]
|
| 81/4 |
2026/07/04 00:31 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 23456
output: 38
thoughts: 464
Thoughts: **Analyzing a KCSAN Report on `struct timekeeper`**
Okay, so I'm looking at this KCSAN report. The size of `struct timekeeper` is roughly 304 bytes. A `memcpy` of that size, even with single-byte accesses, is well within the `KCSAN_SEQLOCK_REGION_MAX` limit, so why is KCSAN flagging it as a race? That's the question.
I vaguely recall seeing something similar before... ah, right. `memcpy` itself might not be fully instrumented for atomic flag checks in the same way regular loads/stores are. That could be a reason, but there's probably a more specific cause here.
Let me think⦠the core issue is likely in `ktime_get_coarse_ts64`. Specifically, lines 2767 and 2768: `now = tk_xtime_coarse(tk);` and `mono = tk->wall_to_monotonic;`. These are just plain reads. If KCSAN is reporting a race, it means it's detecting a *concurrent write*. And that write is⦠the `memcpy`.
Now, if this race is benign, we usually use `data_race()` or `READ_ONCE`. But, with seqlocks, proper retry logic should make those unnecessary. So, it shouldn't be a problem here.
Wait, it's coming back to me. KCSAN has a "seqlock" mode specifically to validate seqlock usage. It's designed to flag races that are *not* retried, because that's a bug. And this *is* retried, so itβs likely that it is working as expected.
I remember a commit related to this⦠`de66768832a8`. "seqlock: Annotate seqlock_t and seqcount_t for KCSAN". Let me look at that commit. (While I can't look at it directly, I *can* check the current seqlock.h, which I've already done).
Actually, let's go straight to the source. I should look at `kcsan_atomic_next` in `include/linux/kcsan-checks.h`. That's where I'll find what the check is.
|
| 82/4 |
2026/07/04 00:31 |
tool |
grepper |
0m
Args: map[Expression:kcsan_atomic_next PathPrefix:include/linux/kcsan-checks.h]
Results: map[Output:include/linux/kcsan-checks.h=108=void kcsan_flat_atomic_end(void);
--
include/linux/kcsan-checks.h-110-/**
include/linux/kcsan-checks.h:111: * kcsan_atomic_next - consider following accesses as atomic
include/linux/kcsan-checks.h-112- *
--
include/linux/kcsan-checks.h-117- */
include/linux/kcsan-checks.h:118:void kcsan_atomic_next(int n);
include/linux/kcsan-checks.h-119-
--
include/linux/kcsan-checks.h=202=static inline void kcsan_flat_atomic_end(void) { }
include/linux/kcsan-checks.h:203:static inline void kcsan_atomic_next(int n) { }
include/linux/kcsan-checks.h-204-static inline void kcsan_set_access_mask(unsigned long mask) { }
--
include/linux/kcsan-checks.h=232=static inline void __kcsan_disable_current(void) { }
--
include/linux/kcsan-checks.h-529- kcsan_set_access_mask(0); \
include/linux/kcsan-checks.h:530: kcsan_atomic_next(1); \
include/linux/kcsan-checks.h-531- } while (0)
]
|
| 83/4 |
2026/07/04 00:31 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 24874
output: 35
thoughts: 143
Thoughts: **Investigating Timekeeping Sequence Counters**
Okay, first things first, I need to confirm something. I need to make absolutely sure that `ktime_get_coarse_ts64` is indeed utilizing `read_seqcount_begin` or if it's employing another mechanism for its sequence counter. Let me quickly verify... Yup, confirmed. It *is* using `read_seqcount_begin`.
Alright, I'm now looking at the `kernel/time/timekeeping.c` file and I am seeing something interesting in lines 2760 and 2765. I see that there is a `timekeeper` struct being used:
```c
2760: struct timekeeper *tk = &tk_core.timekeeper;
...
2765: seq = read_seqcount_begin(&tk_core.seq);
```
Okay, so I see a `tk_core` variable being used. And it seems like this `tk_core` is a `struct tk_data`. So, my next step is going to be to check the definition of that `struct tk_data`.
|
| 84/4 |
2026/07/04 00:31 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/time/timekeeping.c Name:tk_data]
Results: map[SourceCode: 44: /*
45: * The most important data for readout fits into a single 64 byte
46: * cache line.
47: */
48: struct tk_data {
49: seqcount_raw_spinlock_t seq;
50: struct timekeeper timekeeper;
51: struct timekeeper shadow_timekeeper;
52: raw_spinlock_t lock;
53: } ____cacheline_aligned;
SourceFile:kernel/time/timekeeping.c]
|
| 85/4 |
2026/07/04 00:31 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 25459
output: 31
thoughts: 962
Thoughts: **KCSAN and `memcpy` in `timekeeping.c` - Deeper Dive**
Okay, so I'm revisiting this KCSAN report related to `memcpy` within the `timekeeping.c` code, specifically around the seqlock implementation (`tk_core.seq`). Let me piece this together. `tk_core.seq` is a `seqcount_raw_spinlock_t`, and the problem seems to be centered around potential KCSAN reports during read/write sequences. The concern is that a "plain" access like `memcpy` might trigger KCSAN warnings, even within an ostensibly atomic section.
The writer side has: `write_seqcount_begin` -> `kcsan_nestable_atomic_begin()`, a `memcpy`, and then `write_seqcount_end` -> `kcsan_nestable_atomic_end()`. The reader side involves `read_seqcount_begin` -> `kcsan_atomic_next(1000)`, some reads, and finally `read_seqcount_retry`.
The logic is that if `kcsan_atomic_next(1000)` is active, the reads are considered atomic. Similarly, if `kcsan_nestable_atomic_begin()` is active, the `memcpy` accesses should be considered atomic. KCSAN *shouldn't* report a race between two atomic accesses. So why am I seeing a report?
My initial thought was that maybe `memcpy` *isn't* covered by `kcsan_nestable_atomic_begin()`, but I recall that KCSAN often has special handling for `memcpy`. Then, I considered the possibility of `memcpy` being called from a context with disabled KCSAN instrumentation, but that's not the case here. It's a standard `memcpy`.
Now, I'm digging into `timekeeping.c` and I remember seeing some comments about `memcpy`. Aha! There's a comment about a possible optimization of switching pointers instead of a `memcpy`, but they chose to stick with the `memcpy`. The seqlock protects against a race but the seqlock *must* retry because the race might be non-benign! If the `memcpy` isn't atomic, the reader could end up with mixed data. The code in `ktime_get_coarse_ts64` appears to use the data after the retry. The code reads the values into local variables, *then* checks `read_seqcount_retry`. This means the data is only used if the read was consistent.
Specifically, `now = tk_xtime_coarse(tk)` returns a `struct timespec64` by value. No issue there. And then I have `mono = tk->wall_to_monotonic;`. `wall_to_monotonic` is also a `struct timespec64`. So if a `memcpy` operation is halfway through, `mono.tv_sec` could be from the new timekeeper and `mono.tv_nsec` from the old one, right? But the `read_seqcount_retry` will catch it. The important part is that the retried value is used *after* the seqlock check, so it should be safe.
Could this crash? No, there are no pointers involved in the values being read. They're all basic integer types. Okay, one more thing. I'm going to do another search for "KCSAN" in `kernel/time/timekeeping.c` to look at comments in context.
|
| 86/4 |
2026/07/04 00:31 |
tool |
grepper |
0m
Error:
Args: map[Expression:KCSAN PathPrefix:kernel/time/timekeeping.c]
Results:
|
| 87/4 |
2026/07/04 00:31 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 27235
output: 27
thoughts: 492
Thoughts: **Analyzing Potential KCSAN Issues in Timekeeping**
Okay, I'm diving into this KCSAN situation. First, I'm noting that there are no direct comments mentioning KCSAN. That's fine; let's get to the root of the issue.
Ideally, I'd have a KCSAN report to reference, but since I'm the one investigating, I need to hunt for the information in the code and history. My initial thought is to go back and check `include/linux/seqlock.h` again, maybe there's something relevant. I'm also wondering if there's a specific reason `memcpy` might not be covered, which seems odd given how prevalent it is.
Ah, I've got something! Thinking about how KCSAN and instrumentation works, it's hitting me: `kcsan_nestable_atomic_begin()` and `kcsan_nestable_atomic_end()` rely on accesses *being* instrumented. `memcpy` is instrumented, but maybe it isn't utilizing the "nestable atomic" state when it's called?
Right, KCSAN's instrumentation of `memcpy` usually calls `check_access`. So if `kcsan_nestable_atomic_begin()` is active, `check_access` should definitely see it.
Hang on... I've found a commit in a different part of the kernel that is likely relevant. I've seen situations where `memcpy` gets replaced with something else to suppress KCSAN reports that are false positives, but this seems to be a real raceβa benign one, though.
Hmm, this is getting interesting. In `kernel/time/timekeeping.c`, I've spotted a function, `timekeeping_update_from_shadow`, that uses `memcpy`. And if I look at `ktime_get_coarse_ts64`, it's a pretty straightforward reader.
I need to quickly check if `tk_core.seq` is a `seqcount_t` or something else. Ah! It's a `seqcount_raw_spinlock_t`. So, when we call `read_seqcount_begin(&tk_core.seq)`, it uses the `seqcount_t` that is inside this structure.
Now I've found a commit, `946976628888`, which is titled "timekeeping: Annotate data races in ktime_get_coarse_ts64()". Let me check if it exists in my setup.
|
| 88/4 |
2026/07/04 00:31 |
tool |
git-show |
0m
Error:
Args:
Results:
|
| 89/4 |
2026/07/04 00:31 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 28344
output: 33
thoughts: 15
Thoughts: Okay, here's my take on those initial thoughts:
**Hash Guessed, Title Time**
Alright, the hash is in. That was the first hurdle, and I think I've made a decent stab at the solution. Now that I have what appears to be a valid guess for the key, my next logical step is to pivot to something more easily searchable: the title. It seems like the obvious next step to validate or refute my initial assumption. I need to see if my hash guess provides me with anything useful in relation to the work I'm focusing on.
|
| 90/4 |
2026/07/04 00:31 |
tool |
git-log |
0m
Args: map[MessageRegexps:[ktime_get_coarse_ts64 data race]]
Results:
|
| 91/4 |
2026/07/04 00:32 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 28525
output: 27
thoughts: 362
Thoughts: **Analyzing `ktime_get_coarse_ts64` for Potential KCSAN Issues**
Okay, so I'm revisiting this `ktime_get_coarse_ts64` function. Let's see... nothing immediately jumps out, so let's dig into the code again. Right, here it is:
```c
2764: do {
2765: seq = read_seqcount_begin(&tk_core.seq);
2766:
2767: now = tk_xtime_coarse(tk);
2768: mono = tk->wall_to_monotonic;
2769: } while (read_seqcount_retry(&tk_core.seq, seq));
```
Aha! There's `tk_xtime_coarse(tk)`. Let me look at that function definition:
```c
static inline struct timespec64 tk_xtime_coarse(const struct timekeeper *tk)
{
return (struct timespec64) { .tv_sec = tk->xtime_sec, .tv_nsec = tk->coarse_nsec };
}
```
Okay, so that's just a struct initializer. Now, the potential problem KCSAN is reporting a race condition, it's likely because of how it sees the access to `tk->xtime_sec` conflicting with a `memcpy` operation.
But wait a minute, is it possible that simply using `read_seqcount_begin` is *not* enough to protect these "plain" memory accesses from KCSAN? The thing is, `read_seqcount_begin` only guards the *next* 1000 memory accesses. If `tk_xtime_coarse` is inlined, that should mitigate the issue.
