| 0/0 |
2026/10/13 07:54 |
flow |
assessment-security |
41m
Results: map[DenialOfService:true Explanation:Based on the analysis of the bug report and the kernel source code, here is the
assessment of the vulnerability:
**Root Cause Analysis:**
The bug is a NULL pointer dereference in the core scheduling code
(`kernel/sched/core.c`). It occurs due to a race condition between the
scheduler's `pick_next_task()` function and CPU hotplug operations.
When `CONFIG_SCHED_CORE` is enabled, `pick_next_task()` performs a core-wide
task selection. It iterates over the SMT siblings of the current CPU using
`smt_mask = cpu_smt_mask(cpu)` (which points to the CPU's
`topology_sibling_cpumask`). It does this in two passes:
1. **First loop** (`for_each_cpu_wrap`): It calls `pick_task()` for each sibling
and stores the selected task in `rq_i->core_pick`.
2. **Second loop** (`for_each_cpu`): It reads `rq_i->core_pick` for each sibling
and checks if it matches the core cookie using `cookie_equals(p, cookie)`.
During CPU hotplug (specifically when a CPU is coming online),
`set_cpu_sibling_map()` modifies the `topology_sibling_cpumask` to add the new
CPU. Crucially, this modification happens *without* holding the runqueue locks.
If a CPU is running `pick_next_task()` concurrently with another CPU coming
online, it can read the `smt_mask` in the first loop and see only itself. Then,
the booting CPU modifies the mask. In the second loop, the scheduling CPU reads
the mask again and sees the newly added CPU. It then accesses `rq_i->core_pick`
for the new CPU. Since the new CPU was not visited in the first loop, its
`core_pick` is uninitialized (i.e., `NULL`). This `NULL` pointer is passed to
`cookie_equals()`, which dereferences it when calling `task_cpu(p)`, resulting
in a kernel Oops.
**Security Impact:**
* **Exploitable:** No. The faulting address is `0xdffffc0000000002` (KASAN
shadow for `0x10-0x17`), which corresponds to reading `p->cpu` at offset `0x14`
from a `NULL` pointer. Because `mmap_min_addr` prevents mapping page 0 in modern
Linux systems, an attacker cannot place controlled data at this address.
Therefore, it cannot be exploited for privilege escalation or arbitrary code
execution.
* **Denial of Service:** Yes. Dereferencing the NULL pointer will cause a kernel
Oops/panic, crashing the system or deadlocking the runqueues.
* **Accessible From Unprivileged Processes / User Namespaces:** No. Triggering
this race condition strictly requires performing CPU hotplug operations (e.g.,
writing to `/sys/devices/system/cpu/cpuX/online`). CPU hotplug is a highly
privileged operation that requires global `CAP_SYS_ADMIN`. It cannot be
triggered by unprivileged users or from within a user namespace.
* **Remote / Peripheral / Filesystem Triggers:** No. The bug is tied to local
CPU topology management and cannot be triggered over the network, via untrusted
filesystems, or by plugging in standard peripheral devices. Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/10/13 07:54 |
action |
syz-repro-to-c-repro |
0m
Results:
|
| 2/1 |
2026/10/13 07:54 |
action |
kernel-checkouter |
0m
Results: map[KernelSrc:/app/workdir/cache/src/1381de6e1e7dcca4e4d7893fa5b5f1002799874c]
|
| 3/1 |
2026/10/13 07:54 |
action |
kernel-builder |
18m
Results: map[KernelObj:/app/workdir/cache/build/d40834185cbeee6c271be4c49f828563c2b2667e]
|
| 4/1 |
2026/10/13 08:12 |
action |
codesearch-prepare |
12m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/10/13 08:25 |
agent |
expert |
11m
Model:
Results: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Instruction: You are an experienced Linux kernel security engineer. Your task is to analyze given kernel bug report
and determine its security impact based on the following dimensions.
Use the provided tools to examine the source code, check for capability checks (e.g., capable(), ns_capable()),
and understand the nature of the bug. Analyze the given kernel build and configuration.
You can check the kernel config by grepping ".config" file; you can check kernel cmdline by grepping
".config" file for "CONFIG_CMDLINE=". Assume sysctl parameters have default values.
But analyze for the corresponding production build w/o debugging tools enabled (like KASAN, KMSAN, UBSAN).
Try different strategies when analyzing the bug:
- think of ways in which the vulnerable code is unreachable
- or the other way around: try to come up with different ideas of how an unprivileged user can reach the bug
If still unsure err on the side of the bug being non-exploitable/not-accessible.
In the final reply, provide a reasoning for your assessment.
Analysis dimensions:
* Exploitable:
Determine if the bug can result in memory corruption, elevated privileges, or an information leak.
Memory safety issues are almost always exploitable (KASAN or UBSAN reports for use-after-free, out-of-bounds;
refcounting issues, corrupted lists, etc). When kernel is crashing on a completely wild pointer access
(e.g. user-space address, or non-canonical address, but not on NULL or address corresponding to KASAN shadow
for NULL address), including both data accesses and control transfers, that also usually implies possibility
of exploitation. Such reports usually say "unable to handle kernel paging request".
Uses of uninitialized values detected by KMSAN may be exploitable b/c attacker frequently can affect uninit
values with spraying techniques. However, for these exploitability depends on how exactly the uninit value
is used in the code, and what it affects.
Information leaks are exploitable on their own and should be classified as such. A bug that copies kernel
memory contents to userspace (e.g. an out-of-bounds read whose result is returned to the caller, or
uninitialized stack/heap bytes written to a user buffer) is exploitable: it can reveal kernel pointer
values and defeat KASLR, expose sensitive data such as cryptographic keys or other processes' memory, and
serves as a necessary building block in most modern kernel privilege-escalation exploit chains. Do not classify
an information leak as non-exploitable solely because it does not directly cause a memory write or control-flow
hijack; the leak itself is the exploit primitive.
Think of what happens after the bug is triggered. Some bugs cause kernel panic and halt execution,
they are harder to exploit. For example, BUG reports halts the kernel. However, WARNING reports don't halt
execution in production builds. Debug bug detection tools (like KASAN, KMSAN, KCSAN, UBSAN) are also not enabled
in production builds, so attacker can freely exploit these bugs w/o being detected by these tools.
If you see an integer overflow, think how the overflowed value used later (if it's used as allocation size,
or an array index). If you see an out-of-bounds read, think if it's followed by an out-of-bounds write as well.
Some KCSAN data-races may be exploitable by skilled attackers as well. Think what data structures got corrupted
as the result of data races and how. However, note that kernel has lots of "benign" data races that don't lead
to any runtime misbehavior at all.
* Denial Of Service:
Determine if the bug can result in denial-of-service. Most bugs can, since they cause system crash,
hangs, deadlocks, or resource leaks. This is mostly applicable to WARNING bugs that won't cause system crash
in production. For these think what will be consequences of the violation of the kernel assumptions flagged
by the WARNING. In some cases the unexpected condition is also properly handled by the normal control flow
(e.g. with "if (WARN_ON(...))"), these won't cause denial-of-service. If the condition is not handled,
then it may or may not cause denial-of-service.
* Accessible From Unprivileged Processes:
Determine if the bug can be reached from a typical (non-root) user process that does NOT have any special capabilities
(like CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON) or access to device nodes restricted to root.
Assume that unprivileged_bpf_disabled=1, that is eBPF loading is not accessible. However, cBPF (classical BPF)
is still accessible to non-root processes.
Assume that user namespaces are not accessible, that is, the process cannot get the mentioned capabilities even
within a new user namespace (checked by ns_capable() function in the kernel sources).
* Accessible From User Namespaces:
Determine if the bug can be reached within a user-namespace where the process has all capabilities
(including CAP_SYS_ADMIN, CAP_NET_ADMIN, CAP_NET_RAW, CAP_PERFMON). Such capabilities are checked with ns_capable()
function in the kernel sources.
* VM Guest Trigger:
Determine if the bug can be triggered from the context of a typical KVM guest (e.g., set up by a QEMU VMM).
Consider accesses to standard Linux host paravirtualized features (virtio-blk, virtio-net, etc.),
and handling of VM exits in the KVM code.
* VM Host Trigger in The Confidential Computing Context:
Determine if the bug can be triggered in a confidential computing guest kernel from the context of a KVM host.
Consider access to standard Linux guest paravirtualized features (virtio-blk, virtio-net, etc.).
* Ethernet Network Trigger:
Determine if the bug can be triggered by processing ingress network Ethernet traffic, either directly (network stack)
or via drivers exposed to network data.
* Other Remote Trigger:
Determine if the bug can be triggered by processing remote traffic other than Ethernet (Wifi, Bluetooth, NFC, etc).
* Peripheral Trigger:
Determine if the bug can be triggered via an untrusted peripheral device that can be physically plugged
into a system, such as a USB device or a niche hardware driver handling external hardware inputs.
This is particularly important for mobile and desktop environments where users can plug in unknown devices.
* Malicious Filesystem Trigger:
Determine if the bug can be triggered by the kernel mounting and parsing a malicious filesystem image.
This is highly critical for Desktop and Mobile environments where external media or downloaded images
might be auto-mounted.
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 kernel bug report is:
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000002: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
CPU: 0 UID: 0 PID: 12282 Comm: syz.7.1759 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
RIP: 0010:task_cpu include/linux/sched.h:2284 [inline]
RIP: 0010:is_task_rq_idle kernel/sched/core.c:6204 [inline]
RIP: 0010:cookie_equals kernel/sched/core.c:6209 [inline]
RIP: 0010:pick_next_task kernel/sched/core.c:6403 [inline]
RIP: 0010:__schedule+0x2083/0x6920 kernel/sched/core.c:7199
Code: 0f 85 70 3c 00 00 4d 8b ac 24 d0 0e 00 00 48 c7 c0 40 20 8f 95 48 ba 00 00 00 00 00 fc ff df 49 8d 7d 14 48 89 f9 48 c1 e9 03 <0f> b6 0c 11 48 89 fa 83 e2 07 83 c2 03 38 ca 7c 08 84 c9 0f 85 32
RSP: 0000:ffffc900073cf890 EFLAGS: 00010003
RAX: ffffffff958f2040 RBX: 0000000000000001 RCX: 0000000000000002
RDX: dffffc0000000000 RSI: 1ffffffff1cd5324 RDI: 0000000000000014
RBP: ffffc900073cf9e0 R08: 0000000000000001 R09: 0000000000000000
R10: ffff88802b03cbd8 R11: 0000000000000000 R12: ffff88802b13c040
R13: 0000000000000000 R14: ffff88802b03c040 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff88809574a000(0063) knlGS:00000000f5456b40
CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 00000000f71c6400 CR3: 000000004fb11000 CR4: 0000000000352ef0
Call Trace:
<TASK>
__schedule_loop kernel/sched/core.c:7361 [inline]
schedule+0xdd/0x2c0 kernel/sched/core.c:7376
schedule_timeout+0x127/0x280 kernel/time/sleep_timeout.c:99
snd_rawmidi_write+0x492/0xc60 sound/core/rawmidi.c:1619
do_loop_readv_writev fs/read_write.c:851 [inline]
do_loop_readv_writev fs/read_write.c:836 [inline]
vfs_writev+0x5ed/0xdd0 fs/read_write.c:1060
do_writev+0x28a/0x340 fs/read_write.c:1104
do_syscall_32_irqs_on arch/x86/entry/syscall_32.c:79 [inline]
__do_fast_syscall_32+0x13a/0x8b0 arch/x86/entry/syscall_32.c:291
do_fast_syscall_32+0x32/0x70 arch/x86/entry/syscall_32.c:316
entry_SYSENTER_compat_after_hwframe+0x84/0x8e
RIP: 0023:0xf743efec
Code: Unable to access opcode bytes at 0xf743efc2.
RSP: 002b:00000000f545650c EFLAGS: 00000292 ORIG_RAX: 0000000000000092
RAX: ffffffffffffffda RBX: 0000000000000006 RCX: 0000000080000840
RDX: 0000000000000002 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000292 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
</TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:task_cpu include/linux/sched.h:2284 [inline]
RIP: 0010:is_task_rq_idle kernel/sched/core.c:6204 [inline]
RIP: 0010:cookie_equals kernel/sched/core.c:6209 [inline]
RIP: 0010:pick_next_task kernel/sched/core.c:6403 [inline]
RIP: 0010:__schedule+0x2083/0x6920 kernel/sched/core.c:7199
Code: 0f 85 70 3c 00 00 4d 8b ac 24 d0 0e 00 00 48 c7 c0 40 20 8f 95 48 ba 00 00 00 00 00 fc ff df 49 8d 7d 14 48 89 f9 48 c1 e9 03 <0f> b6 0c 11 48 89 fa 83 e2 07 83 c2 03 38 ca 7c 08 84 c9 0f 85 32
RSP: 0000:ffffc900073cf890 EFLAGS: 00010003
RAX: ffffffff958f2040 RBX: 0000000000000001 RCX: 0000000000000002
RDX: dffffc0000000000 RSI: 1ffffffff1cd5324 RDI: 0000000000000014
RBP: ffffc900073cf9e0 R08: 0000000000000001 R09: 0000000000000000
R10: ffff88802b03cbd8 R11: 0000000000000000 R12: ffff88802b13c040
R13: 0000000000000000 R14: ffff88802b03c040 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff88809574a000(0063) knlGS:00000000f5456b40
CS: 0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 00000000f743f000 CR3: 000000004fb11000 CR4: 0000000000352ef0
----------------
Code disassembly (best guess):
0: 0f 85 70 3c 00 00 jne 0x3c76
6: 4d 8b ac 24 d0 0e 00 mov 0xed0(%r12),%r13
d: 00
e: 48 c7 c0 40 20 8f 95 mov $0xffffffff958f2040,%rax
15: 48 ba 00 00 00 00 00 movabs $0xdffffc0000000000,%rdx
1c: fc ff df
1f: 49 8d 7d 14 lea 0x14(%r13),%rdi
23: 48 89 f9 mov %rdi,%rcx
26: 48 c1 e9 03 shr $0x3,%rcx
* 2a: 0f b6 0c 11 movzbl (%rcx,%rdx,1),%ecx <-- trapping instruction
2e: 48 89 fa mov %rdi,%rdx
31: 83 e2 07 and $0x7,%edx
34: 83 c2 03 add $0x3,%edx
37: 38 ca cmp %cl,%dl
39: 7c 08 jl 0x43
3b: 84 c9 test %cl,%cl
3d: 0f .byte 0xf
3e: 85 32 test %esi,(%rdx)
Reply: Based on the analysis of the bug report and the kernel source code, here is the assessment of the vulnerability:
**Root Cause Analysis:**
The bug is a NULL pointer dereference in the core scheduling code (`kernel/sched/core.c`). It occurs due to a race condition between the scheduler's `pick_next_task()` function and CPU hotplug operations.
When `CONFIG_SCHED_CORE` is enabled, `pick_next_task()` performs a core-wide task selection. It iterates over the SMT siblings of the current CPU using `smt_mask = cpu_smt_mask(cpu)` (which points to the CPU's `topology_sibling_cpumask`). It does this in two passes:
1. **First loop** (`for_each_cpu_wrap`): It calls `pick_task()` for each sibling and stores the selected task in `rq_i->core_pick`.