Alright, let me refresh my memory. Hang on... I seem to recall a relevant commit. Yes, I think it was... ah, here it is! Commit `306067756f6b`: "timekeeping: Use READ_ONCE() for coarse time accessors." I need to check if that commit exists in the source tree. This is probably the key.
|
| 92/4 |
2026/07/04 00:32 |
tool |
git-show |
0m
Error:
Args:
Results:
|
| 93/4 |
2026/07/04 00:32 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 29440
output: 34
thoughts: 38
Thoughts: Okay, here's my summary, in the first person, as requested:
**Debugging My Git Hash Guessing Game**
Ugh, I'm just terrible at this. I'm failing miserably at guessing the relevant commit hashes. I need a more efficient approach than this blind guessing game. Okay, time to be more strategic. Let's see... I'm looking for a specific bit of code related to timekeeping, specifically involving `tk_xtime_coarse`. Now, I have a hunch that `READ_ONCE` might be involved somehow β maybe it's used within that function, or in a related context. So, rather than flailing about, let's leverage some of the tools at my disposal. I'll use `grepper` to see where `READ_ONCE` is used, and then cross-reference those usages with my target function. Hopefully, this will give me some clues about which commits are relevant and point me towards the correct hash. Time to dig in!
|
| 94/4 |
2026/07/04 00:32 |
tool |
grepper |
0m
Args: map[Expression:tk_xtime_coarse PathPrefix:kernel/time/timekeeping.c]
Results: map[Output:kernel/time/timekeeping.c=203=static inline struct timespec64 tk_xtime(const struct timekeeper *tk)
--
kernel/time/timekeeping.c-211-
kernel/time/timekeeping.c:212:static inline struct timespec64 tk_xtime_coarse(const struct timekeeper *tk)
kernel/time/timekeeping.c-213-{
--
kernel/time/timekeeping.c=2657=void ktime_get_coarse_real_ts64(struct timespec64 *ts)
--
kernel/time/timekeeping.c-2664-
kernel/time/timekeeping.c:2665: *ts = tk_xtime_coarse(tk);
kernel/time/timekeeping.c-2666- } while (read_seqcount_retry(&tk_core.seq, seq));
--
kernel/time/timekeeping.c=2679=void ktime_get_coarse_real_ts64_mg(struct timespec64 *ts)
--
kernel/time/timekeeping.c-2687- seq = read_seqcount_begin(&tk_core.seq);
kernel/time/timekeeping.c:2688: *ts = tk_xtime_coarse(tk);
kernel/time/timekeeping.c-2689- offset = tk_core.timekeeper.offs_real;
--
kernel/time/timekeeping.c=2758=void ktime_get_coarse_ts64(struct timespec64 *ts)
--
kernel/time/timekeeping.c-2766-
kernel/time/timekeeping.c:2767: now = tk_xtime_coarse(tk);
kernel/time/timekeeping.c-2768- mono = tk->wall_to_monotonic;
]
|
| 95/4 |
2026/07/04 00:32 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 30192
output: 35
thoughts: 26
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Diving into the Kernel's Timekeeping**
Okay, so I need to understand `tk_xtime_coarse`. Right, it's a kernel function, which strongly suggests we're dealing with *internal* timekeeping details. Let's get right into the source code, specifically `kernel/time/timekeeping.c`. I need the precise definition of `tk_xtime_coarse`. I'm guessing this function likely provides a "coarse" (meaning less precise) representation of the system's time, possibly for efficiency or to avoid certain locking overheads. I'm hoping I can immediately see its implementation to get the full picture of how the time is calculated. This is crucial for understanding how the kernel deals with time and how that influences things upstream.
|
| 96/4 |
2026/07/04 00:32 |
tool |
read-file |
0m
Args: map[File:kernel/time/timekeeping.c FirstLine:212 LineCount:5]
Results: map[Contents: 212: static inline struct timespec64 tk_xtime_coarse(const struct timekeeper *tk)
213: {
214: struct timespec64 ts;
215:
216: ts.tv_sec = tk->xtime_sec;
]
|
| 97/4 |
2026/07/04 00:32 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 30524
output: 36
thoughts: 7
Thoughts: Okay, here's the summary, assuming I'm the expert and the "thoughts" are my own:
**Developing a More Robust Output**
Alright, this feels a little sparse. I need to flesh this out, expand on the core idea, and really make sure the output has the depth and nuance it deserves. I'm aiming for clarity and precision, but right now, it's just scratching the surface. I've got a good foundation, but it lacks the necessary detail for a truly insightful analysis. It needs more substance, more concrete examples, or a deeper dive into the technical aspects of the process. I have a feeling I'll be able to improve on the quality with more effort.
|
| 98/4 |
2026/07/04 00:32 |
tool |
read-file |
0m
Args: map[File:kernel/time/timekeeping.c FirstLine:212 LineCount:10]
Results: map[Contents: 212: static inline struct timespec64 tk_xtime_coarse(const struct timekeeper *tk)
213: {
214: struct timespec64 ts;
215:
216: ts.tv_sec = tk->xtime_sec;
217: ts.tv_nsec = tk->coarse_nsec;
218: return ts;
219: }
220:
221: /*
]
|
| 99/4 |
2026/07/04 00:32 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 30857
output: 35
thoughts: 548
Thoughts: **Debugging Seqlock KCSAN Reports**
Okay, so I'm looking at this KCSAN report and the first thing that jumps out is that it doesn't seem to be using `READ_ONCE`. But, hold on, I think I've figured out the issue. KCSAN's seqlock checks are designed to flag when a reader might see inconsistent data and crucially *doesn't* retry. It's supposed to catch those sneaky data races. Now, it seems like in the context of seqlocks, the "plain" data race is actually expected, but it should be suppressed by `kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX)`.
Wait a minute, did I misread something in `include/linux/seqlock.h`? Let's double check `read_seqcount_begin`. Okay, here it is:
```c
297: #define read_seqcount_begin(s) \
298: ({ \
299: seqcount_lockdep_reader_access(seqprop_const_ptr(s)); \
300: raw_read_seqcount_begin(s); \
301: })
```
And then `raw_read_seqcount_begin`:
```c
#define raw_read_seqcount_begin(s) \
raw_read_seqcount(seqprop_const_ptr(s))
```
And finally, `raw_read_seqcount`:
```c
314: #define raw_read_seqcount(s) \
315: ({ \
316: unsigned __seq = seqprop_sequence(s); \
317: \
318: kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
319: __seq; \
320: })
```
Aha! `seqprop_sequence(s)` reads the sequence. If the sequence is odd, it indicates a writer is active, which is important to know. But, *that's it*? `read_seqcount_begin` doesn't wait for the sequence to become even! It just returns the value. If it's odd, the `read_seqcount_retry` will fail down the line, right?
Wait, actually... isn't `read_seqcount_begin` *supposed* to wait for an even value when starting a read? Yes, it is! Let's verify. Ok, let's look at `seqprop_sequence`. Let's see...
|
| 100/4 |
2026/07/04 00:32 |
tool |
grepper |
0m
Args: map[Expression:#define seqprop_sequence PathPrefix:include/linux/seqlock.h]
Results: map[Output:include/linux/seqlock.h=230=SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex)
--
include/linux/seqlock.h-261-#define seqprop_const_ptr(s) __seqprop(s, const_ptr)(s)
include/linux/seqlock.h:262:#define seqprop_sequence(s) __seqprop(s, sequence)(s)
include/linux/seqlock.h-263-#define seqprop_preemptible(s) __seqprop(s, preemptible)(s)
]
|
| 101/4 |
2026/07/04 00:32 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 32198
output: 33
thoughts: 36
Thoughts: Okay, here's my interpretation of that thought process, as if it were my own:
**Finding the Sequence Count**
Alright, let's see... I'm looking at `__seqprop(s, sequence)` for a `seqcount_t` type. The standard way this would work is with `seqcount_sequence`. Hmm, so I need to find where that is defined. Let me start digging around for the implementation of `seqcount_sequence`. Shouldn't be too hard to track down. It's probably related to the core sequence handling logic. I'll start with the obvious places, checking headers and source files where these sequence counts are used. Let's get cracking!
|
| 102/4 |
2026/07/04 00:32 |
tool |
grepper |
0m
Error:
Args: map[Expression:seqcount_sequence PathPrefix:include/linux/seqlock.h]
Results:
|
| 103/4 |
2026/07/04 00:32 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 32430
output: 37
thoughts: 26
Thoughts: Here's my take on those thoughts, summarized as if they were my own:
**Initial Assessment and Exploration**
Hmm, this output... something about it feels... familiar. My gut says it was probably generated by the `__seqprop` function. I've encountered that pattern before. Okay, let's not jump to conclusions, but that's where I'll start digging. To really understand what's happening here, I need to refresh my memory of how `__seqprop` is defined and operates. Time to dive into the code and see exactly what it's doing. Let's see what we can find.
|
| 104/4 |
2026/07/04 00:32 |
tool |
read-file |
0m
Args: map[File:include/linux/seqlock.h FirstLine:140 LineCount:50]
Results: map[Contents: 140: * @lockname: "LOCKNAME" part of seqcount_LOCKNAME_t
141: * @locktype: LOCKNAME canonical C data type
142: * @preemptible: preemptibility of above locktype
143: * @lockbase: prefix for associated lock/unlock
144: */
145: #define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockbase) \
146: static __always_inline seqcount_t * \
147: __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s) \
148: { \
149: return &s->seqcount; \
150: } \
151: \
152: static __always_inline const seqcount_t * \
153: __seqprop_##lockname##_const_ptr(const seqcount_##lockname##_t *s) \
154: { \
155: return &s->seqcount; \
156: } \
157: \
158: static __always_inline unsigned \
159: __seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s) \
160: { \
161: unsigned seq = smp_load_acquire(&s->seqcount.sequence); \
162: \
163: if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
164: return seq; \
165: \
166: if (preemptible && unlikely(seq & 1)) { \
167: __SEQ_LOCK(lockbase##_lock(s->lock)); \
168: __SEQ_LOCK(lockbase##_unlock(s->lock)); \
169: \
170: /* \
171: * Re-read the sequence counter since the (possibly \
172: * preempted) writer made progress. \
173: */ \
174: seq = smp_load_acquire(&s->seqcount.sequence); \
175: } \
176: \
177: return seq; \
178: } \
179: \
180: static __always_inline bool \
181: __seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s) \
182: { \
183: if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
184: return preemptible; \
185: \
186: /* PREEMPT_RT relies on the above LOCK+UNLOCK */ \
187: return false; \
188: } \
189: \
]
|
| 105/4 |
2026/07/04 00:32 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 33454
output: 26
thoughts: 464
Thoughts: **Analyzing a Seqlock KCSAN Race Condition**
Okay, let's break this down. My initial thought was: the `__seqprop_##lockname##_sequence` function uses `smp_load_acquire` which, if `CONFIG_PREEMPT_RT` isn't enabled, just returns the sequence number immediately. It *doesn't* wait for an even value. That's a key detail.
Now, `read_seqcount_begin` is *supposed* to ensure it's an even sequence number before entering the critical section, right? If it doesn't wait, a reader could sneak in while a writer is active. The subsequent `read_seqcount_retry` would fail, and it'd loop β which is a perfectly valid seqlock implementation, employing speculative execution.
But here's the kicker: if the reader enters the critical section concurrently with a writer, it's a race condition. KCSAN, the kernel Concurrency Sanitizer, *will* detect this.
My next line of thought was about mitigating this. If we use `kcsan_atomic_next(1000)` inside `read_seqcount_begin`, we mark the accesses inside the critical section as atomic. If the writer also marks *its* accesses as atomic (which `write_seqcount_begin` does with `kcsan_nestable_atomic_begin`), KCSAN *should* remain silent.
Wait, here's a potential wrinkle! `memcpy` is suspect. It *might not* be covered by `kcsan_nestable_atomic_begin()` if it's a large `memcpy` and KCSAN's instrumentation for `memcpy` doesn't check the nestable atomic count.
Actually, a simpler explanation emerges...Is `tk_core.seq` always used with `write_seqcount_begin`?
In `timekeeping_update_from_shadow`:
```c
802: write_seqcount_begin(&tkd->seq);
...
835: memcpy(&tkd->timekeeper, tk, sizeof(*tk));
836: write_seqcount_end(&tkd->seq);
```
Yes, it is. The memcpy is protected.
Aha! Maybe I'm onto something. I stumbled on a commit that could be the solution: `fde86d90cb8b`, "kcsan: seqlock: fix kcsan_atomic_next() usage". Time to check if this commit exists!
|
| 106/4 |
2026/07/04 00:32 |
tool |
git-log |
0m
Args: map[MessageRegexps:[kcsan seqlock]]
Results: map[Output:183ec5f26b2f kcsan, seqlock: Fix incorrect assumption in read_seqbegin()
5c1806c41ce0 kcsan, seqlock: Support seqcount_latch_t
d16317de9b41 seqlock/latch: Provide raw_read_seqcount_latch_retry()
b968a08f242d compiler.h, seqlock.h: Remove unnecessary kcsan.h includes
bf07132f96d4 seqlock: Require WRITE_ONCE surrounding raw_seqcount_barrier
88ecd153be95 seqlock, kcsan: Add annotations for KCSAN
]
|
| 107/4 |
2026/07/04 00:32 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 34684
output: 26
thoughts: 18
Thoughts: Okay, here's my interpretation of that.
**Analyzing `183ec5f26b2f`**
Alright, let's dive into `183ec5f26b2f`. Just seeing the string gives me a starting point. It's likely a hash, maybe an identifier of some kind. Immediately, I need to start thinking about the context. What's the system? What's the data being hashed? Is it a file checksum, a database key, a unique user ID? Is there a known algorithm associated with it? I'll probably need to identify what type of hash it is. Is it MD5, SHA-1, SHA-256, or something else? If I can quickly determine the hash algorithm, I can rule out entire classes of possibilities. From there, I'll need to see if it can be cracked or reversed. Is there known vulnerabilities with the hashing algorithm? Are rainbow tables a possibility? I'll need to think about how large this identifier is.
Is it a truncated hash or a full-length one? And then, assuming there isn't obvious information like plaintext or a reference, I might have to go through a sequence of steps. First, look for a standard library/program that deals with that hash. Is it used to identify a file, a user, or something else. I'll need to trace its origins, the information in its proximity, and any relevant logs. I'll have to see where it appears and what it's linked to. I might need to cross-reference it with a larger dataset or existing vulnerability databases. This will take some time, but ultimately, the solution is in the data, the process, and the surrounding environment, all of which will make this identifier's purpose clear.
|
| 108/4 |
2026/07/04 00:33 |
tool |
git-show |
0m
Args:
Results: map[Output:commit 183ec5f26b2fc97a4a9871865bfe9b33c41fddb2
Author: Marco Elver <elver@google.com>
Date: Mon Nov 4 16:43:09 2024 +0100
kcsan, seqlock: Fix incorrect assumption in read_seqbegin()
During testing of the preceding changes, I noticed that in some cases,
current->kcsan_ctx.in_flat_atomic remained true until task exit. This is
obviously wrong, because _all_ accesses for the given task will be
treated as atomic, resulting in false negatives i.e. missed data races.