2. **Second loop** (`for_each_cpu`): It reads `rq_i->core_pick` for each sibling and checks if it matches the core cookie using `cookie_equals(p, cookie)`.
During CPU hotplug (specifically when a CPU is coming online), `set_cpu_sibling_map()` modifies the `topology_sibling_cpumask` to add the new CPU. Crucially, this modification happens *without* holding the runqueue locks.
If a CPU is running `pick_next_task()` concurrently with another CPU coming online, it can read the `smt_mask` in the first loop and see only itself. Then, the booting CPU modifies the mask. In the second loop, the scheduling CPU reads the mask again and sees the newly added CPU. It then accesses `rq_i->core_pick` for the new CPU. Since the new CPU was not visited in the first loop, its `core_pick` is uninitialized (i.e., `NULL`). This `NULL` pointer is passed to `cookie_equals()`, which dereferences it when calling `task_cpu(p)`, resulting in a kernel Oops.
**Security Impact:**
* **Exploitable:** No. The faulting address is `0xdffffc0000000002` (KASAN shadow for `0x10-0x17`), which corresponds to reading `p->cpu` at offset `0x14` from a `NULL` pointer. Because `mmap_min_addr` prevents mapping page 0 in modern Linux systems, an attacker cannot place controlled data at this address. Therefore, it cannot be exploited for privilege escalation or arbitrary code execution.
* **Denial of Service:** Yes. Dereferencing the NULL pointer will cause a kernel Oops/panic, crashing the system or deadlocking the runqueues.
* **Accessible From Unprivileged Processes / User Namespaces:** No. Triggering this race condition strictly requires performing CPU hotplug operations (e.g., writing to `/sys/devices/system/cpu/cpuX/online`). CPU hotplug is a highly privileged operation that requires global `CAP_SYS_ADMIN`. It cannot be triggered by unprivileged users or from within a user namespace.
* **Remote / Peripheral / Filesystem Triggers:** No. The bug is tied to local CPU topology management and cannot be triggered over the network, via untrusted filesystems, or by plugging in standard peripheral devices.
|
| 6/2 |
2026/10/13 08:25 |
llm |
expert |
0m
Model:
Tokens:
input: 7965
output: 31
thoughts: 324
|
| 7/2 |
2026/10/13 08:25 |
tool |
grepper |
0m
Args: map[Expression:cookie_equals PathPrefixes:[kernel/sched/core.c]]
Results: map[Output:kernel/sched/core.c=6202=static inline bool is_task_rq_idle(struct task_struct *t)
--
kernel/sched/core.c-6206-
kernel/sched/core.c:6207:static inline bool cookie_equals(struct task_struct *a, unsigned long cookie)
kernel/sched/core.c-6208-{
--
kernel/sched/core.c=6245=pick_next_task(struct rq *rq, struct rq_flags *rf)
--
kernel/sched/core.c-6402-
kernel/sched/core.c:6403: if (!cookie_equals(p, cookie)) {
kernel/sched/core.c-6404- p = NULL;
]
|
| 8/2 |
2026/10/13 08:25 |
llm |
expert |
0m
Model:
Tokens:
input: 8505
output: 36
thoughts: 20
|
| 9/2 |
2026/10/13 08:25 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:6390 LineCount:30]
Results: map[Contents:6390: max = p;
6391: }
6392:
6393: cookie = rq->core->core_cookie = max->core_cookie;
6394:
6395: /*
6396: * For each thread: try and find a runnable task that matches @max or
6397: * force idle.
6398: */
6399: for_each_cpu(i, smt_mask) {
6400: rq_i = cpu_rq(i);
6401: p = rq_i->core_pick;
6402:
6403: if (!cookie_equals(p, cookie)) {
6404: p = NULL;
6405: if (cookie)
6406: p = sched_core_find(rq_i, cookie);
6407: if (!p)
6408: p = idle_sched_class.pick_task(rq_i, rf);
6409: }
6410:
6411: rq_i->core_pick = p;
6412: rq_i->core_dl_server = NULL;
6413:
6414: if (p == rq_i->idle) {
6415: if (rq_i->nr_running) {
6416: rq->core->core_forceidle_count++;
6417: if (!fi_before)
6418: rq->core->core_forceidle_seq++;
6419: }
]
|
| 10/2 |
2026/10/13 08:25 |
llm |
expert |
0m
Model:
Tokens:
input: 8974
output: 36
thoughts: 19
Thoughts: **My Thought Process**
Okay, my immediate next step is to dive into the code. I need to understand the specifics of how these two functions, `cookie_equals` and `is_task_rq_idle`, are implemented. This will involve reading the actual source code for each. My goal is to grasp their individual logic, what inputs they take, what outputs they produce, and how they interact, if at all. Ultimately, I'm looking to build a comprehensive mental model of their behavior and purpose within the larger system.
|
| 11/2 |
2026/10/13 08:25 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:6200 LineCount:20]
Results: map[Contents:6200:
6201: #ifdef CONFIG_SCHED_CORE
6202: static inline bool is_task_rq_idle(struct task_struct *t)
6203: {
6204: return (task_rq(t)->idle == t);
6205: }
6206:
6207: static inline bool cookie_equals(struct task_struct *a, unsigned long cookie)
6208: {
6209: return is_task_rq_idle(a) || (a->core_cookie == cookie);
6210: }
6211:
6212: static inline bool cookie_match(struct task_struct *a, struct task_struct *b)
6213: {
6214: if (is_task_rq_idle(a) || is_task_rq_idle(b))
6215: return true;
6216:
6217: return a->core_cookie == b->core_cookie;
6218: }
6219:
]
|
| 12/2 |
2026/10/13 08:25 |
llm |
expert |
0m
Model:
Tokens:
input: 9309
output: 38
thoughts: 58
Thoughts: **My Thought Process Regarding `task_rq` and `is_task_rq_idle`**
Okay, so I'm looking at this `is_task_rq_idle` function, and my immediate reaction is to understand its dependencies. The first thing I notice is that it calls `task_rq(t)`. This tells me that the state of the task's run queue is fundamental to determining if the task itself is idle.
My next step, naturally, is to locate the definition of `task_rq`. I know that core scheduling structures and functions are typically found in the `kernel/sched/` directory, specifically within header files. So, my mental search immediately lands me on `kernel/sched/sched.h` or, if it's a more general scheduler-related definition, perhaps `include/linux/sched.h`. These are the canonical locations for such definitions in the Linux kernel.
At this point, my primary objective becomes to *examine* the implementation of `task_rq`. I need to understand what information it retrieves and how it structures it, because that will directly inform how `is_task_rq_idle` uses that information to make its own determination. I'm not just looking for *where* it's defined, but *what* it does.
|
| 13/2 |
2026/10/13 08:25 |
tool |
grepper |
0m
Args: map[Expression:task_rq PathPrefixes:[include/linux/sched.h kernel/sched/]]
Results: map[Output:
Full output is too long, showing 200 out of 1275 lines.
Use more precise expression or PathPrefixes if possible.
Number of matching lines per file (17 files in total):
include/linux/sched.h:2
kernel/sched/core.c:86
kernel/sched/core_sched.c:2
kernel/sched/cputime.c:2
kernel/sched/deadline.c:18
kernel/sched/ext/ext.c:60
kernel/sched/ext/idle.c:3
kernel/sched/ext/internal.h:3
kernel/sched/ext/sub.c:12
kernel/sched/fair.c:25
kernel/sched/idle.c:2
kernel/sched/psi.c:2
kernel/sched/rt.c:5
kernel/sched/sched.h:45
kernel/sched/stats.h:3
kernel/sched/stop_task.c:2
kernel/sched/syscalls.c:9
include/linux/sched.h=835=struct task_struct {
--
include/linux/sched.h-916- * Clamp values requested for a scheduling entity.
include/linux/sched.h:917: * Must be updated with task_rq_lock() held.
include/linux/sched.h-918- */
--
include/linux/sched.h-921- * Effective clamp values used for a scheduling entity.
include/linux/sched.h:922: * Must be updated with task_rq_lock() held.
include/linux/sched.h-923- */
--
kernel/sched/core.c=269=static inline bool __sched_core_less(const struct task_struct *a,
--
kernel/sched/core.c-278- /* flip prio, so high prio is leftmost */
kernel/sched/core.c:279: if (prio_less(b, a, !!task_rq(a)->core->core_forceidle_count))
kernel/sched/core.c-280- return true;
--
kernel/sched/core.c=561=int task_llc(const struct task_struct *p)
--
kernel/sched/core.c-594- *
kernel/sched/core.c:595: * System-calls and anything external will use task_rq_lock() which acquires
kernel/sched/core.c-596- * both p->pi_lock and rq->lock. As a consequence the state they change is
--
kernel/sched/core.c-620- * ON_RQ_MIGRATING state is used for migration without holding both
kernel/sched/core.c:621: * rq->locks. It indicates task_cpu() is not stable, see task_rq_lock().
kernel/sched/core.c-622- *
--
kernel/sched/core.c-654- * - for migration called under rq->lock:
kernel/sched/core.c:655: * [ see task_on_rq_migrating() in task_rq_lock() ]
kernel/sched/core.c-656- *
--
kernel/sched/core.c=723=void double_rq_lock(struct rq *rq1, struct rq *rq2)
--
kernel/sched/core.c-739-/*
kernel/sched/core.c:740: * ___task_rq_lock - lock the rq @p resides on.
kernel/sched/core.c-741- */
kernel/sched/core.c:742:struct rq *___task_rq_lock(struct task_struct *p, struct rq_flags *rf)
kernel/sched/core.c-743-{
--
kernel/sched/core.c-748- for (;;) {
kernel/sched/core.c:749: rq = task_rq(p);
kernel/sched/core.c-750- raw_spin_rq_lock(rq);
kernel/sched/core.c:751: if (likely(rq == task_rq(p) && !task_on_rq_migrating(p))) {
kernel/sched/core.c-752- rq_pin_lock(rq, rf);
--
kernel/sched/core.c-762-/*
kernel/sched/core.c:763: * task_rq_lock - lock p->pi_lock and lock the rq @p resides on.
kernel/sched/core.c-764- */
kernel/sched/core.c:765:struct rq *_task_rq_lock(struct task_struct *p, struct rq_flags *rf)
kernel/sched/core.c-766-{
--
kernel/sched/core.c-770- raw_spin_lock_irqsave(&p->pi_lock, rf->flags);
kernel/sched/core.c:771: rq = task_rq(p);
kernel/sched/core.c-772- raw_spin_rq_lock(rq);
kernel/sched/core.c-773- /*
kernel/sched/core.c:774: * move_queued_task() task_rq_lock()
kernel/sched/core.c-775- *
kernel/sched/core.c-776- * ACQUIRE (rq->lock)
kernel/sched/core.c:777: * [S] ->on_rq = MIGRATING [L] rq = task_rq()
kernel/sched/core.c-778- * WMB (__set_task_cpu()) ACQUIRE (rq->lock);
kernel/sched/core.c:779: * [S] ->cpu = new_cpu [L] task_rq()
kernel/sched/core.c-780- * [L] ->on_rq
--
kernel/sched/core.c-782- *
kernel/sched/core.c:783: * If we observe the old CPU in task_rq_lock(), the acquire of
kernel/sched/core.c-784- * the old rq->lock will fully serialize against the stores.
kernel/sched/core.c-785- *
kernel/sched/core.c:786: * If we observe the new CPU in task_rq_lock(), the address
kernel/sched/core.c:787: * dependency headed by '[L] rq = task_rq()' and the acquire
kernel/sched/core.c-788- * will pair with the WMB to ensure we then also see migrating.
kernel/sched/core.c-789- */
kernel/sched/core.c:790: if (likely(rq == task_rq(p) && !task_on_rq_migrating(p))) {
kernel/sched/core.c-791- rq_pin_lock(rq, rf);
--
kernel/sched/core.c=1534=void set_load_weight(struct task_struct *p, bool update_load)
--
kernel/sched/core.c-1551- if (update_load && p->sched_class->reweight_task)
kernel/sched/core.c:1552: p->sched_class->reweight_task(task_rq(p), p, &lw);
kernel/sched/core.c-1553- else
--
kernel/sched/core.c=1681=static void uclamp_update_util_min_rt_default(struct task_struct *p)
--
kernel/sched/core.c-1686- /* Protect updates to p->uclamp_* */
kernel/sched/core.c:1687: guard(task_rq_lock)(p);
kernel/sched/core.c-1688- __uclamp_update_util_min_rt_default(p);
--
kernel/sched/core.c=1935=uclamp_update_active(struct task_struct *p)
--
kernel/sched/core.c-1948- */
kernel/sched/core.c:1949: rq = task_rq_lock(p, &rf);
kernel/sched/core.c-1950-
kernel/sched/core.c-1951- /*
kernel/sched/core.c:1952: * Setting the clamp bucket is serialized by task_rq_lock().
kernel/sched/core.c-1953- * If the task is not yet RUNNABLE and its task_struct is not
--
kernel/sched/core.c-1959-
kernel/sched/core.c:1960: task_rq_unlock(rq, p, &rf);
kernel/sched/core.c-1961-}
--
kernel/sched/core.c=2086=static void uclamp_fork(struct task_struct *p)
--
kernel/sched/core.c-2090- /*
kernel/sched/core.c:2091: * We don't need to hold task_rq_lock() when updating p->uclamp_* here
kernel/sched/core.c-2092- * as the task is still at its early fork stages.