Debugging led to fs/dcache.c, where we can see this usage of seqlock:
struct dentry *d_lookup(const struct dentry *parent, const struct qstr *name)
{
struct dentry *dentry;
unsigned seq;
do {
seq = read_seqbegin(&rename_lock);
dentry = __d_lookup(parent, name);
if (dentry)
break;
} while (read_seqretry(&rename_lock, seq));
[...]
As can be seen, read_seqretry() is never called if dentry != NULL;
consequently, current->kcsan_ctx.in_flat_atomic will never be reset to
false by read_seqretry().
Give up on the wrong assumption of "assume closing read_seqretry()", and
rely on the already-present annotations in read_seqcount_begin/retry().
Fixes: 88ecd153be95 ("seqlock, kcsan: Add annotations for KCSAN")
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20241104161910.780003-6-elver@google.com
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 45eee0e5dca0..5298765d6ca4 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -810,11 +810,7 @@ static __always_inline void write_seqcount_latch_end(seqcount_latch_t *s)
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
- unsigned ret = read_seqcount_begin(&sl->seqcount);
-
- kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */
- kcsan_flat_atomic_begin();
- return ret;
+ return read_seqcount_begin(&sl->seqcount);
}
/**
@@ -830,12 +826,6 @@ static inline unsigned read_seqbegin(const seqlock_t *sl)
*/
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
- /*
- * Assume not nested: read_seqretry() may be called multiple times when
- * completing read critical section.
- */
- kcsan_flat_atomic_end();
-
return read_seqcount_retry(&sl->seqcount, start);
}
]
|
| 109/4 |
2026/07/04 00:33 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 36012
output: 24
thoughts: 60
Thoughts: Here's my thinking about this commit:
**Analyzing Seqlock Implementations**
Okay, so this commit is centered on `seqlock_t`, which is important for understanding concurrent access patterns. But then I see that `tk_core.seq` uses `seqcount_t`, specifically wrapped in a `seqcount_raw_spinlock_t`. That's interesting, because there are subtle differences in their guarantees. I need to make sure I understand how this specific implementation works and how it relates to the wider synchronization landscape. Before getting too deep I should take a look at `88ecd153be95` for context as well. I need to fully understand all related changes for a complete picture.
|
| 110/4 |
2026/07/04 00:33 |
tool |
git-show |
0m
Args:
Results: map[Output:commit 88ecd153be9519f259b87a9f6f4c8383a8b3bbf1
Author: Marco Elver <elver@google.com>
Date: Thu Nov 14 19:02:59 2019 +0100
seqlock, kcsan: Add annotations for KCSAN
Since seqlocks in the Linux kernel do not require the use of marked
atomic accesses in critical sections, we teach KCSAN to assume such
accesses are atomic. KCSAN currently also pretends that writes to
`sequence` are atomic, although currently plain writes are used (their
corresponding reads are READ_ONCE).
Further, to avoid false positives in the absence of clear ending of a
seqlock reader critical section (only when using the raw interface),
KCSAN assumes a fixed number of accesses after start of a seqlock
critical section are atomic.
=== Commentary on design around absence of clear begin/end markings ===
Seqlock usage via seqlock_t follows a predictable usage pattern, where
clear critical section begin/end is enforced. With subtle special cases
for readers needing to be flat atomic regions, e.g. because usage such
as in:
- fs/namespace.c:__legitimize_mnt - unbalanced read_seqretry
- fs/dcache.c:d_walk - unbalanced need_seqretry
But, anything directly accessing seqcount_t seems to be unpredictable.
Filtering for usage of read_seqcount_retry not following 'do { .. }
while (read_seqcount_retry(..));':
$ git grep 'read_seqcount_retry' | grep -Ev 'while \(|seqlock.h|Doc|\* '
=> about 1/3 of the total read_seqcount_retry usage.
Just looking at fs/namei.c, we conclude that it is non-trivial to
prescribe and migrate to an interface that would force clear begin/end
seqlock markings for critical sections.
As such, we concluded that the best design currently, is to simply
ensure that KCSAN works well with the existing code.
Signed-off-by: Marco Elver <elver@google.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index bcf4cf26b8c8..61232bc223fd 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -37,8 +37,24 @@
#include <linux/preempt.h>
#include <linux/lockdep.h>
#include <linux/compiler.h>
+#include <linux/kcsan.h>
#include <asm/processor.h>
+/*
+ * The seqlock interface does not prescribe a precise sequence of read
+ * begin/retry/end. For readers, typically there is a call to
+ * read_seqcount_begin() and read_seqcount_retry(), however, there are more
+ * esoteric cases which do not follow this pattern.
+ *
+ * As a consequence, we take the following best-effort approach for raw usage
+ * via seqcount_t under KCSAN: upon beginning a seq-reader critical section,
+ * pessimistically mark then next KCSAN_SEQLOCK_REGION_MAX memory accesses as
+ * atomics; if there is a matching read_seqcount_retry() call, no following
+ * memory operations are considered atomic. Usage of seqlocks via seqlock_t
+ * interface is not affected.
+ */
+#define KCSAN_SEQLOCK_REGION_MAX 1000
+
/*
* Version using sequence counter only.
* This can be used when code has its own mutex protecting the
@@ -115,6 +131,7 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
cpu_relax();
goto repeat;
}
+ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
return ret;
}
@@ -131,6 +148,7 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
+ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
return ret;
}
@@ -183,6 +201,7 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
+ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
return ret & ~1;
}
@@ -202,7 +221,8 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s)
*/
static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
{
- return unlikely(s->sequence != start);
+ kcsan_atomic_next(0);
+ return unlikely(READ_ONCE(s->sequence) != start);
}
/**
@@ -225,6 +245,7 @@ static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
static inline void raw_write_seqcount_begin(seqcount_t *s)
{
+ kcsan_nestable_atomic_begin();
s->sequence++;
smp_wmb();
}
@@ -233,6 +254,7 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
{
smp_wmb();
s->sequence++;
+ kcsan_nestable_atomic_end();
}
/**
@@ -271,9 +293,11 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
*/
static inline void raw_write_seqcount_barrier(seqcount_t *s)
{
+ kcsan_nestable_atomic_begin();
s->sequence++;
smp_wmb();
s->sequence++;
+ kcsan_nestable_atomic_end();
}
static inline int raw_read_seqcount_latch(seqcount_t *s)
@@ -398,7 +422,9 @@ static inline void write_seqcount_end(seqcount_t *s)
static inline void write_seqcount_invalidate(seqcount_t *s)
{
smp_wmb();
+ kcsan_nestable_atomic_begin();
s->sequence+=2;
+ kcsan_nestable_atomic_end();
}
typedef struct {
@@ -430,11 +456,21 @@ typedef struct {
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
- return read_seqcount_begin(&sl->seqcount);
+ unsigned ret = read_seqcount_begin(&sl->seqcount);
+
+ kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry */
+ kcsan_flat_atomic_begin();
+ return ret;
}
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
+ /*
+ * Assume not nested: read_seqretry may be called multiple times when
+ * completing read critical section.
+ */
+ kcsan_flat_atomic_end();
+
return read_seqcount_retry(&sl->seqcount, start);
}
]
|
| 111/4 |
2026/07/04 00:33 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 38256
output: 27
thoughts: 473
Thoughts: **KCSAN and Seqlocks: A Deep Dive**
Okay, so KCSAN *does* have annotations for seqlocks, which is good. But if I'm seeing a race reported between `memcpy` and the reads, that's telling me something's off. It's either the annotations aren't functioning correctly, the `memcpy` isn't being properly wrapped by `kcsan_nestable_atomic_begin()`, or the reads aren't protected by `kcsan_atomic_next(1000)`.
Aha! I've spotted a potential culprit. Specifically, the `timekeeping.c` file has a likely issue. Looking at `timekeeping_update_from_shadow`: the `memcpy` operation that is under suspicion is enclosed within a `write_seqcount_begin/end` pair, which is good.
```c
802: write_seqcount_begin(&tkd->seq);
...
835: memcpy(&tkd->timekeeper, tk, sizeof(*tk));
836: write_seqcount_end(&tkd->seq);
```
Then, in `ktime_get_coarse_ts64`: the reads are inside the read section:
```c
2765: seq = read_seqcount_begin(&tk_core.seq);
2766:
2767: now = tk_xtime_coarse(tk);
2768: mono = tk->wall_to_monotonic;
2769: } while (read_seqcount_retry(&tk_core.seq, seq));
```
If KCSAN is flagging this, I suspect that the `memcpy` operation is not instrumented to be "atomic" even when `kcsan_nestable_atomic_begin()` is active. The `memcpy` instrumentation, whether it's via `__msan_memcpy` or something similar, may be failing to check the per-thread `in_flat_atomic` or `atomic_next` state. KCSAN looks like it uses `__tsan_memcpy` or its own version. I think it is the issue.
Wait a second, there's a commit that seems *very* relevant. Commit `662692291681` "kcsan: Add support for instrumented memcpy/memset/memmove". Time to go check that out.
|
| 112/4 |
2026/07/04 00:33 |
tool |
git-show |
0m
Error:
Args:
Results:
|
| 113/4 |
2026/07/04 00:33 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 39338
output: 24
thoughts: 11
Thoughts: Here's your summary:
**Initial Investigation: "kcsan memcpy"**
Okay, so I need to dig into this. My initial thought is to search for "kcsan memcpy". That search query should be a good starting point. I'm expecting that combination of keywords to be specific enough to focus my attention on memory copy issues that are flagged by KCSAN. This should hopefully bring up relevant bug reports, kernel patches, or discussions relating to KCSAN's interaction with `memcpy`. I'm hoping to quickly identify the scope of the problem.
|
| 114/4 |
2026/07/04 00:33 |
tool |
git-log |
0m
Args: map[MessageRegexps:[kcsan memcpy]]
Results: map[Output:61b258b0d2f6 x86: call instrumentation hooks from copy_mc.c
922621a68284 instrumented.h: add instrument_memcpy_before, instrument_memcpy_after
6a86b5b5cd76 bpf: Annotate bpf_long_memcpy with data_race
f9cfb1910ece string: use __builtin_memcpy() in strlcpy/strlcat
6c1c5097781f net: add atomic_long_t to net_device_stats fields
7c201739beef kcsan: Instrument memcpy/memset/memmove with newer Clang
efc72a665a61 lkdtm: Disable return thunks in rodata.c
b82a8b93b4f9 drm/i915: Be wary of data races when reading the active execlists
b4d9145b0154 drm/i915: Be wary of data races when reading the active execlists
]
|
| 115/4 |
2026/07/04 00:33 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 39758
output: 24
thoughts: 16
Thoughts: Okay, let's dive into this. Here's my thought process regarding `7c201739beef`:
**Initial Assessment of `7c201739beef`**
Right, let's see what we're dealing with. The sheer brevity of the identifier suggests this is likely a short-lived artifact or a very granular reference. It's not immediately obvious what it represents without context. Given the hexadecimal nature, it's almost certainly a unique identifier of some kind. This could be a commit hash, a database key, a file ID, or even a session token β possibilities are numerous. My initial task is to recall where this might have its root and contextualize this number. I need to figure out its origin point, its system of derivation.
My immediate next step is to recall the most recent project or tasks I've been working on, because that could provide the immediate context needed. I will need to consider the most recent projects and databases and files I've been working with. I need to quickly cross-reference this against any recent logs or documentation. This is where a good search function and recall of recent project work is invaluable. Where did I come across this string? If I can recall the environment, it'll significantly narrow down the possibilities. From there, I can begin to interpret its significance.
This is the baseline and the place to begin looking into it. The next step is a deep dive into the code base or data context where this identifier originated.
|
| 116/4 |
2026/07/04 00:33 |
tool |
git-show |
0m
Args:
Results: map[Output:commit 7c201739beef1a586d806463f1465429cdce34c5
Author: Marco Elver <elver@google.com>
Date: Mon Sep 12 11:45:40 2022 +0200
kcsan: Instrument memcpy/memset/memmove with newer Clang
With Clang version 16+, -fsanitize=thread will turn
memcpy/memset/memmove calls in instrumented functions into
__tsan_memcpy/__tsan_memset/__tsan_memmove calls respectively.
Add these functions to the core KCSAN runtime, so that we (a) catch data
races with mem* functions, and (b) won't run into linker errors with
such newer compilers.
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index fe12dfe254ec..54d077e1a2dc 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -14,10 +14,12 @@
#include <linux/init.h>
#include <linux/kernel.h>
#include <linux/list.h>
+#include <linux/minmax.h>
#include <linux/moduleparam.h>
#include <linux/percpu.h>
#include <linux/preempt.h>
#include <linux/sched.h>
+#include <linux/string.h>
#include <linux/uaccess.h>
#include "encoding.h"
@@ -1308,3 +1310,51 @@ noinline void __tsan_atomic_signal_fence(int memorder)
}
}
EXPORT_SYMBOL(__tsan_atomic_signal_fence);
+
+#ifdef __HAVE_ARCH_MEMSET
+void *__tsan_memset(void *s, int c, size_t count);
+noinline void *__tsan_memset(void *s, int c, size_t count)
+{
+ /*
+ * Instead of not setting up watchpoints where accessed size is greater
+ * than MAX_ENCODABLE_SIZE, truncate checked size to MAX_ENCODABLE_SIZE.
+ */
+ size_t check_len = min_t(size_t, count, MAX_ENCODABLE_SIZE);
+
+ check_access(s, check_len, KCSAN_ACCESS_WRITE, _RET_IP_);
+ return memset(s, c, count);
+}
+#else
+void *__tsan_memset(void *s, int c, size_t count) __alias(memset);
+#endif
+EXPORT_SYMBOL(__tsan_memset);
+
+#ifdef __HAVE_ARCH_MEMMOVE
+void *__tsan_memmove(void *dst, const void *src, size_t len);
+noinline void *__tsan_memmove(void *dst, const void *src, size_t len)
+{
+ size_t check_len = min_t(size_t, len, MAX_ENCODABLE_SIZE);
+
+ check_access(dst, check_len, KCSAN_ACCESS_WRITE, _RET_IP_);
+ check_access(src, check_len, 0, _RET_IP_);
+ return memmove(dst, src, len);
+}
+#else
+void *__tsan_memmove(void *dst, const void *src, size_t len) __alias(memmove);
+#endif
+EXPORT_SYMBOL(__tsan_memmove);
+
+#ifdef __HAVE_ARCH_MEMCPY
+void *__tsan_memcpy(void *dst, const void *src, size_t len);
+noinline void *__tsan_memcpy(void *dst, const void *src, size_t len)
+{
+ size_t check_len = min_t(size_t, len, MAX_ENCODABLE_SIZE);
+
+ check_access(dst, check_len, KCSAN_ACCESS_WRITE, _RET_IP_);
+ check_access(src, check_len, 0, _RET_IP_);
+ return memcpy(dst, src, len);
+}
+#else
+void *__tsan_memcpy(void *dst, const void *src, size_t len) __alias(memcpy);
+#endif
+EXPORT_SYMBOL(__tsan_memcpy);
]
|
| 117/4 |
2026/07/04 00:33 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 41292
output: 38
thoughts: 78
Thoughts: Here's my take on the thought process, as if it were my own:
**Analyzing the KCSAN memcpy Interaction**
Okay, so I'm digging into this `__tsan_memcpy` function, and it's calling `check_access`. Right, that's the central KCSAN function, the one that does the heavy lifting of race detection. My expectation is that it *should* be properly considering the atomic states, `in_flat_atomic` or `atomic_next`. That's critical for correctly handling synchronization primitives and preventing false positives or negatives.
Let me just quickly verify that `check_access` behavior in `kernel/kcsan/core.c` and make sure my assumptions here are correct. I need to refresh my understanding of how it integrates with the atomic context to ensure the check doesn't accidentally trigger within a protected section.
|
| 118/4 |
2026/07/04 00:33 |
tool |
read-file |
0m
Args: map[File:kernel/kcsan/core.c FirstLine:600 LineCount:100]
Results: map[Contents: 600: }
601:
602: atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_SETUP_WATCHPOINTS]);
603: atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_USED_WATCHPOINTS]);
604:
605: /*
606: * Read the current value, to later check and infer a race if the data
607: * was modified via a non-instrumented access, e.g. from a device.
608: */
609: old = is_reorder_access ? 0 : read_instrumented_memory(ptr, size);
610:
611: /*
612: * Delay this thread, to increase probability of observing a racy
613: * conflicting access.
614: */
615: delay_access(type);
616:
617: /*
618: * Re-read value, and check if it is as expected; if not, we infer a
619: * racy access.
620: */
621: if (!is_reorder_access) {
622: new = read_instrumented_memory(ptr, size);
623: } else {
624: /*
625: * Reordered accesses cannot be used for value change detection,
626: * because the memory location may no longer be accessible and
627: * could result in a fault.
628: */
629: new = 0;
630: access_mask = 0;
631: }
632:
633: diff = old ^ new;
634: if (access_mask)
635: diff &= access_mask;
636:
637: /*
638: * Check if we observed a value change.
639: *
640: * Also check if the data race should be ignored (the rules depend on
641: * non-zero diff); if it is to be ignored, the below rules for
642: * KCSAN_VALUE_CHANGE_MAYBE apply.
643: */
644: if (diff && !kcsan_ignore_data_race(size, type, old, new, diff))
645: value_change = KCSAN_VALUE_CHANGE_TRUE;
646:
647: /* Check if this access raced with another. */
648: if (!consume_watchpoint(watchpoint)) {
649: /*
650: * Depending on the access type, map a value_change of MAYBE to
651: * TRUE (always report) or FALSE (never report).
652: */
653: if (value_change == KCSAN_VALUE_CHANGE_MAYBE) {
654: if (access_mask != 0) {
655: /*
656: * For access with access_mask, we require a
657: * value-change, as it is likely that races on
658: * ~access_mask bits are expected.
659: */
660: value_change = KCSAN_VALUE_CHANGE_FALSE;
661: } else if (size > 8 || is_assert) {
662: /* Always assume a value-change. */
663: value_change = KCSAN_VALUE_CHANGE_TRUE;
664: }
665: }
666:
667: /*
668: * No need to increment 'data_races' counter, as the racing
669: * thread already did.
670: *
671: * Count 'assert_failures' for each failed ASSERT access,
672: * therefore both this thread and the racing thread may
673: * increment this counter.
674: */
675: if (is_assert && value_change == KCSAN_VALUE_CHANGE_TRUE)
676: atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ASSERT_FAILURES]);
677:
678: kcsan_report_known_origin(ptr, size, type, ip,
679: value_change, watchpoint - watchpoints,
680: old, new, access_mask);
681: } else if (value_change == KCSAN_VALUE_CHANGE_TRUE) {
682: /* Inferring a race, since the value should not have changed. */
683:
684: atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_RACES_UNKNOWN_ORIGIN]);
685: if (is_assert)
686: atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_ASSERT_FAILURES]);
687:
688: if (IS_ENABLED(CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN) || is_assert) {
689: kcsan_report_unknown_origin(ptr, size, type, ip,
690: old, new, access_mask);
691: }
692: }
693:
694: /*
695: * Remove watchpoint; must be after reporting, since the slot may be
696: * reused after this point.
697: */
698: remove_watchpoint(watchpoint);
699: atomic_long_dec(&kcsan_counters[KCSAN_COUNTER_USED_WATCHPOINTS]);
]
|
| 119/4 |
2026/07/04 00:33 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 43150
output: 37
thoughts: 12
Thoughts: Here's my thought process, summarized as if I were considering the code:
**Analyzing `check_access` and Its Starting Point**
Okay, so I need to get a clear picture of what's happening with `check_access`. The initial entry point is crucial. I can't really grasp its function without seeing where the process *begins*. Understanding the arguments it accepts and how it's initially invoked will provide immediate context to the overall flow and intended functionality. I need that initial "trigger" to understand where I am at the outset.
|
| 120/4 |
2026/07/04 00:33 |
tool |
read-file |
0m
Args: map[File:kernel/kcsan/core.c FirstLine:530 LineCount:70]
Results: map[Contents: 530: const bool is_assert = (type & KCSAN_ACCESS_ASSERT) != 0;
531: atomic_long_t *watchpoint;
532: u64 old, new, diff;
533: enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
534: bool interrupt_watcher = kcsan_interrupt_watcher;
535: unsigned long ua_flags = user_access_save();
536: struct kcsan_ctx *ctx = get_ctx();
537: unsigned long access_mask = ctx->access_mask;
538: unsigned long irq_flags = 0;
539: bool is_reorder_access;
540:
541: /*
542: * Always reset kcsan_skip counter in slow-path to avoid underflow; see
543: * should_watch().
544: */
545: reset_kcsan_skip();
546:
547: if (!kcsan_is_enabled(ctx))
548: goto out;
549:
550: /*
551: * Check to-ignore addresses after kcsan_is_enabled(), as we may access
552: * memory that is not yet initialized during early boot.
553: */
554: if (!is_assert && kcsan_ignore_address(ptr))
555: goto out;
556:
557: if (!check_encodable((unsigned long)ptr, size)) {
558: atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_UNENCODABLE_ACCESSES]);
559: goto out;
560: }
561:
562: /*
563: * The local CPU cannot observe reordering of its own accesses, and
564: * therefore we need to take care of 2 cases to avoid false positives:
565: *
566: * 1. Races of the reordered access with interrupts. To avoid, if
567: * the current access is reorder_access, disable interrupts.
568: * 2. Avoid races of scoped accesses from nested interrupts (below).
569: */
570: is_reorder_access = find_reorder_access(ctx, ptr, size, type, ip);
571: if (is_reorder_access)
572: interrupt_watcher = false;
573: /*
574: * Avoid races of scoped accesses from nested interrupts (or scheduler).
575: * Assume setting up a watchpoint for a non-scoped (normal) access that
576: * also conflicts with a current scoped access. In a nested interrupt,
577: * which shares the context, it would check a conflicting scoped access.
578: * To avoid, disable scoped access checking.
579: */
580: ctx->disable_scoped++;
581:
582: /*
583: * Save and restore the IRQ state trace touched by KCSAN, since KCSAN's
584: * runtime is entered for every memory access, and potentially useful
585: * information is lost if dirtied by KCSAN.
586: */
587: kcsan_save_irqtrace(current);
588: if (!interrupt_watcher)
589: local_irq_save(irq_flags);
590:
591: watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
592: if (watchpoint == NULL) {
593: /*
594: * Out of capacity: the size of 'watchpoints', and the frequency
595: * with which should_watch() returns true should be tweaked so
596: * that this case happens very rarely.
597: */
598: atomic_long_inc(&kcsan_counters[KCSAN_COUNTER_NO_CAPACITY]);
599: goto out_unlock;
]
|
| 121/4 |
2026/07/04 00:33 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 44458
output: 37
thoughts: 63
Thoughts: **Analyzing the KCSAN Access Check**
Okay, hold on a second. I'm going through this access check, and I'm not spotting the usual suspects: no direct checks for `in_flat_atomic` or `atomic_next` within the `check_access` function itself. That's a little unusual, and it tells me those conditions are being handled elsewhere, either upstream or downstream of this particular function.
My initial assumption is that one of the related functions, `should_watch` or `kcsan_is_enabled`, must contain this key logic. Since `kcsan_is_enabled` seems a bit more likely as a global condition, let me start there. I'll need to confirm that's where the atomic-related safeguards are implemented, or to quickly move on to `should_watch` to narrow down the potential location.
|
| 122/4 |
2026/07/04 00:33 |
tool |
read-file |
0m
Args: map[File:kernel/kcsan/core.c FirstLine:350 LineCount:50]
Results: map[Contents: 350: case 1: return *(const volatile u8 *)ptr;
351: case 2: return *(const volatile u16 *)ptr;
352: case 4: return *(const volatile u32 *)ptr;
353: case 8: return *(const volatile u64 *)ptr;
354: default: return 0; /* Ignore; we do not diff the values. */
355: }
356: }
357:
358: void kcsan_save_irqtrace(struct task_struct *task)
359: {
360: #ifdef CONFIG_TRACE_IRQFLAGS
361: task->kcsan_save_irqtrace = task->irqtrace;
362: #endif
363: }
364:
365: void kcsan_restore_irqtrace(struct task_struct *task)
366: {
367: #ifdef CONFIG_TRACE_IRQFLAGS
368: task->irqtrace = task->kcsan_save_irqtrace;
369: #endif
370: }
371:
372: static __always_inline int get_kcsan_stack_depth(void)
373: {
374: #ifdef CONFIG_KCSAN_WEAK_MEMORY
375: return current->kcsan_stack_depth;
376: #else
377: BUILD_BUG();
378: return 0;
379: #endif
380: }
381:
382: static __always_inline void add_kcsan_stack_depth(int val)
383: {
384: #ifdef CONFIG_KCSAN_WEAK_MEMORY
385: current->kcsan_stack_depth += val;
386: #else
387: BUILD_BUG();
388: #endif
389: }
390:
391: static __always_inline struct kcsan_scoped_access *get_reorder_access(struct kcsan_ctx *ctx)
392: {
393: #ifdef CONFIG_KCSAN_WEAK_MEMORY
394: return ctx->disable_scoped ? NULL : &ctx->reorder_access;
395: #else
396: return NULL;
397: #endif
398: }
399:
]
|
| 123/4 |
2026/07/04 00:33 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 45456
output: 42
thoughts: 16
Thoughts: Here's my thought process, summarized as you requested:
**KCSAN Investigation: Definition Discovery**
Okay, first things first. I need to understand what `kcsan_is_enabled` actually *is*. This is probably a macro, function, or variable that controls the behavior of Kernel Concurrency Sanitizer (KCSAN). My immediate next step is to locate its definition within the codebase. Finding the declaration will give me the most fundamental information about its purpose, potential values, and how it's used. This is ground zero for figuring out how KCSAN is being managed in this particular context. Once I have the definition, I can start tracing its usage to get a full picture.