--
kernel/sched/core.c=2353=unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state)
--
kernel/sched/core.c-2366- */
kernel/sched/core.c:2367: rq = task_rq(p);
kernel/sched/core.c-2368-
--
kernel/sched/core.c-2390- */
kernel/sched/core.c:2391: rq = task_rq_lock(p, &rf);
kernel/sched/core.c-2392- /*
--
kernel/sched/core.c-2410- }
kernel/sched/core.c:2411: task_rq_unlock(rq, p, &rf);
kernel/sched/core.c-2412-
--
kernel/sched/core.c=2461=static void migrate_disable_switch(struct rq *rq, struct task_struct *p)
--
kernel/sched/core.c-2473-
kernel/sched/core.c:2474: scoped_guard (task_rq_lock, p)
kernel/sched/core.c-2475- do_set_cpus_allowed(p, &ac);
--
kernel/sched/core.c=2621=static int migration_cpu_stop(void *data)
--
kernel/sched/core.c-2657- /*
kernel/sched/core.c:2658: * If task_rq(p) != rq, it cannot be migrated here, because we're
kernel/sched/core.c-2659- * holding rq->lock, if p->on_rq == 0 it cannot get enqueued because
--
kernel/sched/core.c-2661- */
kernel/sched/core.c:2662: if (task_rq(p) == rq) {
kernel/sched/core.c-2663- if (is_migration_disabled(p))
--
kernel/sched/core.c=2735=int push_cpu_stop(void *arg)
--
kernel/sched/core.c-2742-
kernel/sched/core.c:2743: if (task_rq(p) != rq)
kernel/sched/core.c-2744- goto out_unlock;
--
kernel/sched/core.c-2761- // XXX validate p is still the highest prio task
kernel/sched/core.c:2762: if (task_rq(p) == rq) {
kernel/sched/core.c-2763- move_queued_task_locked(rq, lowest_rq, p);
--
kernel/sched/core.c=2813=void set_cpus_allowed_force(struct task_struct *p, const struct cpumask *new_mask)
--
kernel/sched/core.c-2824-
kernel/sched/core.c:2825: scoped_guard (__task_rq_lock, p)
kernel/sched/core.c-2826- do_set_cpus_allowed(p, &ac);
--
kernel/sched/core.c=2969=static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flags *rf,
--
kernel/sched/core.c-3003- preempt_disable();
kernel/sched/core.c:3004: task_rq_unlock(rq, p, rf);
kernel/sched/core.c-3005- if (push_task) {
--
kernel/sched/core.c-3057- if (WARN_ON_ONCE(!pending)) {
kernel/sched/core.c:3058: task_rq_unlock(rq, p, rf);
kernel/sched/core.c-3059- return -EINVAL;
--
kernel/sched/core.c-3075- preempt_disable();
kernel/sched/core.c:3076: task_rq_unlock(rq, p, rf);
kernel/sched/core.c-3077- if (!stop_pending) {
--
kernel/sched/core.c-3095- }
kernel/sched/core.c:3096: task_rq_unlock(rq, p, rf);
kernel/sched/core.c-3097-
--
kernel/sched/core.c=3122=static int __set_cpus_allowed_ptr_locked(struct task_struct *p,
--
kernel/sched/core.c-3192-out:
kernel/sched/core.c:3193: task_rq_unlock(rq, p, rf);
kernel/sched/core.c-3194-
--
kernel/sched/core.c=3207=int __set_cpus_allowed_ptr(struct task_struct *p, struct affinity_context *ctx)
--
kernel/sched/core.c-3211-
kernel/sched/core.c:3212: rq = task_rq_lock(p, &rf);
kernel/sched/core.c-3213- /*
--
kernel/sched/core.c=3245=static int restrict_cpus_allowed_ptr(struct task_struct *p,
--
kernel/sched/core.c-3256-
kernel/sched/core.c:3257: rq = task_rq_lock(p, &rf);
kernel/sched/core.c-3258-
--
kernel/sched/core.c-3276-err_unlock:
kernel/sched/core.c:3277: task_rq_unlock(rq, p, &rf);
kernel/sched/core.c-3278- return err;
--
kernel/sched/core.c=3351=void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
--
kernel/sched/core.c-3379- *
kernel/sched/core.c:3380: * Furthermore, all task_rq users should acquire both locks, see
kernel/sched/core.c:3381: * task_rq_lock().
kernel/sched/core.c-3382- */
kernel/sched/core.c-3383- WARN_ON_ONCE(debug_locks && !(lockdep_is_held(&p->pi_lock) ||
kernel/sched/core.c:3384: lockdep_is_held(__rq_lockp(task_rq(p)))));
kernel/sched/core.c-3385-#endif
--
]
|
| 14/2 |
2026/10/13 08:25 |
llm |
expert |
0m
Model:
Tokens:
input: 13379
output: 31
thoughts: 27
Thoughts: **My Thought Process on Locating `task_rq`**
As I delve into the kernel's scheduling mechanisms, one of the crucial data structures that keeps popping up is `task_rq`. I need to understand where this definition resides so I can properly contextualize its usage and behavior. My immediate thought is to consult the source code. Given that `task_rq` is intrinsically linked to the task structure and its role within the scheduler, the most logical place to look would be within the header files that define core scheduling components. Specifically, `kernel/sched/sched.h` seems like the prime candidate for housing such a fundamental definition. Therefore, my next step is to perform a targeted search within that file to pinpoint the exact location and definition of `task_rq`. A quick `grep` command for `task_rq` in `kernel/sched/sched.h` should efficiently reveal what I'm looking for.
|
| 15/2 |
2026/10/13 08:25 |
tool |
grepper |
0m
Args: map[Expression:task_rq PathPrefixes:[kernel/sched/sched.h]]
Results: map[Output:kernel/sched/sched.h=604=extern int sched_group_set_idle(struct task_group *tg, long idle);
kernel/sched/sched.h-605-
kernel/sched/sched.h:606:extern void set_task_rq_fair(struct sched_entity *se,
kernel/sched/sched.h-607- struct cfs_rq *prev, struct cfs_rq *next);
--
kernel/sched/sched.h=1434=static __always_inline struct rq *__this_rq(void)
--
kernel/sched/sched.h-1440-#define this_rq() __this_rq()
kernel/sched/sched.h:1441:#define task_rq(p) cpu_rq(task_cpu(p))
kernel/sched/sched.h-1442-#define cpu_curr(cpu) (cpu_rq(cpu)->curr)
--
kernel/sched/sched.h=1759=static inline struct cfs_rq *task_cfs_rq(const struct task_struct *p)
kernel/sched/sched.h-1760-{
kernel/sched/sched.h:1761: return &task_rq(p)->cfs;
kernel/sched/sched.h-1762-}
--
kernel/sched/sched.h=1764=static inline struct cfs_rq *cfs_rq_of(const struct sched_entity *se)
--
kernel/sched/sched.h-1766- const struct task_struct *p = task_of(se);
kernel/sched/sched.h:1767: struct rq *rq = task_rq(p);
kernel/sched/sched.h-1768-
--
kernel/sched/sched.h=1953=static inline void rq_repin_lock(struct rq *rq, struct rq_flags *rf)
--
kernel/sched/sched.h-1962-
kernel/sched/sched.h:1963:#define __task_rq_lock(...) __acquire_ret(___task_rq_lock(__VA_ARGS__), __rq_lockp(__ret))
kernel/sched/sched.h:1964:extern struct rq *___task_rq_lock(struct task_struct *p, struct rq_flags *rf) __acquires_ret;
kernel/sched/sched.h-1965-
kernel/sched/sched.h:1966:#define task_rq_lock(...) __acquire_ret(_task_rq_lock(__VA_ARGS__), __rq_lockp(__ret))
kernel/sched/sched.h:1967:extern struct rq *_task_rq_lock(struct task_struct *p, struct rq_flags *rf)
kernel/sched/sched.h-1968- __acquires(&p->pi_lock) __acquires_ret;
--
kernel/sched/sched.h=1970=static inline void
kernel/sched/sched.h:1971:__task_rq_unlock(struct rq *rq, struct task_struct *p, struct rq_flags *rf)
kernel/sched/sched.h-1972- __releases(__rq_lockp(rq))
--
kernel/sched/sched.h=1978=static inline void
kernel/sched/sched.h:1979:task_rq_unlock(struct rq *rq, struct task_struct *p, struct rq_flags *rf)
kernel/sched/sched.h-1980- __releases(__rq_lockp(rq), &p->pi_lock)
kernel/sched/sched.h-1981-{
kernel/sched/sched.h:1982: __task_rq_unlock(rq, p, rf);
kernel/sched/sched.h-1983- raw_spin_unlock_irqrestore(&p->pi_lock, rf->flags);
--
kernel/sched/sched.h-1985-
kernel/sched/sched.h:1986:DEFINE_LOCK_GUARD_1(task_rq_lock, struct task_struct,
kernel/sched/sched.h:1987: _T->rq = task_rq_lock(_T->lock, &_T->rf),
kernel/sched/sched.h:1988: task_rq_unlock(_T->rq, _T->lock, &_T->rf),
kernel/sched/sched.h-1989- struct rq *rq; struct rq_flags rf)
kernel/sched/sched.h:1990:DECLARE_LOCK_GUARD_1_ATTRS(task_rq_lock, __acquires(_T->pi_lock), __releases((*(struct task_struct **)_T)->pi_lock))
kernel/sched/sched.h:1991:#define class_task_rq_lock_constructor(_T) WITH_LOCK_GUARD_1_ATTRS(task_rq_lock, _T)
kernel/sched/sched.h-1992-
kernel/sched/sched.h:1993:DEFINE_LOCK_GUARD_1(__task_rq_lock, struct task_struct,
kernel/sched/sched.h:1994: _T->rq = __task_rq_lock(_T->lock, &_T->rf),
kernel/sched/sched.h:1995: __task_rq_unlock(_T->rq, _T->lock, &_T->rf),
kernel/sched/sched.h-1996- struct rq *rq; struct rq_flags rf)
--
kernel/sched/sched.h=2361=static inline struct sched_entity *cfs_rq_se(struct cfs_rq *cfs_rq)
--
kernel/sched/sched.h-2373-/* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
kernel/sched/sched.h:2374:static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
kernel/sched/sched.h-2375-{
--
kernel/sched/sched.h-2380-#ifdef CONFIG_FAIR_GROUP_SCHED
kernel/sched/sched.h:2381: set_task_rq_fair(&p->se, p->se.cfs_rq, tg_cfs_rq(tg, cpu));
kernel/sched/sched.h-2382- p->se.cfs_rq = tg_cfs_rq(tg, cpu);
--
kernel/sched/sched.h-2401-
kernel/sched/sched.h:2402:static inline void set_task_rq(struct task_struct *p, unsigned int cpu) { }
kernel/sched/sched.h-2403-
--
kernel/sched/sched.h=2411=static inline void __set_task_cpu(struct task_struct *p, unsigned int cpu)
kernel/sched/sched.h-2412-{
kernel/sched/sched.h:2413: set_task_rq(p, cpu);
kernel/sched/sched.h-2414-#ifdef CONFIG_SMP
kernel/sched/sched.h-2415- /*
kernel/sched/sched.h:2416: * After ->cpu is set up to a new value, task_rq_lock(p, ...) can be
kernel/sched/sched.h-2417- * successfully executed on another CPU. We must ensure that updates of
--
kernel/sched/sched.h=2520=static inline int task_on_rq_migrating(struct task_struct *p)
--
kernel/sched/sched.h-2532-#define WF_CURRENT_CPU 0x40 /* Prefer to move the wakee to the current CPU. */
kernel/sched/sched.h:2533:#define WF_RQ_SELECTED 0x80 /* ->select_task_rq() was called */
kernel/sched/sched.h-2534-
--
kernel/sched/sched.h=2552=extern const u32 sched_prio_to_wmult[40];
--
kernel/sched/sched.h-2578- * ENQUEUE_MIGRATED - the task was migrated during wakeup
kernel/sched/sched.h:2579: * ENQUEUE_RQ_SELECTED - ->select_task_rq() was called
kernel/sched/sched.h-2580- *
--
kernel/sched/sched.h=2623=struct sched_class {
--
kernel/sched/sched.h-2631- * ttwu_do_activate/activate_task/enqueue_task: rq->lock
kernel/sched/sched.h:2632: * wake_up_new_task/activate_task/enqueue_task: task_rq_lock
kernel/sched/sched.h:2633: * ttwu_runnable/enqueue_task: task_rq_lock
kernel/sched/sched.h-2634- * proxy_task_current: rq->lock
--
kernel/sched/sched.h-2641- * proxy_task_current: rq->lock
kernel/sched/sched.h:2642: * wait_task_inactive: task_rq_lock
kernel/sched/sched.h-2643- * sched_change_begin
--
kernel/sched/sched.h-2659- * ttwu_do_activate: rq->lock
kernel/sched/sched.h:2660: * ttwu_runnable: task_rq_lock
kernel/sched/sched.h:2661: * wake_up_new_task: task_rq_lock
kernel/sched/sched.h-2662- */
--
kernel/sched/sched.h-2682- /*
kernel/sched/sched.h:2683: * select_task_rq: p->pi_lock
kernel/sched/sched.h-2684- * sched_exec: p->pi_lock
kernel/sched/sched.h-2685- */
kernel/sched/sched.h:2686: int (*select_task_rq)(struct task_struct *p, int task_cpu, int flags);
kernel/sched/sched.h-2687-
--
kernel/sched/sched.h-2690- */
kernel/sched/sched.h:2691: void (*migrate_task_rq)(struct task_struct *p, int new_cpu);
kernel/sched/sched.h-2692-
--
kernel/sched/sched.h-2694- * ttwu_do_activate: rq->lock
kernel/sched/sched.h:2695: * wake_up_new_task: task_rq_lock
kernel/sched/sched.h-2696- */
--
kernel/sched/sched.h-2699- /*
kernel/sched/sched.h:2700: * do_set_cpus_allowed: task_rq_lock + sched_change
kernel/sched/sched.h-2701- */
--
kernel/sched/sched.h-2741- /*
kernel/sched/sched.h:2742: * set_load_weight: task_rq_lock + sched_change
kernel/sched/sched.h:2743: * __setscheduler_parms: task_rq_lock + sched_change
kernel/sched/sched.h-2744- */
--
kernel/sched/sched.h-2748- /*
kernel/sched/sched.h:2749: * sched_rr_get_interval: task_rq_lock
kernel/sched/sched.h-2750- */
--
kernel/sched/sched.h-2754- /*
kernel/sched/sched.h:2755: * task_sched_runtime: task_rq_lock
kernel/sched/sched.h-2756- */
--
kernel/sched/sched.h-2760- /*
kernel/sched/sched.h:2761: * sched_change_group: task_rq_lock + sched_change
kernel/sched/sched.h-2762- */
--
kernel/sched/sched.h=3070=static inline void __block_task(struct rq *rq, struct task_struct *p)
--
kernel/sched/sched.h-3097- *
kernel/sched/sched.h:3098: * cpu = select_task_rq();
kernel/sched/sched.h-3099- * set_task_cpu(p, cpu);
--
kernel/sched/sched.h=3121=static inline void attach_task(struct rq *rq, struct task_struct *p)
--
kernel/sched/sched.h-3124-
kernel/sched/sched.h:3125: WARN_ON_ONCE(task_rq(p) != rq);
kernel/sched/sched.h-3126- activate_task(rq, p, ENQUEUE_NOCLOCK);
--
kernel/sched/sched.h=3398=DECLARE_LOCK_GUARD_2_ATTRS(double_raw_spinlock,
--
kernel/sched/sched.h-3407- *
kernel/sched/sched.h:3408: * Note this does not restore interrupts like task_rq_unlock,
kernel/sched/sched.h-3409- * you need to do so manually after calling.
--
kernel/sched/sched.h=4215=extern void balance_callbacks(struct rq *rq, struct balance_callback *head);
--
kernel/sched/sched.h-4222- *
kernel/sched/sched.h:4223: * Typically this must be called while holding task_rq_lock, since most/all
kernel/sched/sched.h-4224- * properties are serialized under those locks. There are currently two
]
|
| 16/2 |
2026/10/13 08:25 |
llm |
expert |
0m
Model:
Tokens:
input: 16896
output: 36
thoughts: 57
Thoughts: **My Thought Process on the `task_rq(p)` Crash**
Okay, so I'm looking at this bug report, and the core of it seems to be a crash related to `task_rq(p)`. My first instinct, as someone who deals with these kernel internals regularly, is to understand what `task_rq(p)` actually *is*. My immediate thought is that it's a lookup mechanism for the runqueue associated with a particular task `p`.
And then, it hits me: `task_rq(p)` is actually defined as `cpu_rq(task_cpu(p))`. This is a crucial deconstruction. It tells me that to get to the runqueue, I first need to determine which CPU the task `p` is currently running on. The crash is specifically occurring *within* the `task_cpu(p)` part of that expression.
This is a critical piece of information. My mind immediately goes to the implementation details. I know `task_cpu(p)` isn't some complex, multi-step process. It's likely a direct, optimized lookup. And indeed, my experience confirms this: `task_cpu(p)` is defined as an inline function right there in `include/linux/sched.h`. This means it's designed to be expanded directly at the call site for maximum performance. The fact that the crash is happening *there* suggests a potential issue with how that inline function is being called, or perhaps an unexpected state of the task `p` itself when `task_cpu(p)` is invoked.
|
| 17/2 |
2026/10/13 08:25 |
tool |
read-file |
0m
Args: map[File:include/linux/sched.h FirstLine:2270 LineCount:20]
Results: map[Contents:2270: #endif /* !CONFIG_PREEMPT_RT */
2271:
2272: static __always_inline bool need_resched(void)
2273: {
2274: return unlikely(tif_need_resched());
2275: }
2276:
2277: /*
2278: * Wrappers for p->thread_info->cpu access. No-op on UP.
2279: */
2280: #ifdef CONFIG_SMP
2281:
2282: static inline unsigned int task_cpu(const struct task_struct *p)
2283: {
2284: return READ_ONCE(task_thread_info(p)->cpu);
2285: }
2286:
2287: extern void set_task_cpu(struct task_struct *p, unsigned int cpu);
2288:
2289: #else
]
|
| 18/2 |
2026/10/13 08:25 |
llm |
expert |
0m
Model:
Tokens:
input: 17243
output: 36
thoughts: 675
Thoughts: **My Thought Process on the Crash**
Okay, I'm looking at this crash and trying to unravel it. The initial point of failure is reported at `task_cpu(p)`. I know `task_cpu(p)` is essentially a `READ_ONCE` operation on a field within `task_thread_info(p)->cpu`. And `task_thread_info(p)` itself is a bit of a chameleon; it's either `(struct thread_info *)p` or `&p->thread_info` depending on the kernel configuration `CONFIG_THREAD_INFO_IN_TASK`.
Now, the really interesting piece of information is the faulting address: `0xdffffc0000000002`. This address immediately flags as a KASAN shadow address, specifically for the memory range `0x10` to `0x17`. What this tells me is that the pointer `p` must be very close to zero, likely `0x14` or something similar, if the access is at offset `0x14` from it.
Let me re-examine the disassembly at the point of the trap:
```
6: 4d 8b ac 24 d0 0e 00 mov 0xed0(%r12),%r13
d: 00
e: 48 c7 c0 40 20 8f 95 mov $0xffffffff958f2040,%rax
15: 48 ba 00 00 00 00 00 movabs $0xdffffc0000000000,%rdx
1c: fc ff df
1f: 49 8d 7d 14 lea 0x14(%r13),%rdi
23: 48 89 f9 mov %rdi,%rcx
26: 48 c1 e9 03 shr $0x3,%rcx
* 2a: 0f b6 0c 11 movzbl (%rcx,%rdx,1),%ecx <-- trapping instruction
```
Looking closely, `%r13` is currently `0`. Therefore, `0x14(%r13)` evaluates to `0x14`. This means the value `p` is being used to derive the pointer for the `task_thread_info` access is effectively `0`. This is a *null pointer dereference*.
My initial suspicion is that `p` is `0` because `rq_i->core_pick` is `NULL`. I'm tracing back to `kernel/sched/core.c:6403`, where the code is `if (!cookie_equals(p, cookie))`. This `cookie_equals` function, in turn, calls `is_task_rq_idle(p)`, which then calls `task_rq(p)`, and ultimately leads to `task_cpu(p)`. If `p` is `NULL` at that point, `task_cpu(NULL)` is going to dereference that `NULL` pointer, and boom, that's our crash.
The crucial question now is: can `rq_i->core_pick` legitimately be `NULL` at this stage of `pick_next_task` execution? I need to dig into the logic of `pick_next_task` to understand the conditions under which `rq_i->core_pick` might be `NULL`.
|
| 19/2 |
2026/10/13 08:25 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:6380 LineCount:40]
Results: map[Contents:6380: core_clock_updated = false;
6381: if (!(rq->clock_update_flags & RQCF_UPDATED))
6382: update_rq_clock(rq);
6383: goto restart;
6384: }
6385:
6386: rq_i->core_pick = p;
6387: rq_i->core_dl_server = rq_i->dl_server;
6388:
6389: if (!max || prio_less(max, p, fi_before))
6390: max = p;
6391: }
6392:
6393: cookie = rq->core->core_cookie = max->core_cookie;
6394:
6395: /*
6396: * For each thread: try and find a runnable task that matches @max or
6397: * force idle.
6398: */
6399: for_each_cpu(i, smt_mask) {
6400: rq_i = cpu_rq(i);
6401: p = rq_i->core_pick;
6402:
6403: if (!cookie_equals(p, cookie)) {
6404: p = NULL;
6405: if (cookie)
6406: p = sched_core_find(rq_i, cookie);
6407: if (!p)
6408: p = idle_sched_class.pick_task(rq_i, rf);
6409: }
6410:
6411: rq_i->core_pick = p;
6412: rq_i->core_dl_server = NULL;
6413:
6414: if (p == rq_i->idle) {
6415: if (rq_i->nr_running) {
6416: rq->core->core_forceidle_count++;
6417: if (!fi_before)
6418: rq->core->core_forceidle_seq++;
6419: }
]
|
| 20/2 |
2026/10/13 08:25 |
llm |
expert |
0m
Model:
Tokens:
input: 18518
output: 36
thoughts: 207
Thoughts: **My Thought Process During Core Picking**
Okay, I'm looking at this `for_each_cpu` loop here, iterating through CPUs in `smt_mask`. My primary goal in this loop is to identify the best task (`p`) to potentially pick for a given CPU's runqueue (`rq_i`), and to track the overall "maximum" priority task encountered so far.
My initial thought when I see `p = pick_task(rq_i, class, max, fi_before);` is to then consider what happens with `p`. I immediately notice the `if (!p) continue;` right after. This is a crucial point for me. It means that if `pick_task` returns `NULL`, indicating no suitable task was found on that particular CPU (`rq_i`), the loop simply moves to the next CPU.
And this is where a specific question pops into my mind: if `p` is `NULL`, the code *doesn't* update `rq_i->core_pick`. This implies that `rq_i->core_pick` must have some initial state *before* this loop begins, or at least that it's not necessarily being assigned a valid task pointer by this specific `if (!p) continue;` block. This is something I need to investigate further. My instinct is to understand what the default or initial value of `rq_i->core_pick` is when no task is picked.
Following that, I see the assignments `rq_i->core_pick = p;` and `rq_i->core_dl_server = rq_i->dl_server;`. These are contingent on `p` not being `NULL` due to the `continue` statement I just analyzed. So, `rq_i->core_pick` will indeed be set to the task `p` that was just picked, *but only if a task was picked*. The `core_dl_server` update seems straightforward, a direct copy.
Then I look at the `if (!max || prio_less(max, p, fi_before)) max = p;` condition. This is my mechanism for tracking the highest priority task found *across all CPUs processed so far*. If `max` is currently `NULL` (meaning this is the first valid task I've encountered), or if the newly picked task `p` has a higher priority than the current `max` (as determined by `prio_less`), then I update `max` to point to `p`. This is essential for maintaining the "best" candidate across the entire set of CPUs being considered in this loop.
So, my immediate follow-up thought after seeing the `continue` is about the *uninitialized* or *pre-existing* state of `rq_i->core_pick`. I need to trace back to understand its initial value to fully grasp the implications of that `continue`. My next step is to look at the beginning of the `pick_next_task` function to see how `rq_i->core_pick` is initialized or what its state is before this loop begins its work.
|
| 21/2 |
2026/10/13 08:25 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:6340 LineCount:40]
Results: map[Contents:6340: core_clock_updated = false;
6341: if (!(rq->clock_update_flags & RQCF_UPDATED))
6342: update_rq_clock(rq);
6343: goto restart;
6344: }
6345:
6346: if (!next->core_cookie) {
6347: rq->core_pick = NULL;
6348: rq->core_dl_server = NULL;
6349: /*
6350: * For robustness, update the min_vruntime_fi for
6351: * unconstrained picks as well.
6352: */
6353: WARN_ON_ONCE(fi_before);
6354: task_vruntime_update(rq, next, false);
6355: goto out_set_next;
6356: }
6357: }
6358:
6359: /*
6360: * For each thread: do the regular task pick and find the max prio task
6361: * amongst them.
6362: *
6363: * Tie-break prio towards the current CPU
6364: */
6365: max = NULL;
6366: for_each_cpu_wrap(i, smt_mask, cpu) {
6367: rq_i = cpu_rq(i);
6368:
6369: /*
6370: * Current cpu always has its clock updated on entrance to
6371: * pick_next_task(). If the current cpu is not the core,
6372: * the core may also have been updated above.
6373: */
6374: if (i != cpu && (rq_i != rq->core || !core_clock_updated))
6375: update_rq_clock(rq_i);
6376:
6377: p = pick_task(rq_i, rf);
6378: if (unlikely(p == RETRY_TASK)) {
6379: /* rq lock may have been dropped, clocks invalidated */
]
|
| 22/2 |
2026/10/13 08:25 |
llm |
expert |
0m
Model:
Tokens:
input: 19329
output: 36
thoughts: 21
Thoughts: **My Thought Process on `pick_task` Execution**
Alright, so the `pick_task` function has just been invoked. My immediate next step, as I observe this event unfolding in the execution flow, is to meticulously examine the subsequent operations within the encompassing loop. I need to understand precisely how the task selection, dictated by `pick_task`, integrates with and influences the remainder of the iterative process.
|
| 23/2 |
2026/10/13 08:25 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:6377 LineCount:40]
Results: map[Contents:6377: p = pick_task(rq_i, rf);
6378: if (unlikely(p == RETRY_TASK)) {
6379: /* rq lock may have been dropped, clocks invalidated */
6380: core_clock_updated = false;
6381: if (!(rq->clock_update_flags & RQCF_UPDATED))
6382: update_rq_clock(rq);
6383: goto restart;
6384: }
6385:
6386: rq_i->core_pick = p;
6387: rq_i->core_dl_server = rq_i->dl_server;
6388:
6389: if (!max || prio_less(max, p, fi_before))
6390: max = p;
6391: }
6392:
6393: cookie = rq->core->core_cookie = max->core_cookie;
6394:
6395: /*
6396: * For each thread: try and find a runnable task that matches @max or
6397: * force idle.
6398: */
6399: for_each_cpu(i, smt_mask) {
6400: rq_i = cpu_rq(i);
6401: p = rq_i->core_pick;
6402:
6403: if (!cookie_equals(p, cookie)) {
6404: p = NULL;
6405: if (cookie)
6406: p = sched_core_find(rq_i, cookie);
6407: if (!p)
6408: p = idle_sched_class.pick_task(rq_i, rf);
6409: }
6410:
6411: rq_i->core_pick = p;
6412: rq_i->core_dl_server = NULL;
6413:
6414: if (p == rq_i->idle) {
6415: if (rq_i->nr_running) {
6416: rq->core->core_forceidle_count++;
]
|
| 24/2 |
2026/10/13 08:25 |
llm |
expert |
0m
Model:
Tokens:
input: 19968
output: 36
thoughts: 28
Thoughts: **My Thought Process Regarding Task Selection**
Okay, so I'm looking at this `pick_task` function, and my immediate thought is about its return value. My primary concern here is whether it's possible for `pick_task` to return `NULL`. This is a critical piece of information for me as an expert in this domain because a `NULL` return would signify a specific state β perhaps no tasks are available, or an error occurred during selection. Understanding this possibility dictates how I'll handle the return value in the subsequent logic. Therefore, my very next step is to dive into the implementation of `pick_task` to confirm its behavior and determine if `NULL` is indeed a valid outcome.
|
| 25/2 |
2026/10/13 08:25 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:6245 LineCount:40]
Results: map[Contents:6245: pick_next_task(struct rq *rq, struct rq_flags *rf)
6246: __must_hold(__rq_lockp(rq))
6247: {
6248: struct task_struct *next, *p, *max;
6249: const struct cpumask *smt_mask;
6250: bool fi_before = false;
6251: bool core_clock_updated = (rq == rq->core);
6252: unsigned long cookie;
6253: int i, cpu, occ = 0;
6254: struct rq *rq_i;
6255: bool need_sync = false;
6256:
6257: if (!sched_core_enabled(rq))
6258: return __pick_next_task(rq, rf);
6259:
6260: cpu = cpu_of(rq);
6261:
6262: /* Stopper task is switching into idle, no need core-wide selection. */
6263: if (cpu_is_offline(cpu)) {
6264: /*
6265: * Reset core_pick so that we don't enter the fastpath when
6266: * coming online. core_pick would already be migrated to
6267: * another cpu during offline.
6268: */
6269: rq->core_pick = NULL;
6270: rq->core_dl_server = NULL;
6271: return __pick_next_task(rq, rf);
6272: }
6273:
6274: rq->core->core_pick_in_flight++;
6275:
6276: /*
6277: * If there were no {en,de}queues since we picked (IOW, the task
6278: * pointers are all still valid), and we haven't scheduled the last
6279: * pick yet, do so now.
6280: *
6281: * rq->core_pick can be NULL if no selection was made for a CPU because
6282: * it was either offline or went offline during a sibling's core-wide
6283: * selection. In this case, do a core-wide selection.
6284: */
]
|
| 26/2 |
2026/10/13 08:25 |
llm |
expert |
0m
Model:
Tokens:
input: 20644
output: 31
thoughts: 21
Thoughts: **My Thought Process for Locating `pick_task`**
Ah, I've just encountered a reference to `pick_task` within this context, and immediately my internal alarm bells are ringing. My expertise tells me that this function isn't defined right here; it must be declared or implemented elsewhere in the codebase. My immediate, almost instinctive, next step is therefore to initiate a search. I need to locate the definition of `pick_task` to understand its signature, its behavior, and how it's intended to integrate with the current code. This isn't just a casual browse; it's a focused, methodical hunt to ensure I have a complete and accurate understanding of the system's architecture and dependencies.
|
| 27/2 |
2026/10/13 08:25 |
tool |
grepper |
0m
Args: map[Expression:pick_task PathPrefixes:[kernel/sched/core.c]]
Results: map[Output:kernel/sched/core.c=429=static void __sched_core_flip(bool enabled)
--
kernel/sched/core.c-446- * A core-wide selection may have the shared rq lock temporarily
kernel/sched/core.c:447: * released by a lock-dropping ->pick_task(). Flipping would
kernel/sched/core.c-448- * rebind rq_lockp() under it. Wait it out.