|
| 124/4 |
2026/07/04 00:33 |
tool |
grepper |
0m
Args: map[Expression:static __always_inline bool kcsan_is_enabled PathPrefix:kernel/kcsan/core.c]
Results: map[Output:kernel/kcsan/core.c=306=static inline void reset_kcsan_skip(void)
--
kernel/kcsan/core.c-314-
kernel/kcsan/core.c:315:static __always_inline bool kcsan_is_enabled(struct kcsan_ctx *ctx)
kernel/kcsan/core.c-316-{
]
|
| 125/4 |
2026/07/04 00:33 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 45771
output: 38
thoughts: 83
Thoughts: **Analyzing the KCSAN Enablement Check**
Okay, so I'm looking at this `kcsan_is_enabled` function. It seems pretty straightforward. It's an inline function, so presumably, the compiler will try to optimize its inclusion. It takes a `kcsan_ctx` pointer as an argument, which makes sense; that's where the KCSAN context information is held. The crucial part here is the return statement: `return ctx->disable_count == 0;`. Essentially, KCSAN is enabled if the `disable_count` within the context is zero. A nonzero value would indicate the feature has been temporarily deactivated, likely for a critical section or some debugging purpose. This is a common and efficient way to control feature enablement within a system. I'm not seeing any immediate red flags, but this feels too simple to be the central point of my interest. I need a broader view, it seems like I am missing some context. Let's dig deeper to see where `should_watch` comes into play.
|
| 126/4 |
2026/07/04 00:33 |
tool |
grepper |
0m
Error:
Args: map[Expression:static __always_inline bool should_watch PathPrefix:kernel/kcsan/core.c]
Results:
|
| 127/4 |
2026/07/04 00:33 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 46128
output: 38
thoughts: 22
Thoughts: Okay, here's my summary, tailored as if I'm thinking it, and assuming my expertise:
**Targeting the Core of the Issue: `kcsan/core.c`**
Alright, let's get to it. My initial thought is to dive right into the heart of the matter. I know this issue relates to KCSan, so I'm going to head directly to the `kernel/kcsan/core.c` file. That's the logical starting point for understanding how the core functionality related to memory safety is handled. My hunch is the root cause lies somewhere in that code. I need to figure out where the `should_watch` function is used. That's where to find the related functionality. I'm going to search for the function name directly.
|
| 128/4 |
2026/07/04 00:33 |
tool |
read-file |
0m
Args: map[File:kernel/kcsan/core.c FirstLine:400 LineCount:100]
Results: map[Contents: 400: static __always_inline bool
401: find_reorder_access(struct kcsan_ctx *ctx, const volatile void *ptr, size_t size,
402: int type, unsigned long ip)
403: {
404: struct kcsan_scoped_access *reorder_access = get_reorder_access(ctx);
405:
406: if (!reorder_access)
407: return false;
408:
409: /*
410: * Note: If accesses are repeated while reorder_access is identical,
411: * never matches the new access, because !(type & KCSAN_ACCESS_SCOPED).
412: */
413: return reorder_access->ptr == ptr && reorder_access->size == size &&
414: reorder_access->type == type && reorder_access->ip == ip;
415: }
416:
417: static inline void
418: set_reorder_access(struct kcsan_ctx *ctx, const volatile void *ptr, size_t size,
419: int type, unsigned long ip)
420: {
421: struct kcsan_scoped_access *reorder_access = get_reorder_access(ctx);
422:
423: if (!reorder_access || !kcsan_weak_memory)
424: return;
425:
426: /*
427: * To avoid nested interrupts or scheduler (which share kcsan_ctx)
428: * reading an inconsistent reorder_access, ensure that the below has
429: * exclusive access to reorder_access by disallowing concurrent use.
430: */
431: ctx->disable_scoped++;
432: barrier();
433: reorder_access->ptr = ptr;
434: reorder_access->size = size;
435: reorder_access->type = type | KCSAN_ACCESS_SCOPED;
436: reorder_access->ip = ip;
437: reorder_access->stack_depth = get_kcsan_stack_depth();
438: barrier();
439: ctx->disable_scoped--;
440: }
441:
442: /*
443: * Pull everything together: check_access() below contains the performance
444: * critical operations; the fast-path (including check_access) functions should
445: * all be inlinable by the instrumentation functions.
446: *
447: * The slow-path (kcsan_found_watchpoint, kcsan_setup_watchpoint) are
448: * non-inlinable -- note that, we prefix these with "kcsan_" to ensure they can
449: * be filtered from the stacktrace, as well as give them unique names for the
450: * UACCESS whitelist of objtool. Each function uses user_access_save/restore(),
451: * since they do not access any user memory, but instrumentation is still
452: * emitted in UACCESS regions.
453: */
454:
455: static noinline void kcsan_found_watchpoint(const volatile void *ptr,
456: size_t size,
457: int type,
458: unsigned long ip,
459: atomic_long_t *watchpoint,
460: long encoded_watchpoint)
461: {
462: const bool is_assert = (type & KCSAN_ACCESS_ASSERT) != 0;
463: struct kcsan_ctx *ctx = get_ctx();
464: unsigned long flags;
465: bool consumed;
466:
467: /*
468: * We know a watchpoint exists. Let's try to keep the race-window
469: * between here and finally consuming the watchpoint below as small as
470: * possible -- avoid unneccessarily complex code until consumed.
471: */
472:
473: if (!kcsan_is_enabled(ctx))
474: return;
475:
476: /*
477: * The access_mask check relies on value-change comparison. To avoid
478: * reporting a race where e.g. the writer set up the watchpoint, but the
479: * reader has access_mask!=0, we have to ignore the found watchpoint.
480: *
481: * reorder_access is never created from an access with access_mask set.
482: */
483: if (ctx->access_mask && !find_reorder_access(ctx, ptr, size, type, ip))
484: return;
485:
486: /*
487: * If the other thread does not want to ignore the access, and there was
488: * a value change as a result of this thread's operation, we will still
489: * generate a report of unknown origin.
490: *
491: * Use CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN=n to filter.
492: */
493: if (!is_assert && kcsan_ignore_address(ptr))
494: return;
495:
496: /*
497: * Consuming the watchpoint must be guarded by kcsan_is_enabled() to
498: * avoid erroneously triggering reports if the context is disabled.
499: */
]
|
| 129/4 |
2026/07/04 00:33 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 47951
output: 32
thoughts: 15
Thoughts: Here's my thought process, as if I were analyzing that prompt:
**Analyzing the Search Query**
Okay, so the instruction is pretty straightforward. I need to "summarize the thought process," and it looks like I'm supposed to do it *from the perspective* of someone actually thinking those thoughts. The key is in the instruction: this isn't just a simple paraphrase, but an emulation of the thought process itself.
The first clue I'm given is a search term I'm told to look for: "`should_watch`". Since I'm presented with this piece of information, I know I'll be looking for `should_watch` in whatever file context follows. I assume the user will give me this file context and I should give a good summary of where the string `should_watch` appears in the provided file. My goal is to understand how the prompt wants me to interpret and respond to that search, by summarizing the results.
|
| 130/4 |
2026/07/04 00:33 |
tool |
grepper |
0m
Args: map[Expression:should_watch PathPrefix:kernel/kcsan/core.c]
Results: map[Output:kernel/kcsan/core.c=103=static atomic_long_t watchpoints[CONFIG_KCSAN_NUM_WATCHPOINTS + NUM_SLOTS-1];
--
kernel/kcsan/core.c-105-/*
kernel/kcsan/core.c:106: * Instructions to skip watching counter, used in should_watch(). We use a
kernel/kcsan/core.c-107- * per-CPU counter to avoid excessive contention.
--
kernel/kcsan/core.c=267=static __always_inline bool
kernel/kcsan/core.c:268:should_watch(struct kcsan_ctx *ctx, const volatile void *ptr, size_t size, int type)
kernel/kcsan/core.c-269-{
--
kernel/kcsan/core.c=527=kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type, unsigned long ip)
--
kernel/kcsan/core.c-542- * Always reset kcsan_skip counter in slow-path to avoid underflow; see
kernel/kcsan/core.c:543: * should_watch().
kernel/kcsan/core.c-544- */
--
kernel/kcsan/core.c-594- * Out of capacity: the size of 'watchpoints', and the frequency
kernel/kcsan/core.c:595: * with which should_watch() returns true should be tweaked so
kernel/kcsan/core.c-596- * that this case happens very rarely.
--
kernel/kcsan/core.c=719=check_access(const volatile void *ptr, size_t size, int type, unsigned long ip)
--
kernel/kcsan/core.c-751-
kernel/kcsan/core.c:752: if (unlikely(should_watch(ctx, ptr, size, type))) {
kernel/kcsan/core.c-753- kcsan_setup_watchpoint(ptr, size, type, ip);
]
|
| 131/4 |
2026/07/04 00:33 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 48736
output: 37
thoughts: 71
Thoughts: **Analyzing the `should_watch` Function**
Okay, let's dive into this `should_watch` function. My initial impression is that it's designed to determine if a memory region, defined by a pointer `ptr` and its `size`, should be monitored or "watched." The `type` argument probably indicates the nature of this memory access or the type of watch needed. The fact that it's `__always_inline` suggests it's performance-critical and the compiler should integrate the function's logic directly into any calling code for speed.
The presence of `kcsan_ctx` as a parameter tells me this likely integrates with KCSan, the Kernel Concurrency Sanitizer. So, this function is a key component in detecting data races or concurrency issues. The `volatile` qualifier on the pointer `ptr` is crucial; it explicitly indicates that the compiler should *not* optimize away accesses to this memory, because its value can change outside the normal program flow, like through an interrupt or another thread. The use of `const volatile void *` further underscores the intention is to inspect the address.
I need to understand what exactly the criteria are for a memory region to be "watched." What specific conditions within the `kcsan_ctx` determine whether a memory region is considered "interesting" enough for KCSan to monitor for potential issues? The core logic of `should_watch` is where the real work happens. I need to get into the heart of the function to see the decision-making process.
|
| 132/4 |
2026/07/04 00:33 |
tool |
read-file |
0m
Args: map[File:kernel/kcsan/core.c FirstLine:267 LineCount:40]
Results: map[Contents: 267: static __always_inline bool
268: should_watch(struct kcsan_ctx *ctx, const volatile void *ptr, size_t size, int type)
269: {
270: /*
271: * Never set up watchpoints when memory operations are atomic.
272: *
273: * Need to check this first, before kcsan_skip check below: (1) atomics
274: * should not count towards skipped instructions, and (2) to actually
275: * decrement kcsan_atomic_next for consecutive instruction stream.
276: */
277: if (is_atomic(ctx, ptr, size, type))
278: return false;
279:
280: if (this_cpu_dec_return(kcsan_skip) >= 0)
281: return false;
282:
283: /*
284: * NOTE: If we get here, kcsan_skip must always be reset in slow path
285: * via reset_kcsan_skip() to avoid underflow.
286: */
287:
288: /* this operation should be watched */
289: return true;
290: }
291:
292: /*
293: * Returns a pseudo-random number in interval [0, ep_ro). Simple linear
294: * congruential generator, using constants from "Numerical Recipes".
295: */
296: static u32 kcsan_prandom_u32_max(u32 ep_ro)
297: {
298: u32 state = this_cpu_read(kcsan_rand_state);
299:
300: state = 1664525 * state + 1013904223;
301: this_cpu_write(kcsan_rand_state, state);
302:
303: return state % ep_ro;
304: }
305:
306: static inline void reset_kcsan_skip(void)
]
|
| 133/4 |
2026/07/04 00:33 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 49782
output: 38
thoughts: 6
Thoughts: Okay, here's my attempt at summarizing the thought process, tailored for an expert audience, and written in the first person:
**Analyzing the `is_atomic` Flag**
Right, let me see. We're talking about an `is_atomic` flag, which suggests some kind of concurrency control or memory management aspect. My immediate thought is, "What kind of data structure or operation are we discussing where atomicity is a critical concern?" Without more context, I'm already mentally running through the usual suspects: shared counters, lock-free data structures, inter-thread communication primitives, maybe even transactional memory.
The presence of this flag strongly implies that whatever's being described can't be safely modified, or accessed concurrently, without some form of synchronization. It's not just about performance; it's about *correctness*. Incorrect handling of an atomic operation can lead to data corruption, race conditions, and all sorts of nasty, unpredictable behavior. I'm thinking about potential consistency models: sequential consistency, relaxed consistency, and how the `is_atomic` flag relates to these. Is this something that's platform-specific? Does this flag map to specific CPU instructions or memory barriers? What's the compiler's role in interpreting this flag?
Essentially, my brain is now asking *why* this flag exists. What are the specific guarantees this flag provides? What are the implications if it's set, and what are the implications if it's *not* set? Are there performance trade-offs? Are there other flags or attributes that this one interacts with? I'm already diving into the nuances of memory models and the possible optimizations and potential pitfalls that arise from the use of atomic operations. I'm practically itching to see the surrounding code!
|
| 134/4 |
2026/07/04 00:34 |
tool |
grepper |
0m
Error:
Args: map[Expression:static __always_inline bool is_atomic PathPrefix:kernel/kcsan/core.c]
Results:
|
| 135/4 |
2026/07/04 00:34 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 50208
output: 37
thoughts: 22
Thoughts: Okay, here's my summary, framed as my internal thought process:
**Diving into the KCSAN Core**
Right, so I'm thinking, where would the function `is_atomic` likely reside in the KCSAN codebase? Given what I know about the kernel sanitizers, and KCSAN's role specifically, it's pretty clear: the core functionality, the heart of the race detection, that's almost certainly going to be in `kernel/kcsan/core.c`. Makes perfect sense. Let's start there.
|
| 136/4 |
2026/07/04 00:34 |
tool |
read-file |
0m
Args: map[File:kernel/kcsan/core.c FirstLine:220 LineCount:50]
Results: map[Contents: 220:
221: ctx->disable_scoped++;
222: list_for_each_entry(scoped_access, &ctx->scoped_accesses, list) {
223: check_access(scoped_access->ptr, scoped_access->size,
224: scoped_access->type, scoped_access->ip);
225: }
226: ctx->disable_scoped--;
227: }
228:
229: /* Rules for generic atomic accesses. Called from fast-path. */
230: static __always_inline bool
231: is_atomic(struct kcsan_ctx *ctx, const volatile void *ptr, size_t size, int type)
232: {
233: if (type & KCSAN_ACCESS_ATOMIC)
234: return true;
235:
236: /*
237: * Unless explicitly declared atomic, never consider an assertion access
238: * as atomic. This allows using them also in atomic regions, such as
239: * seqlocks, without implicitly changing their semantics.
240: */
241: if (type & KCSAN_ACCESS_ASSERT)
242: return false;
243:
244: if (IS_ENABLED(CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC) &&
245: (type & KCSAN_ACCESS_WRITE) && size <= sizeof(long) &&
246: !(type & KCSAN_ACCESS_COMPOUND) && IS_ALIGNED((unsigned long)ptr, size))
247: return true; /* Assume aligned writes up to word size are atomic. */
248:
249: if (ctx->atomic_next > 0) {
250: /*
251: * Because we do not have separate contexts for nested
252: * interrupts, in case atomic_next is set, we simply assume that
253: * the outer interrupt set atomic_next. In the worst case, we
254: * will conservatively consider operations as atomic. This is a
255: * reasonable trade-off to make, since this case should be
256: * extremely rare; however, even if extremely rare, it could
257: * lead to false positives otherwise.
258: */
259: if ((hardirq_count() >> HARDIRQ_SHIFT) < 2)
260: --ctx->atomic_next; /* in task, or outer interrupt */
261: return true;
262: }
263:
264: return ctx->atomic_nest_count > 0 || ctx->in_flat_atomic;
265: }
266:
267: static __always_inline bool
268: should_watch(struct kcsan_ctx *ctx, const volatile void *ptr, size_t size, int type)
269: {
]
|
| 137/4 |
2026/07/04 00:34 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 51211
output: 37
thoughts: 198
Thoughts: **Analyzing Atomic Operations and Watchpoints**
Okay, let me break this down. It seems we're looking at how KCSAN interacts with atomic operations and memory accesses, particularly concerning watchpoints. The function `is_atomic` is the key here. It checks several things within the context `ctx`: `ctx->atomic_next`, `ctx->atomic_nest_count`, and `ctx->in_flat_atomic`. This function's outcome is crucial, as it determines if we consider a section of code to be within an atomic region.
Now, `write_seqcount_begin` is straightforward; it calls `kcsan_nestable_atomic_begin()`, which simply increments `ctx->atomic_nest_count`. `read_seqcount_begin` calls `kcsan_atomic_next(1000)`, setting `ctx->atomic_next`. These are likely related to how KCSAN tracks the start of write and read access.
Ah, here's where it gets interesting. `memcpy` triggers `check_access`. This is the core of how KCSAN monitors memory. `check_access` in turn calls `should_watch`. Crucially, `should_watch` then calls `is_atomic`.
If `is_atomic` returns `true`, then `should_watch` *returns `false`*. That's important. This means that if the code is deemed to be within an atomic block, `check_access` *will not* set up a watchpoint. This is probably an optimization, avoiding spurious alerts within atomic sections. However, I see `check_access` also *checks* for existing watchpoints. This implies the potential for some edge case, or that existing watchpoints might be preserved even within an atomic section.
Let's re-examine `check_access` to fully understand the logic around watchpoint handling. I need to be sure that the logic of *existing* watchpoints is properly handled within atomic contexts.
|
| 138/4 |
2026/07/04 00:34 |
tool |
read-file |
0m
Args: map[File:kernel/kcsan/core.c FirstLine:719 LineCount:50]
Results: map[Contents: 719: check_access(const volatile void *ptr, size_t size, int type, unsigned long ip)
720: {
721: atomic_long_t *watchpoint;
722: long encoded_watchpoint;
723:
724: /*
725: * Do nothing for 0 sized check; this comparison will be optimized out
726: * for constant sized instrumentation (__tsan_{read,write}N).
727: */
728: if (unlikely(size == 0))
729: return;
730:
731: again:
732: /*
733: * Avoid user_access_save in fast-path: find_watchpoint is safe without
734: * user_access_save, as the address that ptr points to is only used to
735: * check if a watchpoint exists; ptr is never dereferenced.
736: */
737: watchpoint = find_watchpoint((unsigned long)ptr, size,
738: !(type & KCSAN_ACCESS_WRITE),
739: &encoded_watchpoint);
740: /*
741: * It is safe to check kcsan_is_enabled() after find_watchpoint in the
742: * slow-path, as long as no state changes that cause a race to be
743: * detected and reported have occurred until kcsan_is_enabled() is
744: * checked.
745: */
746:
747: if (unlikely(watchpoint != NULL))
748: kcsan_found_watchpoint(ptr, size, type, ip, watchpoint, encoded_watchpoint);
749: else {
750: struct kcsan_ctx *ctx = get_ctx(); /* Call only once in fast-path. */
751:
752: if (unlikely(should_watch(ctx, ptr, size, type))) {
753: kcsan_setup_watchpoint(ptr, size, type, ip);
754: return;
755: }
756:
757: if (!(type & KCSAN_ACCESS_SCOPED)) {
758: struct kcsan_scoped_access *reorder_access = get_reorder_access(ctx);
759:
760: if (reorder_access) {
761: /*
762: * reorder_access check: simulates reordering of
763: * the access after subsequent operations.
764: */
765: ptr = reorder_access->ptr;
766: type = reorder_access->type;
767: ip = reorder_access->ip;
768: /*
]
|
| 139/4 |
2026/07/04 00:34 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 52654
output: 37
thoughts: 93
Thoughts: Okay, let me think this through.
**Analyzing the KCSAN Watchpoint Logic**
Hold on a second! I'm seeing a potential issue in this watchpoint implementation. The `check_access` function seems to call `find_watchpoint` *before* it gets to `should_watch`. If `find_watchpoint` actually *finds* a watchpoint, it then triggers `kcsan_found_watchpoint`. Now, that function is going to call `kcsan_is_enabled`, which makes sense. But... and this is the important part... I'm not seeing any call to `is_atomic` within `kcsan_found_watchpoint`! This is where the concurrency concern would come into play!
I need to circle back and confirm this. I need to make sure I haven't missed something. Let me go back and re-examine the code for `kcsan_found_watchpoint` to be absolutely certain. This could be a significant oversight if it means we're potentially missing some race conditions due to atomic operations.
|
| 140/4 |
2026/07/04 00:34 |
tool |
read-file |
0m
Args: map[File:kernel/kcsan/core.c FirstLine:455 LineCount:50]
Results: map[Contents: 455: static noinline void kcsan_found_watchpoint(const volatile void *ptr,
456: size_t size,
457: int type,
458: unsigned long ip,
459: atomic_long_t *watchpoint,
460: long encoded_watchpoint)
461: {
462: const bool is_assert = (type & KCSAN_ACCESS_ASSERT) != 0;
463: struct kcsan_ctx *ctx = get_ctx();
464: unsigned long flags;
465: bool consumed;
466:
467: /*
468: * We know a watchpoint exists. Let's try to keep the race-window
469: * between here and finally consuming the watchpoint below as small as
470: * possible -- avoid unneccessarily complex code until consumed.
471: */
472:
473: if (!kcsan_is_enabled(ctx))
474: return;
475:
476: /*
477: * The access_mask check relies on value-change comparison. To avoid
478: * reporting a race where e.g. the writer set up the watchpoint, but the
479: * reader has access_mask!=0, we have to ignore the found watchpoint.
480: *
481: * reorder_access is never created from an access with access_mask set.
482: */
483: if (ctx->access_mask && !find_reorder_access(ctx, ptr, size, type, ip))
484: return;
485:
486: /*
487: * If the other thread does not want to ignore the access, and there was
488: * a value change as a result of this thread's operation, we will still
489: * generate a report of unknown origin.
490: *
491: * Use CONFIG_KCSAN_REPORT_RACE_UNKNOWN_ORIGIN=n to filter.
492: */
493: if (!is_assert && kcsan_ignore_address(ptr))
494: return;
495:
496: /*
497: * Consuming the watchpoint must be guarded by kcsan_is_enabled() to
498: * avoid erroneously triggering reports if the context is disabled.
499: */
500: consumed = try_consume_watchpoint(watchpoint, encoded_watchpoint);
501:
502: /* keep this after try_consume_watchpoint */
503: flags = user_access_save();
504:
]
|
| 141/4 |
2026/07/04 00:34 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 53783
output: 21
thoughts: 1961
Thoughts: **KCSAN and Seqlocks: Unraveling a Data Race Report**
Okay, so I'm digging into a KCSAN data race report related to seqlocks, and it's got me scratching my head. The core of the issue seems to be how KCSAN interacts with atomic operations within a seqlock context. Let's break it down as I understand it.
First, KCSAN's logic: if an access is deemed atomic, it *shouldn't* set up a watchpoint, but it *should* check for existing watchpoints set by a non-atomic access. A race between atomic and non-atomic is a data race (report it!), while a race between two atomic operations *isn't* (don't report).
With seqlocks, we have a reader (atomic) and a writer (atomic). Theoretically, neither should set a watchpoint, so no race should be detected. But something's clearly not right. The `write_seqcount_begin` and `read_seqcount_begin` functions mark the operations as atomic via calls to `kcsan_atomic_next()`.
My first thought was that one of the operations was being missed and not marked as atomic. Then I revisited the code, specifically `ktime_get_coarse_ts64`. I saw the reader calls `read_seqcount_begin` and the code is making an atomic call.
The `raw_read_seqcount` function is crucial. It contains a call to `kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX)`. Then, reading the sequence value inside of `raw_read_seqcount`, using `smp_load_acquire(&s->seqcount.sequence)`, is an atomic access.
I then started wondering if `tk_core.seq` wasn't actually a `seqcount_t` but something else, but it's a `seqcount_raw_spinlock_t`. And I know the macro for those uses a `__seqprop_raw_spinlock_sequence`.
Looking at the writer side in `timekeeping_update_from_shadow`, `write_seqcount_begin` calls `raw_write_seqcount_begin`, which calls `do_raw_write_seqcount_begin`. The `s->sequence++` inside this function is a plain access, but it's wrapped in `kcsan_nestable_atomic_begin()`, which should make it all fine. I also looked at the `memcpy` of the `timekeeper` struct in the same function, and that's also wrapped.
Then it clicked that `kcsan_nestable_atomic_begin()` is *thread-local*. So, if the *reader* thread is not marked as atomic, it *will* set up a watchpoint, and when the *writer* thread (which is marked as atomic) accesses the memory, KCSAN's `kcsan_found_watchpoint` will be called. Crucially, that function does *not* check if the *current* access is atomic. It only cares if the other, watchpoint-setting access, was not atomic (which, by definition, it wasn't).
However, the reader *is* marked as atomic through `kcsan_atomic_next(1000)` during `read_seqcount_begin`. But this only covers the next 1000 accesses, so the loop in `ktime_get_coarse_ts64` becomes important. It calls `read_seqcount_begin` repeatedly. Each time, `kcsan_atomic_next(1000)` *is* called again.
The aha moment came when I looked at the accesses *inside* the seqlock-protected region. In `ktime_get_coarse_ts64`, the reader accesses `tk->xtime_sec`, `tk->coarse_nsec`, and `tk->wall_to_monotonic`, which are *outside* the `raw_read_seqcount` instrumentation. That's about 4 memory accesses.
I'd seen this recently with the commit `fde86d90cb8b` in my head and was now searching for "kcsan_atomic_next" in the kernel. The critical point is that the `kcsan_atomic_next(1000)` counter is exhausted before *all* of the accesses are covered. KCSAN sees the later accesses as non-atomic and a race is incorrectly reported. The seqlock interface has read-retry loops and non-standard usages that are not covered by the standard instrumentation. The race is a false positive.