--
kernel/sched/core.c=6153=__pick_next_task(struct rq *rq, struct rq_flags *rf)
--
kernel/sched/core.c-6172-
kernel/sched/core.c:6173: p = pick_task_fair(rq, rf);
kernel/sched/core.c-6174- if (unlikely(p == RETRY_TASK))
--
kernel/sched/core.c-6178- if (!p)
kernel/sched/core.c:6179: p = pick_task_idle(rq, rf);
kernel/sched/core.c-6180-
--
kernel/sched/core.c-6188- for_each_active_class(class) {
kernel/sched/core.c:6189: p = class->pick_task(rq, rf);
kernel/sched/core.c-6190- if (unlikely(p == RETRY_TASK))
--
kernel/sched/core.c=6212=static inline bool cookie_match(struct task_struct *a, struct task_struct *b)
--
kernel/sched/core.c-6223- */
kernel/sched/core.c:6224:static inline struct task_struct *pick_task(struct rq *rq, struct rq_flags *rf)
kernel/sched/core.c-6225-{
--
kernel/sched/core.c-6231- for_each_active_class(class) {
kernel/sched/core.c:6232: p = class->pick_task(rq, rf);
kernel/sched/core.c-6233- if (p)
--
kernel/sched/core.c=6245=pick_next_task(struct rq *rq, struct rq_flags *rf)
--
kernel/sched/core.c-6336- if (!need_sync) {
kernel/sched/core.c:6337: next = pick_task(rq, rf);
kernel/sched/core.c-6338- if (unlikely(next == RETRY_TASK)) {
--
kernel/sched/core.c-6376-
kernel/sched/core.c:6377: p = pick_task(rq_i, rf);
kernel/sched/core.c-6378- if (unlikely(p == RETRY_TASK)) {
--
kernel/sched/core.c-6407- if (!p)
kernel/sched/core.c:6408: p = idle_sched_class.pick_task(rq_i, rf);
kernel/sched/core.c-6409- }
]
|
| 28/2 |
2026/10/13 08:25 |
llm |
expert |
0m
Model:
Tokens:
input: 21484
output: 36
thoughts: 19
Thoughts: **My Thought Process on `pick_task`**
Alright, so my first instinct is to dive right into the heart of the scheduler's decision-making process. I need to understand *how* the kernel chooses the next task to run. The most logical place to start, given the function name, is `pick_task` within `kernel/sched/core.c`. This is where the magic happens, where the scheduler weighs the options and selects the most deserving process to get CPU time. I'm expecting to find the core logic here, the algorithms and heuristics that determine preemption, fair scheduling, and potentially other policies. I'll be looking for how it iterates through runnable tasks, what metrics it uses to rank them, and what conditions trigger a task switch. Given my expertise, I anticipate a complex interplay of data structures and logic, and I'm keen to see how the kernel balances competing demands for CPU resources.
|
| 29/2 |
2026/10/13 08:26 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:6224 LineCount:20]
Results: map[Contents:6224: static inline struct task_struct *pick_task(struct rq *rq, struct rq_flags *rf)
6225: {
6226: const struct sched_class *class;
6227: struct task_struct *p;
6228:
6229: rq->dl_server = NULL;
6230:
6231: for_each_active_class(class) {
6232: p = class->pick_task(rq, rf);
6233: if (p)
6234: return p;
6235: }
6236:
6237: BUG(); /* The idle class should always have a runnable task. */
6238: }
6239:
6240: extern void task_vruntime_update(struct rq *rq, struct task_struct *p, bool in_fi);
6241:
6242: static void queue_core_balance(struct rq *rq);
6243:
]
|
| 30/2 |
2026/10/13 08:26 |
llm |
expert |
0m
Model:
Tokens:
input: 21808
output: 36
thoughts: 83
Thoughts: **My Thought Process Regarding Task Picking and Core Pick Initialization**
My initial understanding was that `pick_task` is designed to always return a valid task, throwing a `BUG()` if it fails to do so. This led me to believe that `pick_task` would never return a `NULL` pointer.
However, upon deeper reflection and re-examination, I realized there's a potential nuance I need to consider. While `pick_task` itself might guarantee a return value or a bug, the *state* it operates on could be problematic. Specifically, I'm now thinking about the `rq_i->core_pick` field. My concern is that `rq_i->core_pick` *could* legitimately be `NULL` if it was previously explicitly set to `NULL` by some other part of the system.
This means that even if `pick_task` functions as intended, it might be called in a context where `rq_i->core_pick` has already been cleared, leading to an unexpected state. I need to revisit the logic within `pick_next_task` to understand precisely how it handles or initializes `rq_i->core_pick` to ensure this potential `NULL` state doesn't lead to unexpected behavior or errors further down the line.
|
| 31/2 |
2026/10/13 08:26 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:6276 LineCount:40]
Results: map[Contents:6276: /*
6277: * If there were no {en,de}queues since we picked (IOW, the task
6278: * pointers are all still valid), and we haven't scheduled the last
6279: * pick yet, do so now.
6280: *
6281: * rq->core_pick can be NULL if no selection was made for a CPU because
6282: * it was either offline or went offline during a sibling's core-wide
6283: * selection. In this case, do a core-wide selection.
6284: */
6285: if (rq->core->core_pick_seq == rq->core->core_task_seq &&
6286: rq->core->core_pick_seq != rq->core_sched_seq &&
6287: rq->core_pick) {
6288: WRITE_ONCE(rq->core_sched_seq, rq->core->core_pick_seq);
6289:
6290: next = rq->core_pick;
6291: rq->dl_server = rq->core_dl_server;
6292: rq->core_pick = NULL;
6293: rq->core_dl_server = NULL;
6294: goto out_set_next;
6295: }
6296:
6297: prev_balance(rq, rf);
6298:
6299: smt_mask = cpu_smt_mask(cpu);
6300:
6301: restart:
6302: need_sync |= !!rq->core->core_cookie;
6303:
6304: /* reset state */
6305: rq->core->core_cookie = 0UL;
6306: if (rq->core->core_forceidle_count) {
6307: if (!core_clock_updated) {
6308: update_rq_clock(rq->core);
6309: core_clock_updated = true;
6310: }
6311: sched_core_account_forceidle(rq);
6312: /* reset after accounting force idle */
6313: rq->core->core_forceidle_start = 0;
6314: rq->core->core_forceidle_count = 0;
6315: rq->core->core_forceidle_occupation = 0;
]
|
| 32/2 |
2026/10/13 08:26 |
llm |
expert |
0m
Model:
Tokens:
input: 22572
output: 36
thoughts: 19
Thoughts: **My Thought Process**
Okay, so my primary objective right now is to understand precisely how the `core_pick` field within the `rq_i` structure gets its value. I need to delve into the code, specifically targeting the loop responsible for setting this field. My goal is to gain a comprehensive grasp of the logic that dictates this assignment, as it's crucial for understanding the overall flow and decision-making within this part of the system. I'm assuming this `rq_i` structure is a key component, likely representing a request or a queue item of some sort, and `core_pick` probably signifies the designated processing core or unit. Pinpointing the exact loop will allow me to trace the inputs, conditions, and any algorithms involved in determining this selection. This is essential for debugging, optimization, or even just for a complete mental model of how this mechanism operates.
|
| 33/2 |
2026/10/13 08:26 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:6359 LineCount:40]
Results: map[Contents:6359: /*
6360: * For each thread: do the regular task pick and find the max prio task
6361: * amongst them.
6362: *
6363: * Tie-break prio towards the current CPU
6364: */
6365: max = NULL;
6366: for_each_cpu_wrap(i, smt_mask, cpu) {
6367: rq_i = cpu_rq(i);
6368:
6369: /*
6370: * Current cpu always has its clock updated on entrance to
6371: * pick_next_task(). If the current cpu is not the core,
6372: * the core may also have been updated above.
6373: */
6374: if (i != cpu && (rq_i != rq->core || !core_clock_updated))
6375: update_rq_clock(rq_i);
6376:
6377: p = pick_task(rq_i, rf);
6378: if (unlikely(p == RETRY_TASK)) {
6379: /* rq lock may have been dropped, clocks invalidated */
6380: core_clock_updated = false;
6381: if (!(rq->clock_update_flags & RQCF_UPDATED))
6382: update_rq_clock(rq);
6383: goto restart;
6384: }
6385:
6386: rq_i->core_pick = p;
6387: rq_i->core_dl_server = rq_i->dl_server;
6388:
6389: if (!max || prio_less(max, p, fi_before))
6390: max = p;
6391: }
6392:
6393: cookie = rq->core->core_cookie = max->core_cookie;
6394:
6395: /*
6396: * For each thread: try and find a runnable task that matches @max or
6397: * force idle.
6398: */
]
|
| 34/2 |
2026/10/13 08:26 |
llm |
expert |
0m
Model:
Tokens:
input: 23201
output: 40
thoughts: 308
Thoughts: **My Thought Process Unraveling a Potential Race Condition**
Okay, so I'm looking at this `for_each_cpu_wrap(i, smt_mask, cpu)` loop, and my initial understanding is that it's iterating through all the CPUs specified by `smt_mask`. For each of these CPUs, it's calling `pick_task(rq_i, rf)` and then setting `rq_i->core_pick` to whatever `pick_task` returned, which I'm calling `p`. This seems straightforward enough.
Then, I move to the next loop, which uses `for_each_cpu(i, smt_mask)`. Here, I'm retrieving `rq_i` again and accessing `rq_i->core_pick`. The core of my concern is this: if `rq_i->core_pick` was *just* set to `p` in the preceding loop, how could it possibly be `NULL` when I access it in this second loop? This is where my mind starts to churn.
My first thought is that `pick_task` itself might be the culprit. I recall that `pick_task` can sometimes return `RETRY_TASK`. If `p` is `RETRY_TASK`, the code branches off to `goto restart`. That's a known path, so it's not the immediate source of my confusion about a `NULL` `core_pick`.
The real question, the one that's bugging me, is whether `smt_mask` could somehow change between these two loops. I see that `smt_mask` is derived from `cpu_smt_mask(cpu)`, and importantly, `cpu_smt_mask` returns a *pointer* to a cpumask. This opens up a critical possibility: what if this cpumask that `smt_mask` points to is being modified concurrently by another thread or process?
If the underlying cpumask is modified concurrently, it's conceivable that the set of CPUs iterated over in the second loop (`for_each_cpu(i, smt_mask)`) could be different from the set iterated over in the first loop (`for_each_cpu_wrap(i, smt_mask, cpu)`). Specifically, if a CPU was *not* visited in the first loop (because it wasn't in the `smt_mask` at that precise moment), but then it gets *added* to the `smt_mask` before the second loop starts, its `rq_i->core_pick` would naturally still be in its previous state. And, if that previous state was `NULL` β perhaps because the task was migrated away or the CPU was otherwise de-scheduled β then I'd indeed find a `NULL` `core_pick` in the second loop, even though the `smt_mask` appears the same on the surface. This is the race condition I'm trying to pinpoint. I need to dig into `cpu_smt_mask` to understand its concurrency implications.
|
| 35/2 |
2026/10/13 08:26 |
tool |
grepper |
0m
Args: map[Expression:cpu_smt_mask PathPrefixes:[kernel/sched/ include/linux/topology.h]]
Results: map[Output:include/linux/topology.h=172=static inline int cpu_to_mem(int cpu)
--
include/linux/topology.h-233-/*
include/linux/topology.h:234: * Defining cpu_smt_mask as cpumask_of that CPU helps to get
include/linux/topology.h-235- * rid of lot of ifdeffery all around the codebase in case of
--
include/linux/topology.h-239-#if defined(CONFIG_SCHED_SMT)
include/linux/topology.h:240:# if !defined(cpu_smt_mask)
include/linux/topology.h:241:static inline const struct cpumask *cpu_smt_mask(int cpu)
include/linux/topology.h-242-{
--
include/linux/topology.h-246-#else /* !CONFIG_SCHED_SMT */
include/linux/topology.h:247:static inline const struct cpumask *cpu_smt_mask(int cpu)
include/linux/topology.h-248-{
--
kernel/sched/core.c=405=static void sched_core_lock(int cpu, unsigned long *flags)
--
kernel/sched/core.c-408-{
kernel/sched/core.c:409: const struct cpumask *smt_mask = cpu_smt_mask(cpu);
kernel/sched/core.c-410- int t, i = 0;
--
kernel/sched/core.c=417=static void sched_core_unlock(int cpu, unsigned long *flags)
--
kernel/sched/core.c-420-{
kernel/sched/core.c:421: const struct cpumask *smt_mask = cpu_smt_mask(cpu);
kernel/sched/core.c-422- int t;
--
kernel/sched/core.c=429=static void __sched_core_flip(bool enabled)
--
kernel/sched/core.c-440- for_each_cpu(cpu, &sched_core_mask) {
kernel/sched/core.c:441: const struct cpumask *smt_mask = cpu_smt_mask(cpu);
kernel/sched/core.c-442-
--
kernel/sched/core.c=6245=pick_next_task(struct rq *rq, struct rq_flags *rf)
--
kernel/sched/core.c-6298-
kernel/sched/core.c:6299: smt_mask = cpu_smt_mask(cpu);
kernel/sched/core.c-6300-
--
kernel/sched/core.c=6609=static void sched_core_cpu_starting(unsigned int cpu)
kernel/sched/core.c-6610-{
kernel/sched/core.c:6611: const struct cpumask *smt_mask = cpu_smt_mask(cpu);
kernel/sched/core.c-6612- struct rq *rq = cpu_rq(cpu), *core_rq = NULL;
--
kernel/sched/core.c=6648=static void sched_core_cpu_deactivate(unsigned int cpu)
kernel/sched/core.c-6649-{
kernel/sched/core.c:6650: const struct cpumask *smt_mask = cpu_smt_mask(cpu);
kernel/sched/core.c-6651- struct rq *rq = cpu_rq(cpu), *core_rq = NULL;
--
kernel/sched/core.c=8715=static inline void sched_smt_present_inc(int cpu)
kernel/sched/core.c-8716-{
kernel/sched/core.c:8717: if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
kernel/sched/core.c-8718- static_branch_inc_cpuslocked(&sched_smt_present);
--
kernel/sched/core.c=8721=static inline void sched_smt_present_dec(int cpu)
kernel/sched/core.c-8722-{
kernel/sched/core.c:8723: if (cpumask_weight(cpu_smt_mask(cpu)) == 2)
kernel/sched/core.c-8724- static_branch_dec_cpuslocked(&sched_smt_present);
--
kernel/sched/core_sched.c=242=void __sched_core_account_forceidle(struct rq *rq)
kernel/sched/core_sched.c-243-{
kernel/sched/core_sched.c:244: const struct cpumask *smt_mask = cpu_smt_mask(cpu_of(rq));
kernel/sched/core_sched.c-245- u64 delta, now = rq_clock(rq->core);
--
kernel/sched/ext/idle.c=80=static bool scx_idle_test_and_clear_cpu(int cpu)
--
kernel/sched/ext/idle.c-90- if (sched_smt_active()) {
kernel/sched/ext/idle.c:91: const struct cpumask *smt = cpu_smt_mask(cpu);
kernel/sched/ext/idle.c-92- struct cpumask *idle_smts = idle_cpumask(node)->smt;
--
kernel/sched/ext/idle.c=458=s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
--
kernel/sched/ext/idle.c-631- if (sched_smt_active()) {
kernel/sched/ext/idle.c:632: for_each_cpu_and(cpu, cpu_smt_mask(prev_cpu), allowed) {
kernel/sched/ext/idle.c-633- if (cpu == prev_cpu)
--
kernel/sched/ext/idle.c=711=static void update_builtin_idle(int cpu, bool idle)
--
kernel/sched/ext/idle.c-718- if (sched_smt_active()) {
kernel/sched/ext/idle.c:719: const struct cpumask *smt = cpu_smt_mask(cpu);
kernel/sched/ext/idle.c-720- struct cpumask *idle_smts = idle_cpumask(node)->smt;
--
kernel/sched/fair.c=2404=static inline bool is_core_idle(int cpu)
--
kernel/sched/fair.c-2407-
kernel/sched/fair.c:2408: for_each_cpu(sibling, cpu_smt_mask(cpu)) {
kernel/sched/fair.c-2409- if (cpu == sibling)
--
kernel/sched/fair.c=8821=void __update_idle_core(struct rq *rq)
--
kernel/sched/fair.c-8829-
kernel/sched/fair.c:8830: for_each_cpu(cpu, cpu_smt_mask(core)) {
kernel/sched/fair.c-8831- if (cpu == core)
--
kernel/sched/fair.c=8849=static int select_idle_core(struct task_struct *p, int core, struct cpumask *cpus, int *idle_cpu)
--
kernel/sched/fair.c-8853-
kernel/sched/fair.c:8854: for_each_cpu(cpu, cpu_smt_mask(core)) {
kernel/sched/fair.c-8855- if (!available_idle_cpu(cpu)) {
--
kernel/sched/fair.c-8873-
kernel/sched/fair.c:8874: cpumask_andnot(cpus, cpus, cpu_smt_mask(core));
kernel/sched/fair.c-8875- return -1;
--
kernel/sched/fair.c=8881=static int select_idle_smt(struct task_struct *p, struct sched_domain *sd, int target)
--
kernel/sched/fair.c-8884-
kernel/sched/fair.c:8885: for_each_cpu_and(cpu, cpu_smt_mask(target), p->cpus_ptr) {
kernel/sched/fair.c-8886- if (cpu == target)
--
kernel/sched/fair.c=13617=static int should_we_balance(struct lb_env *env)
--
kernel/sched/fair.c-13663- */
kernel/sched/fair.c:13664: cpumask_andnot(swb_cpus, swb_cpus, cpu_smt_mask(cpu));
kernel/sched/fair.c-13665- continue;
--
kernel/sched/fair.c=14320=static inline int find_new_ilb(void)
--
kernel/sched/fair.c-14341- if (sched_smt_active() && fallback >= 0)
kernel/sched/fair.c:14342: cpumask_andnot(ilb_cpus, ilb_cpus, cpu_smt_mask(ilb_cpu));
kernel/sched/fair.c-14343- continue;
--
kernel/sched/fair.c-14361- cpumask_andnot(ilb_cpus, ilb_cpus,
kernel/sched/fair.c:14362: cpu_smt_mask(ilb_cpu));
kernel/sched/fair.c-14363- continue;
--
kernel/sched/sched.h=1535=static inline bool sched_core_cookie_match(struct rq *rq, struct task_struct *p)
--
kernel/sched/sched.h-1546-
kernel/sched/sched.h:1547: for_each_cpu(cpu, cpu_smt_mask(cpu_of(rq))) {
kernel/sched/sched.h-1548- if (!available_idle_cpu(cpu)) {
--
kernel/sched/topology.c=1576=static void init_sched_groups_capacity(int cpu, struct sched_domain *sd)
--
kernel/sched/topology.c-1590- cores++;
kernel/sched/topology.c:1591: cpumask_andnot(mask, mask, cpu_smt_mask(cpu));
kernel/sched/topology.c-1592- }
--
kernel/sched/topology.c=2037=const struct cpumask *tl_smt_mask(struct sched_domain_topology_level *tl, int cpu)
kernel/sched/topology.c-2038-{
kernel/sched/topology.c:2039: return cpu_smt_mask(cpu);
kernel/sched/topology.c-2040-}
]
|
| 36/2 |
2026/10/13 08:26 |
llm |
expert |
0m
Model:
Tokens:
input: 26289
output: 37
thoughts: 224
Thoughts: **A Deep Dive into `cpu_smt_mask` and Potential Concurrency Concerns**
My initial thought is about the `cpu_smt_mask` function. I know it's defined in `include/linux/topology.h` and its primary purpose is to return a pointer to a `cpumask`. My attention is drawn to how this `cpumask` is fetched and then subsequently used in two different loop constructs.
Specifically, I see this pattern:
```c
smt_mask = cpu_smt_mask(cpu);
```
followed by:
```c
for_each_cpu_wrap(i, smt_mask, cpu) {
// ... operations ...
}
```
and then, further down:
```c
for_each_cpu(i, smt_mask) {
// ... operations ...
}
```
This leads me to a crucial question: is it conceivable that the `smt_mask` could be modified concurrently between these two loop invocations? My immediate intuition is that this is unlikely. The `smt_mask` is typically a static, per-CPU `cpumask`, such as `topology_sibling_cpumask`. This suggests it's pre-defined and generally immutable once initialized for a given CPU.
However, a nagging doubt persists. I need to consider the behavior of `for_each_cpu_wrap` more closely. If this specific macro, defined in `include/linux/cpumask.h`, doesn't guarantee that it iterates over *all* CPUs present within the `smt_mask` it receives, then there might be a subtle divergence in the execution path. This could, in turn, lead to an incomplete or unexpected outcome if the subsequent `for_each_cpu` loop *does* process the entire `smt_mask`. My focus is on ensuring that the sets of CPUs visited by both loops are consistent, and that any potential for `smt_mask` mutation doesn't go unnoticed.
|
| 37/2 |
2026/10/13 08:26 |
tool |
grepper |
0m
Args: map[Expression:for_each_cpu_wrap PathPrefixes:[include/linux/cpumask.h]]
Results: map[Output:include/linux/cpumask.h=368=unsigned int cpumask_random(const struct cpumask *src)
--
include/linux/cpumask.h-383-/**
include/linux/cpumask.h:384: * for_each_cpu_wrap - iterate over every cpu in a mask, starting at a specified location
include/linux/cpumask.h-385- * @cpu: the (optionally unsigned) integer iterator
--
include/linux/cpumask.h-392- */
include/linux/cpumask.h:393:#define for_each_cpu_wrap(cpu, mask, start) \
include/linux/cpumask.h-394- for_each_set_bit_wrap(cpu, cpumask_bits(mask), small_cpumask_bits, start)
--
include/linux/cpumask.h=1123=extern const DECLARE_BITMAP(cpu_all_bits, NR_CPUS);
--
include/linux/cpumask.h-1145-#define for_each_possible_cpu_wrap(cpu, start) \
include/linux/cpumask.h:1146: for_each_cpu_wrap((cpu), cpu_possible_mask, (start))
include/linux/cpumask.h-1147-#define for_each_online_cpu_wrap(cpu, start) \
include/linux/cpumask.h:1148: for_each_cpu_wrap((cpu), cpu_online_mask, (start))
include/linux/cpumask.h-1149-#endif
]
|
| 38/2 |
2026/10/13 08:26 |
llm |
expert |
0m
Model:
Tokens:
input: 26948
output: 34
thoughts: 1164
Thoughts: **My Thought Process on `pick_next_task` and CPU Masks**
Okay, so I'm digging into this `pick_next_task` function, specifically how it iterates over CPUs and picks tasks. My initial understanding is that `for_each_cpu_wrap` is designed to iterate through all the CPUs specified in `smt_mask`, starting from a given point and wrapping around. The key takeaway here, as I see it, is that it *should* visit the exact same set of CPUs as a standard `for_each_cpu` loop over the same mask, just in a potentially different sequence.
Then I hit a snag, or rather, a potential problem: what happens if `pick_task` returns `RETRY_TASK`? Looking at the code snippet provided, it's clear that if `RETRY_TASK` is returned, the execution jumps to a `restart` label. This `restart` action implies a complete re-evaluation of the `smt_mask` and a fresh start on the first loop. This seems like a robust way to handle situations where the state might have changed mid-operation, forcing a re-assessment.
My next thought is about the `rq_i->core_pick` field. I'm wondering if this field could somehow become `NULL`. Could `pick_task` itself be the culprit, returning `NULL`? I quickly check the definition of `pick_task` and see that it has a `BUG()` call if it *doesn't* return a task. This implies that `pick_task` itself won't return `NULL` in the sense of a failed lookup that proceeds. However, I notice that `class->pick_task` *can* return `NULL`. But then again, the `BUG()` statement implies that the idle class *should* always provide a runnable task. So, the possibility of `pick_task` returning `NULL` for all classes seems to be ruled out by that `BUG()`.
My thinking then shifts to error pointers. `pick_task` can return `RETRY_TASK`, which I recognize as an error pointer (`(struct task_struct *)-1ul`). This is distinct from a `NULL` return.
The next crucial point of contemplation is concurrent modification. What if `rq_i->core_pick` is being altered by another process while I'm working within this core's locks? I look back at the main `for_each_cpu_wrap` loop and see that the code is holding `rq` locks for all CPUs within the core. This suggests that concurrent modification of `rq_i->core_pick` *within the core* is likely prevented by these locks.
Then I focus on the second loop, the one using `for_each_cpu(i, smt_mask)`. Here, I retrieve `p = rq_i->core_pick`. The condition `if (!cookie_equals(p, cookie))` implies that if `p` is `NULL` at this point, it means `rq_i->core_pick` was `NULL`. This brings me back to the original question: how could `rq_i->core_pick` become `NULL` if `pick_task` doesn't return `NULL` (except for `RETRY_TASK`, which leads to `restart`)?
This leads me to a critical suspicion: the `smt_mask` itself. I observe that `smt_mask` is evaluated once at the beginning: `smt_mask = cpu_smt_mask(cpu);`. Both loops use this *same* pointer. My concern is that while the pointer remains the same, the underlying bitmask that `smt_mask` points to *could* be modified concurrently. If the cpumask bits change between the `for_each_cpu_wrap` and the `for_each_cpu` loop, then the second loop might iterate over a CPU that wasn't initially in the `smt_mask` (or vice-versa), and that CPU's `rq_i->core_pick` might legitimately be `NULL` if it wasn't selected in the first loop.
The core of my concern is whether `cpu_smt_mask` is protected from concurrent modification. I know that CPU hotplug operations can modify these masks, and I'm trying to ascertain if holding the `rq` locks for the core's CPUs is sufficient to prevent such modifications during the execution of `pick_next_task`. This is where I would then proceed to investigate where `cpu_smt_mask` is modified to understand the protection mechanisms.
|
| 39/2 |
2026/10/13 08:26 |
tool |
grepper |
0m
Args: map[Expression:topology_sibling_cpumask PathPrefixes:[arch/x86/]]
Results: map[Output:arch/x86/events/amd/power.c=223=static int power_cpu_exit(unsigned int cpu)
--
arch/x86/events/amd/power.c-234- */
arch/x86/events/amd/power.c:235: target = cpumask_any_but(topology_sibling_cpumask(cpu), cpu);
arch/x86/events/amd/power.c-236- if (target < nr_cpumask_bits) {
--
arch/x86/events/amd/power.c=243=static int power_cpu_init(unsigned int cpu)
--
arch/x86/events/amd/power.c-255- */
arch/x86/events/amd/power.c:256: target = cpumask_any_but(topology_sibling_cpumask(cpu), cpu);
arch/x86/events/amd/power.c-257- if (target >= nr_cpumask_bits)
--
arch/x86/events/intel/core.c=6428=static void intel_pmu_cpu_starting(int cpu)
--
arch/x86/events/intel/core.c-6482- if (!(x86_pmu.flags & PMU_FL_NO_HT_SHARING)) {
arch/x86/events/intel/core.c:6483: for_each_cpu(i, topology_sibling_cpumask(cpu)) {
arch/x86/events/intel/core.c-6484- struct intel_shared_regs *pc;
--
arch/x86/events/intel/core.c-6500- if (x86_pmu.flags & PMU_FL_EXCL_CNTRS) {
arch/x86/events/intel/core.c:6501: for_each_cpu(i, topology_sibling_cpumask(cpu)) {
arch/x86/events/intel/core.c-6502- struct cpu_hw_events *sibling;
--
arch/x86/include/asm/topology.h=192=static inline int topology_get_logical_id(u32 apicid, enum x86_topology_domains at_level)
--
arch/x86/include/asm/topology.h-202-#define topology_core_cpumask(cpu) (per_cpu(cpu_core_map, cpu))
arch/x86/include/asm/topology.h:203:#define topology_sibling_cpumask(cpu) (per_cpu(cpu_sibling_map, cpu))
arch/x86/include/asm/topology.h-204-
--
arch/x86/kernel/cpu/microcode/core.c=438=static void __load_primary(unsigned int cpu)
arch/x86/kernel/cpu/microcode/core.c-439-{
arch/x86/kernel/cpu/microcode/core.c:440: struct cpumask *secondaries = topology_sibling_cpumask(cpu);
arch/x86/kernel/cpu/microcode/core.c-441- enum sibling_ctrl ctrl;
--
arch/x86/kernel/cpu/microcode/core.c=698=static bool setup_cpus(void)
--
arch/x86/kernel/cpu/microcode/core.c-736- */
arch/x86/kernel/cpu/microcode/core.c:737: ctrl.ctrl_cpu = cpumask_first(topology_sibling_cpumask(cpu));
arch/x86/kernel/cpu/microcode/core.c-738- per_cpu(ucode_ctrl, cpu) = ctrl;
--
arch/x86/kernel/process.c=528=void speculative_store_bypass_ht_init(void)
--
arch/x86/kernel/process.c-548- */
arch/x86/kernel/process.c:549: for_each_cpu(cpu, topology_sibling_cpumask(this_cpu)) {
arch/x86/kernel/process.c-550- if (cpu == this_cpu)
--
arch/x86/kernel/smpboot.c=694=void set_cpu_sibling_map(int cpu)
--
arch/x86/kernel/smpboot.c-704- if (!has_mp) {
arch/x86/kernel/smpboot.c:705: cpumask_set_cpu(cpu, topology_sibling_cpumask(cpu));
arch/x86/kernel/smpboot.c-706- cpumask_set_cpu(cpu, cpu_llc_shared_mask(cpu));
--
arch/x86/kernel/smpboot.c-720- if ((i == cpu) || (has_smt && match_smt(c, o)))
arch/x86/kernel/smpboot.c:721: link_mask(topology_sibling_cpumask, cpu, i);
arch/x86/kernel/smpboot.c-722-
--
arch/x86/kernel/smpboot.c-732-
arch/x86/kernel/smpboot.c:733: threads = cpumask_weight(topology_sibling_cpumask(cpu));
arch/x86/kernel/smpboot.c-734- if (threads > __max_smt_threads)
--
arch/x86/kernel/smpboot.c-736-
arch/x86/kernel/smpboot.c:737: for_each_cpu(i, topology_sibling_cpumask(cpu))
arch/x86/kernel/smpboot.c-738- cpu_data(i).smt_active = threads > 1;
--
arch/x86/kernel/smpboot.c-741- * This needs a separate iteration over the cpus because we rely on all
arch/x86/kernel/smpboot.c:742: * topology_sibling_cpumask links to be set-up.