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tool |
grepper |
0m
Args: map[Expression:kcsan_atomic_next]
Results: map[Output:include/linux/kcsan-checks.h=108=void kcsan_flat_atomic_end(void);
--
include/linux/kcsan-checks.h-110-/**
include/linux/kcsan-checks.h:111: * kcsan_atomic_next - consider following accesses as atomic
include/linux/kcsan-checks.h-112- *
--
include/linux/kcsan-checks.h-117- */
include/linux/kcsan-checks.h:118:void kcsan_atomic_next(int n);
include/linux/kcsan-checks.h-119-
--
include/linux/kcsan-checks.h=202=static inline void kcsan_flat_atomic_end(void) { }
include/linux/kcsan-checks.h:203:static inline void kcsan_atomic_next(int n) { }
include/linux/kcsan-checks.h-204-static inline void kcsan_set_access_mask(unsigned long mask) { }
--
include/linux/kcsan-checks.h=232=static inline void __kcsan_disable_current(void) { }
--
include/linux/kcsan-checks.h-529- kcsan_set_access_mask(0); \
include/linux/kcsan-checks.h:530: kcsan_atomic_next(1); \
include/linux/kcsan-checks.h-531- } while (0)
--
include/linux/seqlock.h=230=SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex)
--
include/linux/seqlock.h-278- \
include/linux/seqlock.h:279: kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
include/linux/seqlock.h-280- __seq; \
--
include/linux/seqlock.h-317- \
include/linux/seqlock.h:318: kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \
include/linux/seqlock.h-319- __seq; \
--
include/linux/seqlock.h=389=static inline int do___read_seqcount_retry(const seqcount_t *s, unsigned start)
include/linux/seqlock.h-390-{
include/linux/seqlock.h:391: kcsan_atomic_next(0);
include/linux/seqlock.h-392- return unlikely(READ_ONCE(s->sequence) != start);
--
include/linux/seqlock.h=659=static __always_inline unsigned read_seqcount_latch(const seqcount_latch_t *s)
include/linux/seqlock.h-660-{
include/linux/seqlock.h:661: kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);
include/linux/seqlock.h-662- return raw_read_seqcount_latch(s);
--
include/linux/seqlock.h=687=read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start)
include/linux/seqlock.h-688-{
include/linux/seqlock.h:689: kcsan_atomic_next(0);
include/linux/seqlock.h-690- return raw_read_seqcount_latch_retry(s, start);
--
kernel/kcsan/core.c=268=should_watch(struct kcsan_ctx *ctx, const volatile void *ptr, size_t size, int type)
--
kernel/kcsan/core.c-274- * should not count towards skipped instructions, and (2) to actually
kernel/kcsan/core.c:275: * decrement kcsan_atomic_next for consecutive instruction stream.
kernel/kcsan/core.c-276- */
--
kernel/kcsan/core.c=892=EXPORT_SYMBOL(kcsan_flat_atomic_end);
kernel/kcsan/core.c-893-
kernel/kcsan/core.c:894:void kcsan_atomic_next(int n)
kernel/kcsan/core.c-895-{
--
kernel/kcsan/core.c-897-}
kernel/kcsan/core.c:898:EXPORT_SYMBOL(kcsan_atomic_next);
kernel/kcsan/core.c-899-
]
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2026/07/04 00:34 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 58004
output: 30
thoughts: 853
Thoughts: **KCSAN Race Detection Analysis**
Okay, let's break this down. First, I noticed `kcsan_atomic_next(0)` within `do___read_seqcount_retry`. That makes sense; it's the expected way to signal the end of an atomic region. Good. I thought I had a handle on it.
Then, wait a minute, I think I've found it. In `ktime_get_coarse_ts64`, the potential issue seemed to be if `tk_xtime_coarse` wasn't inlined, the accesses within it might slip *outside* the coverage of `kcsan_atomic_next`. But wait, that's wrong. `kcsan_atomic_next` is per-context, so it should extend to called functions. Scratch that.
Aha! Now I see a *real* problem. The `memcpy` in `timekeeping_update_from_shadow`! That's not just a simple memory access; it's a bulk operation. KCSAN instruments `memcpy` through `__tsan_memcpy`, and that's the key. Looking at `kernel/kcsan/core.c`, the `check_access` within `__tsan_memcpy` is limited. Specifically, it only checks the first `MAX_ENCODABLE_SIZE` bytes, not the whole `memcpy`. So, if the total `len` of bytes to copy is large, KCSAN is only checking a small slice!
However, if KCSAN only checks a small slice, then it is NOT a race, if it only checks the first few bytes, it might MISS a race, but it shouldn't report a FALSE race.
Then, there is the user's question, "Is this race benign? Does seqlock correctly protect against it such that no harmful behavior (like a crash or incorrect result after the loop) can occur?" Well, yes, it *is* benign. The reader retries, the data doesn't contain pointers, and the result is only used after a successful read.
But then, "Why is KCSAN reporting it if it's protected by a seqlock?" It's because KCSAN, by default, reports *all* data races on "plain" accesses unless explicitly suppressed. Seqlocks are designed to handle this, but KCSAN still flags it. The seqlock annotations should prevent the report, but something's going wrong. The reasons for failure could be: the access count exceeds `KCSAN_SEQLOCK_REGION_MAX` (unlikely), incorrect writer-side markings (seems okay), or a KCSAN bug.
The actual reason, I think I have it now. In `timekeeping_update_from_shadow`, the `memcpy` updates the *entire* `struct timekeeper`. The reader accesses *multiple* fields. KCSAN sees multiple races across multiple fields.
Wait, let's see if a commit solves it. I need to check the commit `95f8a14fa38a` "timekeeping: Annotate data races in timekeeping_update_from_shadow()".
|
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2026/07/04 00:34 |
tool |
git-log |
0m
Args: map[MessageRegexps:[timekeeping_update_from_shadow data race]]
Results:
|
| 145/4 |
2026/07/04 00:34 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 59582
output: 31
thoughts: 28
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Analyzing Potential Data Races in Kernel Timekeeping**
Okay, let's circle back to that `data_race` issue. My instinct is to revisit `kernel/time/timekeeping.c`, focusing on the original keyword search. I want to see the surrounding code, the execution flow, and how the potentially problematic shared variables are accessed and modified. It's about putting the pieces together to find the actual condition. This likely requires analyzing any locks, atomic operations, or memory barriers that might be in place (or absent!). The goal is to identify if the current implementation adequately protects critical sections from concurrent access. We need to assess the specific race, and its potential impact on the system, considering the architecture.
|
| 146/4 |
2026/07/04 00:34 |
tool |
grepper |
0m
Args: map[Expression:data_race PathPrefix:kernel/time/timekeeping.c]
Results: map[Output:kernel/time/timekeeping.c=583=u64 notrace ktime_get_boot_fast_ns(void)
--
kernel/time/timekeeping.c-586-
kernel/time/timekeeping.c:587: return (ktime_get_mono_fast_ns() + ktime_to_ns(data_race(tk->offs_boot)));
kernel/time/timekeeping.c-588-}
--
kernel/time/timekeeping.c=600=u64 notrace ktime_get_tai_fast_ns(void)
--
kernel/time/timekeeping.c-603-
kernel/time/timekeeping.c:604: return (ktime_get_mono_fast_ns() + ktime_to_ns(data_race(tk->offs_tai)));
kernel/time/timekeeping.c-605-}
--
kernel/time/timekeeping.c=890=bool ktime_expiry_to_cycles(enum clocksource_ids id, ktime_t expires_ns, u64 *cycles)
--
kernel/time/timekeeping.c-905- */
kernel/time/timekeeping.c:906: if (data_race(tk->cs_id) != id)
kernel/time/timekeeping.c-907- return false;
]
|
| 147/4 |
2026/07/04 00:34 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 60149
output: 35
thoughts: 486
Thoughts: **Analyzing KCSAN Reports and Timekeeping in the Kernel**
Okay, so I'm looking at these `data_race()` calls and they're showing up in these "fast" accessors, specifically ones that don't seem to be using seqlocks in a way KCSAN approves of, or at least they are used in a way that KCSAN doesn't like. I was just wondering, is `ktime_get_mono_fast_ns` playing tricks with timekeepers? And then there's `ktime_get_coarse_ts64`, which does use the standard seqlock.
Wait, I think I've seen something like this before. Right, there was a similar KCSAN report for `timekeeping.c`. The root cause? `memcpy`! KCSAN doesn't treat `memcpy` as "atomic," even if it's within a `kcsan_nestable_atomic_begin()` region. It seems `memcpy` is a "complex" access, and KCSAN handles those differently.
Hang on, there might be something even more subtle. Looks like `kcsan_atomic_next(1000)` only covers the *next* 1000 accesses. If the reader gets preempted or interrupted, and the interrupt handler makes a bunch of accesses, that 1000 can get eaten up really quickly. Although, `ktime_get_coarse_ts64` is pretty short.
Hold on, this is key. `ktime_get_coarse_ts64` is often called from contexts where KCSAN is actually enabled.
Let's revisit that `memcpy` again:
```c
835: memcpy(&tkd->timekeeper, tk, sizeof(*tk));
```
I wonder... if I switched this to a loop of `WRITE_ONCE`, would KCSAN be happy? Probably, but `memcpy` is so much faster.
Actually, according to the kernel developers, this data race is actually benign. They are using seqlocks correctly for this and this KCSAN report is actually a "false positive" in the sense that the code is technically correct, but KCSAN is correct in identifying the race.
Right! I remember, there's a commit, `306067756f6b`. I've tried this before, but let me search for the title "timekeeping: Use READ_ONCE() for coarse time accessors". Let me search for "coarse" and "READ_ONCE" in `kernel/time/timekeeping.c`.
|
| 148/4 |
2026/07/04 00:35 |
tool |
grepper |
0m
Error:
Args: map[Expression:READ_ONCE.*coarse PathPrefix:kernel/time/timekeeping.c]
Results:
|
| 149/4 |
2026/07/04 00:35 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 61252
output: 45
thoughts: 22
Thoughts: Okay, here's the summary, assuming I'm a seasoned kernel developer:
**Delving into `ktime_get_coarse_real_ts64`**
Alright, let's dissect this function, `ktime_get_coarse_real_ts64`. My initial thought is to immediately consider its context within the broader kernel timekeeping architecture. We're dealing with "coarse" time, which implies an emphasis on speed and potential trade-offs in accuracy. I need to keep that in mind. I bet this is used for time measurements where microsecond-level precision isn't paramount, and where overhead must be minimized, such as performance profiling, debugging, or less critical system monitoring.
The name suggests we're dealing with a 64-bit timestamp, which is standard these days, giving us plenty of headroom for expressing time. The "real" part is interesting; that means this function retrieves the actual wall-clock time, not a monotonically increasing time based on some internal counter. That indicates it's likely sourced from the RTC or a similar clock source that is subject to external adjustments like NTP. That gives us a point to consider drift and how the system might handle it.
Now, I'm already mentally walking through the possible implementations. I need to look up the underlying clock source being used by `ktime_get_coarse_real_ts64`. Is it the TSC, a high-resolution timer, or something else? Understanding the hardware dependency is key. I'll need to check the relevant source code. I need to mentally model the effects of things like frequency scaling and interrupts. Also, I'm thinking about potential issues that might arise with multiple CPUs needing to access the same underlying time source: are we looking at spinlocks or atomic operations?
I should also make sure to check its documentation - is it safe in interrupt context? Does it rely on any locks? Are there any guarantees about its monotonicity, even though it's coarse? What's the impact of this function on things like power management?
It's probably best to verify how this function relates to other, more precise time retrieval methods within the kernel, such as `ktime_get_real_ts64` or `ktime_get_ns`. This way I can determine its role in the overall timekeeping hierarchy and the implicit trade-offs made in its design. Ultimately, my analysis will come down to a solid understanding of how it fits into the timing ecosystem and how it manages the hardware involved in making these time calls.
|
| 150/4 |
2026/07/04 00:35 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/time/timekeeping.c Name:ktime_get_coarse_real_ts64]
Results: map[SourceCode:2657: void ktime_get_coarse_real_ts64(struct timespec64 *ts)
2658: {
2659: struct timekeeper *tk = &tk_core.timekeeper;
2660: unsigned int seq;
2661:
2662: do {
2663: seq = read_seqcount_begin(&tk_core.seq);
2664:
2665: *ts = tk_xtime_coarse(tk);
2666: } while (read_seqcount_retry(&tk_core.seq, seq));
2667: }
SourceFile:kernel/time/timekeeping.c]
|
| 151/4 |
2026/07/04 00:35 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 62053
output: 27
thoughts: 755
Thoughts: **Analyzing a Data Race Report in the Kernel**
Okay, so I'm looking at this KCSAN report, and it flags a potential data race. My initial thought is, "Wait, why isn't this using `READ_ONCE`?" I vaguely recall a discussion on the LKML about this specific scenario, and I think Thomas Gleixner argued that seqlocks are sufficient and that adding `READ_ONCE` everywhere isn't desirable.
Let's break down the implications for our user's question:
1. **Benign Race?** Yes, I believe this is benign. The seqlock retry mechanism acts as our safeguard. If a reader gets caught in the middle of a `memcpy` by the writer, the reader detects this and retries. The data being accessed (`xtime_sec`, `coarse_nsec`, `wall_to_monotonic`) are all fundamental numeric types. There aren't any pointers involved, so a "torn read" won't cause a crash.
2. **Seqlock Protection?** Absolutely, the seqlock is doing its job here. Seqlocks are designed to allow high-performance concurrent reads by permitting races against writers, but they provide a mechanism to detect and retry in the event of a race.
3. **KCSAN Report - Why?** KCSAN flags this because it sees "plain" memory accesses β non-atomic loads and stores, even if they're happening within the context of a higher-level synchronization primitive like a seqlock. The core memory operations are still plain loads and stores (or in this case, a `memcpy`). KCSAN is designed to detect all such races because they can, in principle, lead to compiler optimizations that break the code, though the `smp_rmb()` in `read_seqcount_retry` and the loop structure generally prevent such optimizations from being harmful here. KCSAN has annotations to suppress seqlock reports, but they might not cover all cases, especially when `memcpy` is involved.
Now, I'm curious about the use of `memcpy`. Let me check, the comment specifies that `memcpy` is used for performance reasons, as it states "We could avoid this memcpy() by switching pointers, but that has the downside that the reader side does not longer benefit from the cacheline optimized data layout of the timekeeper and requires another indirection."
Aha! One more thought. The reason KCSAN is reporting this might be because `memcpy` isn't explicitly "marked" as a data race. If they used `data_race(memcpy(...))`, KCSAN would likely be silent. However, `data_race()` is usually reserved for cases where we truly *don't* care about the race. We *do* care here, but we're handling it using the seqlock.
The "ideal" solution to appease KCSAN within a seqlock critical section would be to use `READ_ONCE()` and `WRITE_ONCE()`. However, kernel maintainers often avoid this approach for large structures or to avoid code clutter, preferring to rely on the seqlock's retry logic and potentially KCSAN's seqlock-specific annotations.
So, if KCSAN is *still* reporting this, it could indicate a bug in KCSAN itself or insufficient annotations within `seqlock.h` for handling `memcpy`.
Wait, I need to check for commit `70164962667b` "kcsan: Add data\_race() to timekeeping\_update\_from\_shadow()". I should see if that commit is there.
|
| 152/4 |
2026/07/04 00:35 |
tool |
git-show |
0m
Error:
Args:
Results:
|
| 153/4 |
2026/07/04 00:35 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 63655
output: 19
thoughts: 18
Thoughts: Okay, here's my thought process as if I were considering this search, rendered as a first-person summary, keeping in mind I'm an expert:
**Investigating a Potential Data Race**
Alright, let's get down to brass tacks. I need to pinpoint the source of this suspected data race, and the specific function call that's flaggedβ`memcpy`βis a good starting point. My initial strategy is clear: I'm going to launch a comprehensive search across the entire kernel source code. Specifically, I will search for the string: `data_race(memcpy`.
The rationale is straightforward. This type of search will hopefully lead me directly to any existing instances where the data race detector is explicitly flagging a potential issue related to `memcpy` within the kernel. I can then analyse any of the findings with a keen eye for the surrounding code context. I'll be looking for things like mutexes, spinlocks, or other synchronization primitives used (or *not* used) around the `memcpy` call. Hopefully, I'll find an existing explanation of the race if one exists.
Essentially, I'm trying to shortcut the process and zero in on known points of contention involving `memcpy`. It's a targeted and efficient way to begin my investigation, rather than a broad sweep of the entire kernel looking for `memcpy` calls in general. This should provide me with an immediate set of locations to evaluate and determine the root cause, and if the data race is real.
|
| 154/4 |
2026/07/04 00:35 |
tool |
grepper |
0m
Args: map[Expression:data_race\(memcpy]
Results: map[Output:mm/vma_init.c=41=static void vm_area_init_from(const struct vm_area_struct *src,
--
mm/vma_init.c-58- */
mm/vma_init.c:59: data_race(memcpy(&dest->shared, &src->shared, sizeof(dest->shared)));
mm/vma_init.c-60- memcpy(&dest->vm_userfaultfd_ctx, &src->vm_userfaultfd_ctx,
--
net/ipv4/tcp_ipv4.c=1341=static int __tcp_md5_do_add(struct sock *sk, const union tcp_md5_addr *addr,
--
net/ipv4/tcp_ipv4.c-1357- */
net/ipv4/tcp_ipv4.c:1358: data_race(memcpy(key->key, newkey, newkeylen));
net/ipv4/tcp_ipv4.c-1359-
]
|
| 155/4 |
2026/07/04 00:35 |
llm |
codeexpert |
0m
Model:
Tokens:
input: 64262
output: 794
thoughts: 2470
Thoughts: **Analysis of the Reported KCSAN Data Race**
Okay, let's break this down. KCSAN is flagging a data race, and my initial thought is, "Where exactly is this happening?" I know it's not in `timekeeping.c`, which is the first place I checked. The clue came in `include/linux/seqlock.h`. The `read_seqcount_begin` macro uses `kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX)`. The key observation here is that this only works correctly if the reader is *actually* calling `read_seqcount_begin`. Some timekeeping functions use variants like `raw_read_seqcount_begin`, so I have to be careful. Ah, `ktime_get_coarse_ts64` *does* use `read_seqcount_begin`, that's promising.
Now, the crucial detail: The definition of `raw_read_seqcount` gives us the mechanism. `smp_load_acquire` is treated as an atomic access by KCSAN. If the sequence number is odd, a writer is active, which means it will be a race.
The commit `de66768832a8` is a significant hint. It's about annotating seqlocks for KCSAN. I think I have seen the contents of `seqlock.h` before.
So here's what I'm concluding. This is likely a benign race. The seqlock retry mechanism protects us, forcing a retry if a writer was active during the read. KCSAN is flagging it because, technically, it's a data race on plain accesses. A `memcpy` is particularly prone to being reported because it's a large, multi-access operation. KCSAN may not treat `memcpy`'s operation as fully covered by the `atomic_next` annotation (it's often implemented with multi-access, or the reader-side annotation could be exhausted.) KCSAN's philosophy is that they report all data races, even benign ones, to catch potential compiler-induced issues.
Wait, should I rule out any harmful behavior? If `tv_sec` is 64-bit on a 32-bit system, a torn read *could* lead to a bad time value (like 136 years in the future). However, the `read_seqcount_retry` will prevent that garbage data from ever being used.
How could a bad value cause a crash *before* the retry? The `ktime_get_coarse_ts64` code assigns values. No division by zero, no dereference so no crash.
What about `tk->xtime_sec`? It's updated by `memcpy`, and the reader might get garbage. But that value is just an integer, so at worst, it's a bad time value that's fixed by the retry.
So the race seems indeed benign.
The KCSAN flag likely comes from its overzealous data race detection. It sees two threads accessing the same memory concurrently, one of them writing.
Although, KCSAN *does* try to ignore races in seqlock regions, but it's not perfect. `kcsan_atomic_next(1000)` is heuristic. `memcpy` could be handled differently.
Let's check if the reader uses `seqcount_t` or `seqlock_t` for the critical field. It is a `seqcount_raw_spinlock_t`, which contains a `seqcount_t`. `read_seqcount_begin` is used.
I must distinguish between `read_seqbegin` and `read_seqcount_begin`. `read_seqbegin` is for `seqlock_t`, and `read_seqcount_begin` is for `seqcount_t`. The right data, `tk_core.seq`, is a `seqcount_t`.
`read_seqcount_begin` is defined as using `raw_read_seqcount_begin`, which calls `raw_read_seqcount`, which contains the KCSAN annotation. So the reader *is* annotated. The writer is also annotated via `write_seqcount_begin`.
If KCSAN still reports this, it's probably because `memcpy` is involved. `memcpy` might be handled as a built-in or library function. KCSAN's instrumentation could have some imperfections with the `atomic_next` or `atomic_nest_count` mechanism in some compiler versions. Specifically, `__tsan_memcpy` calls `check_access` for the source and destination. If `check_access` is called while `atomic_nest_count
|
| 156/2 |
2026/07/04 00:35 |
llm |
expert |
0m
Model:
Tokens:
input: 38667
output: 15
thoughts: 2089
Thoughts: **My Analysis of the Timekeeping Race**
Okay, the `codeexpert` initially flagged this as **BENIGN**, and I generally agree, but let's be thorough, especially since it concerns the kernel's timekeeping. The core of the issue is a data race between `timekeeping_update_from_shadow` (the writer) and `ktime_get_coarse_ts64` (the reader) on `tk_core.timekeeper`. The `codeexpert` is right, in principle.
The first thing I needed to confirm was the basic safety net. The `codeexpert` pointed out it's simple numeric types. I saw that `xtime_sec` is `time64_t`, `coarse_nsec` is `u32`, and `wall_to_monotonic` is `struct timespec64`. Even on 32-bit, those reads/writes aren't atomic, but, as the report states, `seqlock` handles torn reads.
My immediate thought was to verify that there are no "Marking Required for Correctness" issues. I needed to ensure the compiler isn't going to get clever and break the `seqlock` logic with optimizations. The `codeexpert` assures me the `read_seqcount_begin` and `read_seqcount_retry` macros have the necessary barriers, which is the key.
However, I had to ensure that `memcpy` wasn't subject to the same concerns. Can the compiler do something nasty with it, such as "fuse" or "hoist" it out of the critical section? But, `memcpy` is just a function call (or a builtin behaving like one). The `write_seqcount_begin/end` have barriers, so hoisting across those is not possible.
Alright, I can see how it's safe now. The report shows the value of `coarse_nsec` changed during the race, but this is handled by the retry mechanism. **BENIGN** seems correct, so far.
But, the recommended annotations are `data_race()` or `kcsan_nestable_atomic_begin/end`. The kernel already uses `data_race()` in some parts of `timekeeping.c`. This is reasonable; KCSAN wants explicit marking.
The real test is if the compiler can do something that violates the memory ordering guarantees the `seqlock` provides. It would be bad if the compiler hoisted a load above a barrier. But, if the compiler hoists `tk->xtime_sec` out of the loop in `ktime_get_coarse_ts64`, the `seqlock` will break. Does the presence of `read_seqcount_begin` stop it? Yes, it has `smp_rmb()`. Likewise, `read_seqcount_retry` has it too. So, a load cannot be hoisted above or below. So the compiler has to obey the order of the reads and sequence checks. Each iteration has the necessary barriers. Therefore, it is safe.
So, I will conclude, again, it is **BENIGN**.
Alright, before I declare victory, let me check the other reads to ensure I'm not missing anything. I need to make sure `tk->wall_to_monotonic`, `tk->xtime_sec`, and `tk->coarse_nsec` read correctly.
The stack trace and code snippets are essential. I see `ktime_get_coarse_ts64` calls `tk_xtime_coarse`, and it's reading `tk->coarse_nsec`. The `timekeeping_update_from_shadow` write updates `tk` via `memcpy`. The writer holds `tk_core.lock` and uses `write_seqcount_begin/end`, while the reader is lockless, using `read_seqcount_begin/retry`. Classic `seqlock`.
Now, the final, crucial question: could a "torn" value be used in a way that causes a crash *before* the retry? The `tk_xtime_coarse` only copies to a local variable. The crucial call `set_normalized_timespec64` is called *after* the loop. So, the values used in the function call `now` and `mono` will be consistent. The loop only exits when `read_seqcount_retry` returns `false`.
So, it *is* perfectly safe. So the conclusion: it is **BENIGN**.
Why did KCSAN flag this? As I suspected, KCSAN is being overly cautious because it's instrumenting the accesses. KCSAN encourages marking, and it flags things within seqlocks, even if they're handled correctly by the synchronization primitives, unless they're explicitly marked. That's fine
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Args:
Results:
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Model:
Tokens:
input: 41809
output: 717
thoughts: 30
Thoughts: Okay, here's my take on summarizing this analysis, keeping in mind my expertise:
**My Summary of the Analysis (It's Benign!)**
Alright, so the user wants a summary of the analysis, and thankfully, I've got a definitive answer: it's *benign*. My primary focus, as always, is clarity and efficiency. So, I'm going to jump right into the reasoning behind this, including any recommended annotations that will help the user fully understand the context. Essentially, I'm aiming to provide a clear path to understanding the data.
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Results: map[Explanation:The data race occurs between `timekeeping_update_from_shadow()` and
`ktime_get_coarse_ts64()` on the fields of `tk_core.timekeeper` (specifically
`coarse_nsec`, `xtime_sec`, and `wall_to_monotonic`).
### Synchronization Context
* **Writer (`timekeeping_update_from_shadow`):** Holds the `tk_core.lock` (raw
spinlock) and uses `write_seqcount_begin(&tkd->seq)` and
`write_seqcount_end(&tkd->seq)` to protect the update. The update is performed
using a `memcpy()` of the entire `struct timekeeper`.
* **Reader (`ktime_get_coarse_ts64`):** A lockless reader that uses the
`read_seqcount_begin(&tk_core.seq)` and `read_seqcount_retry(&tk_core.seq, seq)`
loop to ensure data consistency.
### Final Classification: **BENIGN**
### Reasoning
The race is a classic example of a seqlock-protected data access. While KCSAN
correctly identifies that a concurrent read and write are occurring on plain
memory locations (which is technically a data race in the C memory model), the
logic is structurally tolerant to this:
1. **Retry Logic:** The `seqlock` mechanism ensures that if the reader accesses
the data while the writer is modifying it, the sequence counter check
(`read_seqcount_retry`) will fail, and the reader will discard the potentially
torn/inconsistent data and retry the operation.
2. **Safety of Torn Reads:** The fields being read are simple numeric types
(integers representing seconds and nanoseconds). Even if a read is "torn" (e.g.,
reading half of a 64-bit value from the old state and half from the new state),
it results in a garbage numeric value that is never used for anything other than
local calculations before being discarded by the retry logic. There are no
pointers involved that could lead to a Use-After-Free or NULL pointer
dereference.
3. **Barriers:** The `seqcount` macros include the necessary memory barriers
(`smp_rmb()` for readers, `smp_wmb()` for writers) to prevent the compiler from
hoisting loads out of the retry loop or reordering the sequence counter updates
relative to the data update.
KCSAN reports this because `memcpy()` and the plain loads in `tk_xtime_coarse()`
are not explicitly marked with `data_race()` or `READ_ONCE()`/`WRITE_ONCE()`. In
high-performance paths like timekeeping, the kernel often avoids these
annotations to prevent any potential overhead or to keep the code clean, relying
on the `seqlock` to handle the concurrency.
### Recommended Annotations
To silence the KCSAN report, the `memcpy` in `timekeeping_update_from_shadow()`
could be wrapped in `data_race()`, and the reads in `ktime_get_coarse_ts64()`
(and the inlined `tk_xtime_coarse()`) could also be wrapped in `data_race()`.
```c
/* kernel/time/timekeeping.c */
data_race(memcpy(&tkd->timekeeper, tk, sizeof(*tk)));
```]
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