arch/x86/kernel/smpboot.c-743- */
--
arch/x86/kernel/smpboot.c-758- if (cpumask_first(
arch/x86/kernel/smpboot.c:759: topology_sibling_cpumask(i)) == i)
arch/x86/kernel/smpboot.c-760- c->booted_cores++;
--
arch/x86/kernel/smpboot.c=1160=static __init void disable_smp(void)
--
arch/x86/kernel/smpboot.c-1166-
arch/x86/kernel/smpboot.c:1167: cpumask_set_cpu(0, topology_sibling_cpumask(0));
arch/x86/kernel/smpboot.c-1168- cpumask_set_cpu(0, topology_core_cpumask(0));
--
arch/x86/kernel/smpboot.c=1297=static void recompute_smt_state(void)
--
arch/x86/kernel/smpboot.c-1302- for_each_online_cpu (cpu) {
arch/x86/kernel/smpboot.c:1303: int threads = cpumask_weight(topology_sibling_cpumask(cpu));
arch/x86/kernel/smpboot.c-1304-
--
arch/x86/kernel/smpboot.c=1311=static void remove_siblinginfo(int cpu)
--
arch/x86/kernel/smpboot.c-1320- */
arch/x86/kernel/smpboot.c:1321: if (cpumask_weight(topology_sibling_cpumask(cpu)) == 1)
arch/x86/kernel/smpboot.c-1322- cpu_data(sibling).booted_cores--;
--
arch/x86/kernel/smpboot.c-1327-
arch/x86/kernel/smpboot.c:1328: for_each_cpu(sibling, topology_sibling_cpumask(cpu)) {
arch/x86/kernel/smpboot.c:1329: cpumask_clear_cpu(cpu, topology_sibling_cpumask(sibling));
arch/x86/kernel/smpboot.c:1330: if (cpumask_weight(topology_sibling_cpumask(sibling)) == 1)
arch/x86/kernel/smpboot.c-1331- cpu_data(sibling).smt_active = false;
--
arch/x86/kernel/smpboot.c-1339- cpumask_clear(cpu_l2c_shared_mask(cpu));
arch/x86/kernel/smpboot.c:1340: cpumask_clear(topology_sibling_cpumask(cpu));
arch/x86/kernel/smpboot.c-1341- cpumask_clear(topology_core_cpumask(cpu));
]
|
| 40/2 |
2026/10/13 08:26 |
llm |
expert |
0m
Model:
Tokens:
input: 30513
output: 35
thoughts: 629
Thoughts: **The Inner Workings of CPU Hotplug and Scheduler Synchronization**
My current line of thought is focused on a potential race condition involving CPU hotplug operations and the scheduler's task selection mechanism. Specifically, I'm scrutinizing how `topology_sibling_cpumask` is modified within `remove_siblinginfo` during CPU hotplug events, and how this might impact the `pick_next_task` function.
My initial understanding was that `remove_siblinginfo` operates under the `cpu_hotplug_lock`. However, I'm realizing that `pick_next_task`, being part of the scheduler, doesn't run with this lock. It instead operates under the `rq->lock`, with interrupts disabled. This is where the complication arises: `pick_next_task` accesses `cpu_smt_mask`, and I need to understand if concurrent modifications to this mask, driven by hotplug, can lead to unexpected behavior.
I've traced `remove_siblinginfo` back to its origin within `smpboot.c`, triggered when a CPU goes offline. This raises a critical question: when a CPU goes offline, it ceases scheduling, but could the hotplug thread itself be acquiring its `rq->lock`? My current understanding suggests that `remove_siblinginfo` is invoked from functions like `takedown_cpu`, which execute either on the CPU that is going down or after it has already shut down. This implies that direct contention on the `rq->lock` during the `remove_siblinginfo` call itself might not be the primary concern.
The core of my concern lies in the potential for `topology_sibling_cpumask` (or the underlying `smt_mask`) to be modified while `pick_next_task` is iterating over it. If `remove_siblinginfo` clears bits in `topology_sibling_cpumask(sibling)`, then a subsequent `for_each_cpu(i, smt_mask)` loop within `pick_next_task` might encounter fewer CPUs than an earlier `for_each_cpu_wrap(i, smt_mask, cpu)` loop. My initial thought was that this reduction in visited CPUs wouldn't cause `rq_i->core_pick` to be `NULL` for the remaining CPUs, as they would have already been processed. This is because `remove_siblinginfo` only *clears* bits, it doesn't add them.
However, I then considered the opposite scenario: CPU bring-up. When a CPU comes online, `set_cpu_sibling_map` is called, which *adds* bits to the sibling mask. This is where the real danger lies. If a bit is added to the `smt_mask` *after* `for_each_cpu_wrap` has completed its iteration but *before* the subsequent `for_each_cpu` loop begins, the `for_each_cpu` loop will visit a CPU that was *not* seen by `for_each_cpu_wrap`. For this newly added CPU, `rq_i->core_pick` would not have been initialized in the first loop. It would therefore retain its previous (potentially `NULL`) value, leading to the observed problem.
My next step is to investigate the synchronization mechanisms surrounding `set_cpu_sibling_map` and its potential concurrency with `pick_next_task`. I know `pick_next_task` locks all `rq`s within a core, and it determines which `rq`s to lock using the `smt_mask`. The critical question is: how does it know to *wait* or synchronize if the `smt_mask` is being modified concurrently by `set_cpu_sibling_map`? This leads me to examine the `sched_core_lock` and its role in this synchronization.
|
| 41/2 |
2026/10/13 08:27 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:405 LineCount:20]
Results: map[Contents: 405: static void sched_core_lock(int cpu, unsigned long *flags)
406: __context_unsafe(/* acquires multiple */)
407: __acquires(&runqueues.__lock) /* overapproximation */
408: {
409: const struct cpumask *smt_mask = cpu_smt_mask(cpu);
410: int t, i = 0;
411:
412: local_irq_save(*flags);
413: for_each_cpu(t, smt_mask)
414: raw_spin_lock_nested(&cpu_rq(t)->__lock, i++);
415: }
416:
417: static void sched_core_unlock(int cpu, unsigned long *flags)
418: __context_unsafe(/* releases multiple */)
419: __releases(&runqueues.__lock) /* overapproximation */
420: {
421: const struct cpumask *smt_mask = cpu_smt_mask(cpu);
422: int t;
423:
424: for_each_cpu(t, smt_mask)
]
|
| 42/2 |
2026/10/13 08:27 |
llm |
expert |
0m
Model:
Tokens:
input: 31495
output: 36
thoughts: 165
Thoughts: **My Thought Process Regarding `sched_core_lock` and Task Selection**
Okay, so my initial thought is that `sched_core_lock` is designed to lock *all* the run queues (`rq`s) within the SMT mask. This immediately raises a flag in my mind: what happens if this `smt_mask` β the set of CPUs considered part of the same SMT core β changes *after* `sched_core_lock` has been acquired but *before* `pick_next_task` completes, or even *during* `pick_next_task` itself? That seems like a potential race condition, a classic scenario where state can be inconsistent, leading to unpredictable behavior.
But then I pause and re-evaluate. Where is `pick_next_task` actually called from? My memory tells me it's invoked from `__schedule`. Now, `__schedule` is a critical function, and it's known to hold the run queue lock (`rq->lock`) for the *current* CPU. This is a key piece of information.
Here's where I need to be precise: does `__schedule` *also* call `sched_core_lock`? Because if it does, my initial concern about the `smt_mask` changing during `pick_next_task` would be partially mitigated, as `sched_core_lock` would, in theory, keep everything locked.
However, I'm fairly certain now that `__schedule` *doesn't* call `sched_core_lock` directly. It only acquires the `rq->lock` for the specific CPU it's currently running on. This brings me back to my original concern about the `smt_mask`.
So, the crucial question becomes: if SMT scheduling is enabled (`sched_core_enabled(rq)` returns true), does `__schedule` somehow implicitly hold *all* the `rq` locks within the SMT core, even though it only explicitly acquires the lock for the current CPU? That seems counterintuitive, but it's the only way my initial worry about the `smt_mask` changing during `pick_next_task` would be completely unfounded *if* `sched_core_lock` was the mechanism ensuring that.
To resolve this, I need to dive into the implementation of `__schedule` itself and see how it handles SMT core locking, or if it relies on something else to ensure atomicity across the core when `sched_core_enabled` is true. That's my next step: examining the `__schedule` code.
|
| 43/2 |
2026/10/13 08:27 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:6550 LineCount:40]
Results: map[Contents:6550: static bool steal_cookie_task(int cpu, struct sched_domain *sd)
6551: {
6552: int i;
6553:
6554: for_each_cpu_wrap(i, sched_domain_span(sd), cpu + 1) {
6555: if (i == cpu)
6556: continue;
6557:
6558: if (need_resched())
6559: break;
6560:
6561: if (try_steal_cookie(cpu, i))
6562: return true;
6563: }
6564:
6565: return false;
6566: }
6567:
6568: static void sched_core_balance(struct rq *rq)
6569: __must_hold(__rq_lockp(rq))
6570: {
6571: struct sched_domain *sd;
6572: int cpu = cpu_of(rq);
6573:
6574: guard(preempt)();
6575: guard(rcu)();
6576:
6577: raw_spin_rq_unlock_irq(rq);
6578: for_each_domain(cpu, sd) {
6579: if (need_resched())
6580: break;
6581:
6582: if (steal_cookie_task(cpu, sd))
6583: break;
6584: }
6585: raw_spin_rq_lock_irq(rq);
6586: }
6587:
6588: static DEFINE_PER_CPU(struct balance_callback, core_balance_head);
6589:
]
|
| 44/2 |
2026/10/13 08:27 |
llm |
expert |
0m
Model:
Tokens:
input: 32191
output: 30
thoughts: 16
Thoughts: **My Thought Process on `__schedule`**
Alright, so my immediate instinct here is to dive into the specifics of how scheduling is being handled. I need to pinpoint the exact location where `__schedule` is being invoked, and my mental map of the codebase points me towards line 7199. This isn't just a random guess; it's based on my experience with the project's architecture and common patterns for asynchronous operations. I'm anticipating that the logic for task prioritization, resource allocation, or perhaps event dispatching will reside in or around that section of code. My goal is to understand the *why* and *how* of that particular scheduling mechanism, and line 7199 feels like the most promising entry point for that investigation.
|
| 45/2 |
2026/10/13 08:27 |
tool |
grepper |
0m
Args: map[Expression:__schedule PathPrefixes:[kernel/sched/core.c]]
Results: map[Output:kernel/sched/core.c=561=int task_llc(const struct task_struct *p)
--
kernel/sched/core.c-579- *
kernel/sched/core.c:580: * Normal scheduling state is serialized by rq->lock. __schedule() takes the
kernel/sched/core.c-581- * local CPU's rq->lock, it optionally removes the task from the runqueue and
--
kernel/sched/core.c=2255=static void block_task(struct rq *rq, struct task_struct *p, unsigned long task_state)
--
kernel/sched/core.c-2267- /*
kernel/sched/core.c:2268: * __schedule() ttwu()
kernel/sched/core.c-2269- * prev_state = prev->state; if (p->on_rq && ...)
--
kernel/sched/core.c-2273- *
kernel/sched/core.c:2274: * Where __schedule() and ttwu() have matching control dependencies.
kernel/sched/core.c-2275- *
--
kernel/sched/core.c=4274=int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
--
kernel/sched/core.c-4331- *
kernel/sched/core.c:4332: * __schedule() (switch to task 'p')
kernel/sched/core.c-4333- * LOCK rq->lock smp_rmb();
--
kernel/sched/core.c-4340- * Pairs with the LOCK+smp_mb__after_spinlock() on rq->lock in
kernel/sched/core.c:4341: * __schedule(). See the comment for smp_mb__after_spinlock().
kernel/sched/core.c-4342- *
--
kernel/sched/core.c-4355- *
kernel/sched/core.c:4356: * __schedule() (switch to task 'p') try_to_wake_up()
kernel/sched/core.c-4357- * STORE p->on_cpu = 1 LOAD p->on_rq
--
kernel/sched/core.c-4359- *
kernel/sched/core.c:4360: * __schedule() (put 'p' to sleep)
kernel/sched/core.c-4361- * LOCK rq->lock smp_rmb();
--
kernel/sched/core.c-4365- * Pairs with the LOCK+smp_mb__after_spinlock() on rq->lock in
kernel/sched/core.c:4366: * __schedule(). See the comment for smp_mb__after_spinlock().
kernel/sched/core.c-4367- *
--
kernel/sched/core.c-4369- * schedule()'s block_task() has 'happened' and p will no longer
kernel/sched/core.c:4370: * care about it's own p->state. See the comment in __schedule().
kernel/sched/core.c-4371- */
--
kernel/sched/core.c-4392- * STORE p->cpu = @cpu
kernel/sched/core.c:4393: * __schedule() (switch to task 'p')
kernel/sched/core.c-4394- * LOCK rq->lock
--
kernel/sched/core.c=4447=static bool __task_needs_rq_lock(struct task_struct *p)
--
kernel/sched/core.c-4469- /*
kernel/sched/core.c:4470: * Ensure the task has finished __schedule() and will not be referenced
kernel/sched/core.c-4471- * anymore. Again, see try_to_wake_up() for a longer comment.
--
kernel/sched/core.c=5100=static inline void finish_task(struct task_struct *prev)
--
kernel/sched/core.c-5116-/*
kernel/sched/core.c:5117: * Only called from __schedule context
kernel/sched/core.c-5118- *
--
kernel/sched/core.c=5161=static void balance_push(struct rq *rq);
--
kernel/sched/core.c-5168- * that queued it (only later, when it's safe to drop rq->lock again),
kernel/sched/core.c:5169: * balance_push_callback is specifically targeted at __schedule().
kernel/sched/core.c-5170- *
--
kernel/sched/core.c=5180=__splice_balance_callbacks(struct rq *rq, bool split)
--
kernel/sched/core.c-5192- *
kernel/sched/core.c:5193: * In that case it would be possible for __schedule() to interleave
kernel/sched/core.c-5194- * and observe the list empty.
--
kernel/sched/core.c=5341=static struct rq *finish_task_switch(struct task_struct *prev)
--
kernel/sched/core.c-5353- * preempt_disable(); // 1
kernel/sched/core.c:5354: * __schedule()
kernel/sched/core.c-5355- * raw_spin_lock_irq(&rq->lock) // 2
--
kernel/sched/core.c=6059=static inline unsigned long get_preempt_disable_ip(struct task_struct *p)
--
kernel/sched/core.c-6070- */
kernel/sched/core.c:6071:static noinline void __schedule_bug(struct task_struct *prev)
kernel/sched/core.c-6072-{
--
kernel/sched/core.c=6099=static inline void schedule_debug(struct task_struct *prev, bool preempt)
--
kernel/sched/core.c-6118- if (unlikely(in_atomic_preempt_off())) {
kernel/sched/core.c:6119: __schedule_bug(prev);
kernel/sched/core.c-6120- preempt_count_set(PREEMPT_DISABLED);
--
kernel/sched/core.c=6721=pick_next_task(struct rq *rq, struct rq_flags *rf)
--
kernel/sched/core.c-6729-/*
kernel/sched/core.c:6730: * Constants for the sched_mode argument of __schedule().
kernel/sched/core.c-6731- *
--
kernel/sched/core.c-6740-/*
kernel/sched/core.c:6741: * Helper function for __schedule()
kernel/sched/core.c-6742- *
--
kernel/sched/core.c=6823=static inline void proxy_release_rq_lock(struct rq *rq, struct rq_flags *rf)
--
kernel/sched/core.c-6834- *
kernel/sched/core.c:6835: * After we later reaquire the rq lock, we will force __schedule()
kernel/sched/core.c-6836- * to pick_again, so the callbacks will get re-established.
--
kernel/sched/core.c=6863=static void proxy_migrate_task(struct rq *rq, struct rq_flags *rf,
--
kernel/sched/core.c-6876- * or cfs_rq->curr, etc after we have migrated it to another cpu,
kernel/sched/core.c:6877: * and before we pick_again in __schedule.
kernel/sched/core.c-6878- *
--
kernel/sched/core.c=6921=find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
--
kernel/sched/core.c-6954- * we don't know if only at this level. So, let's
kernel/sched/core.c:6955: * just bail out completely and let __schedule()
kernel/sched/core.c-6956- * figure things out (pick_again loop).
--
kernel/sched/core.c=7065=find_proxy_task(struct rq *rq, struct task_struct *donor, struct rq_flags *rf)
--
kernel/sched/core.c-7072-/*
kernel/sched/core.c:7073: * __schedule() is the main scheduler function.
kernel/sched/core.c-7074- *
--
kernel/sched/core.c-7110- */
kernel/sched/core.c:7111:static void __sched notrace __schedule(int sched_mode)
kernel/sched/core.c-7112-{
--
kernel/sched/core.c=7294=void __noreturn do_task_dead(void)
--
kernel/sched/core.c-7301-
kernel/sched/core.c:7302: __schedule(SM_NONE);
kernel/sched/core.c-7303- BUG();
--
kernel/sched/core.c=7347=static void sched_update_worker(struct task_struct *tsk)
--
kernel/sched/core.c-7356-
kernel/sched/core.c:7357:static __always_inline void __schedule_loop(int sched_mode)
kernel/sched/core.c-7358-{
--
kernel/sched/core.c-7360- preempt_disable();
kernel/sched/core.c:7361: __schedule(sched_mode);
kernel/sched/core.c-7362- sched_preempt_enable_no_resched();
--
kernel/sched/core.c=7366=asmlinkage __visible void __sched schedule(void)
--
kernel/sched/core.c-7375- sched_submit_work(tsk);
kernel/sched/core.c:7376: __schedule_loop(SM_NONE);
kernel/sched/core.c-7377- sched_update_worker(tsk);
--
kernel/sched/core.c=7391=void __sched schedule_idle(void)
--
kernel/sched/core.c-7401- do {
kernel/sched/core.c:7402: __schedule(SM_IDLE);
kernel/sched/core.c-7403- } while (need_resched());
--
kernel/sched/core.c=7438=void __sched notrace schedule_rtlock(void)
kernel/sched/core.c-7439-{
kernel/sched/core.c:7440: __schedule_loop(SM_RTLOCK_WAIT);
kernel/sched/core.c-7441-}
--
kernel/sched/core.c=7445=static void __sched notrace preempt_schedule_common(void)
--
kernel/sched/core.c-7462- preempt_latency_start(1);
kernel/sched/core.c:7463: __schedule(SM_PREEMPT);
kernel/sched/core.c-7464- preempt_latency_stop(1);
--
kernel/sched/core.c=7527=asmlinkage __visible void __sched notrace preempt_schedule_notrace(void)
--
kernel/sched/core.c-7555- prev_ctx = exception_enter();
kernel/sched/core.c:7556: __schedule(SM_PREEMPT);
kernel/sched/core.c-7557- exception_exit(prev_ctx);
--
kernel/sched/core.c=7594=asmlinkage __visible void __sched preempt_schedule_irq(void)
--
kernel/sched/core.c-7605- local_irq_enable();
kernel/sched/core.c:7606: __schedule(SM_PREEMPT);
kernel/sched/core.c-7607- local_irq_disable();
--
kernel/sched/core.c=7665=void rt_mutex_schedule(void)
--
kernel/sched/core.c-7667- lockdep_assert(current->sched_rt_mutex);
kernel/sched/core.c:7668: __schedule_loop(SM_NONE);
kernel/sched/core.c-7669-}
]
|
| 46/2 |
2026/10/13 08:27 |
llm |
expert |
0m
Model:
Tokens:
input: 35438
output: 36
thoughts: 18
Thoughts: **My Thought Process for Analyzing `__schedule`**
Alright, diving into `kernel/sched/core.c` to understand `__schedule`. This is where the core scheduling logic lives, so I'm expecting some intricate details about how the kernel decides which process runs next. My immediate goal is to trace the flow of execution within this function and grasp the decision-making points.
First, I'll be looking for the initial state. What conditions are we checking right at the beginning? Is this function being called because a task voluntarily yielded, because its timeslice expired, or because of some interrupt or event? Understanding the entry point will give me context for the subsequent steps.
Then, I'll focus on the actual selection of the next task. This is the heart of the scheduler. I need to see how it iterates through runnable tasks, what criteria it uses to rank them (priority, fairness, load balancing considerations?), and how it ultimately picks one. I'll be paying close attention to any data structures involved β runqueues, priority lists, etc. β and how they are accessed and modified.
Crucially, I'll be examining the context switch mechanism. Once a task is chosen, how does the kernel actually switch to it? This involves saving the state of the current task and restoring the state of the next. I'll be looking for the calls to assembly code or specific architecture-dependent functions that handle this low-level magic.
I also anticipate encountering logic related to task states. What happens to the task that *was* running? Is it put back on a runqueue, moved to a waiting state, or something else? Similarly, what about tasks that are *not* runnable? The scheduler needs to manage this lifecycle efficiently.
Finally, I'll be thinking about any edge cases or special conditions. What happens when there are no runnable tasks? Is there a "idle" task? How does the scheduler handle special kernel threads or tasks with specific scheduling policies? I'll also keep an eye out for any locking mechanisms to ensure thread-safe access to shared data structures. My aim is to get a comprehensive, step-by-step understanding of the entire `__schedule` function's execution path and its implications for system performance.
|
| 47/2 |
2026/10/13 08:27 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:7111 LineCount:40]
Results: map[Contents:7111: static void __sched notrace __schedule(int sched_mode)
7112: {
7113: struct task_struct *prev, *next;
7114: /*
7115: * On PREEMPT_RT kernel, SM_RTLOCK_WAIT is noted
7116: * as a preemption by schedule_debug() and RCU.
7117: */
7118: bool preempt = sched_mode > SM_NONE;
7119: bool is_switch = false;
7120: unsigned long *switch_count;
7121: unsigned long prev_state;
7122: struct rq_flags rf;
7123: struct rq *rq;
7124: int cpu;
7125:
7126: /* Trace preemptions consistently with task switches */
7127: trace_sched_entry_tp(sched_mode == SM_PREEMPT);
7128:
7129: cpu = smp_processor_id();
7130: rq = cpu_rq(cpu);
7131: prev = rq->curr;
7132:
7133: schedule_debug(prev, preempt);
7134:
7135: klp_sched_try_switch(prev);
7136:
7137: local_irq_disable();
7138: rcu_note_context_switch(preempt);
7139: migrate_disable_switch(rq, prev);
7140:
7141: /*
7142: * Make sure that signal_pending_state()->signal_pending() below
7143: * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
7144: * done by the caller to avoid the race with signal_wake_up():
7145: *
7146: * __set_current_state(@state) signal_wake_up()
7147: * schedule() set_tsk_thread_flag(p, TIF_SIGPENDING)
7148: * wake_up_state(p, state)
7149: * LOCK rq->lock LOCK p->pi_state
7150: * smp_mb__after_spinlock() smp_mb__after_spinlock()
]
|
| 48/2 |
2026/10/13 08:27 |
llm |
expert |
0m
Model:
Tokens:
input: 36112
output: 36
thoughts: 11
Thoughts: **My Thought Process for Scheduling**
Alright, my immediate focus is on delving into the remainder of the `__schedule` section. I need to understand its entirety to get a complete picture of what's involved.
|
| 49/2 |
2026/10/13 08:27 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:7151 LineCount:40]
Results: map[Contents:7151: * if (signal_pending_state()) if (p->state & @state)
7152: *
7153: * Also, the membarrier system call requires a full memory barrier
7154: * after coming from user-space, before storing to rq->curr; this
7155: * barrier matches a full barrier in the proximity of the membarrier
7156: * system call exit.
7157: */
7158: rq_lock(rq, &rf);
7159: smp_mb__after_spinlock();
7160:
7161: hrtick_schedule_enter(rq);
7162:
7163: /* Promote REQ to ACT */
7164: rq->clock_update_flags <<= 1;
7165: update_rq_clock(rq);
7166: rq->clock_update_flags = RQCF_UPDATED;
7167:
7168: switch_count = &prev->nivcsw;
7169:
7170: /* Task state changes only considers SM_PREEMPT as preemption */
7171: preempt = sched_mode == SM_PREEMPT;
7172:
7173: /*
7174: * We must load prev->state once (task_struct::state is volatile), such
7175: * that we form a control dependency vs deactivate_task() below.
7176: */
7177: prev_state = READ_ONCE(prev->__state);
7178: if (sched_mode == SM_IDLE) {
7179: /* SCX must consult the BPF scheduler to tell if rq is empty */
7180: if (!rq->nr_running && !scx_enabled()) {
7181: next = prev;
7182: rq->next_class = &idle_sched_class;
7183: goto picked;
7184: }
7185: } else if (!preempt && prev_state) {
7186: /*
7187: * We pass task_is_blocked() as the should_block arg
7188: * in order to keep mutex-blocked tasks on the runqueue
7189: * for slection with proxy-exec (without proxy-exec
7190: * task_is_blocked() will always be false).
]
|
| 50/2 |
2026/10/13 08:27 |
llm |
expert |
0m
Model:
Tokens:
input: 36788
output: 36
thoughts: 11
Thoughts: **My Thought Process for Scheduling**
Alright, my immediate focus is on delving into the remainder of the `__schedule` section. I need to understand its entirety to get a complete picture of what's involved.
|
| 51/2 |
2026/10/13 08:27 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:7191 LineCount:40]
Results: map[Contents:7191: */
7192: try_to_block_task(rq, prev, &prev_state,
7193: !task_is_blocked(prev));
7194: switch_count = &prev->nvcsw;
7195: }
7196:
7197: pick_again:
7198: assert_balance_callbacks_empty(rq);
7199: next = pick_next_task(rq, &rf);
7200: rq->next_class = next->sched_class;
7201: if (sched_proxy_exec()) {
7202: struct task_struct *prev_donor = rq->donor;
7203:
7204: rq_set_donor(rq, next);
7205: next->blocked_donor = NULL;
7206: if (unlikely(next->is_blocked)) {
7207: next = find_proxy_task(rq, next, &rf);
7208: if (!next) {
7209: zap_balance_callbacks(rq);
7210: goto pick_again;
7211: }
7212: if (next == rq->idle) {
7213: zap_balance_callbacks(rq);
7214: goto keep_resched;
7215: }
7216: }
7217: if (rq->donor == prev_donor && prev != next) {
7218: struct task_struct *donor = rq->donor;
7219: /*
7220: * When transitioning like:
7221: *
7222: * prev next
7223: * donor: B B
7224: * curr: A B or C
7225: *
7226: * then put_prev_set_next_task() will not have done
7227: * anything, since B == B. However, A might have
7228: * missed a RT/DL balance opportunity due to being
7229: * on_cpu.
7230: */
]
|
| 52/2 |
2026/10/13 08:27 |
llm |
expert |
0m
Model:
Tokens:
input: 37403
output: 36
thoughts: 93
Thoughts: **The Scheduling Conundrum**
My mind is racing as I dive into the scheduler's inner workings. I've just noticed that `__schedule` invokes `pick_next_task(rq, &rf)`, and I'm keenly aware that `__schedule` is holding the lock for the *current* CPU. This immediately raises a flag because I know `pick_next_task` can potentially engage in core-wide scheduling if `sched_core_enabled(rq)` evaluates to true. The critical question forming in my thoughts is this: does `pick_next_task`, in its core-wide capacity, acquire the locks for the *other* CPUs within that core? I need to revisit the `pick_next_task` function's implementation immediately to confirm this crucial detail.
|
| 53/2 |
2026/10/13 08:27 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:6245 LineCount:40]
Results: map[Contents:6245: pick_next_task(struct rq *rq, struct rq_flags *rf)
6246: __must_hold(__rq_lockp(rq))
6247: {
6248: struct task_struct *next, *p, *max;
6249: const struct cpumask *smt_mask;
6250: bool fi_before = false;
6251: bool core_clock_updated = (rq == rq->core);
6252: unsigned long cookie;
6253: int i, cpu, occ = 0;
6254: struct rq *rq_i;
6255: bool need_sync = false;
6256:
6257: if (!sched_core_enabled(rq))
6258: return __pick_next_task(rq, rf);
6259:
6260: cpu = cpu_of(rq);
6261:
6262: /* Stopper task is switching into idle, no need core-wide selection. */
6263: if (cpu_is_offline(cpu)) {
6264: /*
6265: * Reset core_pick so that we don't enter the fastpath when
6266: * coming online. core_pick would already be migrated to
6267: * another cpu during offline.
6268: */
6269: rq->core_pick = NULL;
6270: rq->core_dl_server = NULL;
6271: return __pick_next_task(rq, rf);
6272: }
6273:
6274: rq->core->core_pick_in_flight++;
6275:
6276: /*
6277: * If there were no {en,de}queues since we picked (IOW, the task
6278: * pointers are all still valid), and we haven't scheduled the last
6279: * pick yet, do so now.
6280: *
6281: * rq->core_pick can be NULL if no selection was made for a CPU because
6282: * it was either offline or went offline during a sibling's core-wide
6283: * selection. In this case, do a core-wide selection.
6284: */
]
|
| 54/2 |
2026/10/13 08:27 |
llm |
expert |
0m
Model:
Tokens:
input: 38144
output: 28
thoughts: 116
Thoughts: **My Thought Process Regarding Task Scheduling and Locking**
My initial thought process was triggered by a seemingly innocuous observation: the `pick_next_task` function, a critical component in our scheduling logic, appears to operate without explicit lock acquisition. This immediately raised a red flag because, in a multi-core environment, such a function *must* assume that the necessary locks protecting all CPUs within a core are already held by the caller. The implication is that the responsibility for acquiring these core-wide locks rests *outside* of `pick_next_task` itself.
This led me to question the underlying assumption. I needed to verify if this assumption was indeed valid. My next step was to trace the locking mechanism. I started by investigating where `sched_core_lock` is acquired, or more broadly, how `__schedule` (the function that likely orchestrates the actual scheduling decisions) obtains the necessary locks for its operation.
To my surprise, my initial check revealed that `__schedule` *itself* doesn't directly call `sched_core_lock`. This presented a bit of a puzzle. If `__schedule` isn't explicitly acquiring these core-level locks, and `pick_next_task` relies on them being held, then where *is* the locking happening?
My attention then shifted to `rq_lock` within `__schedule`. I hypothesized that perhaps `rq_lock` is the key. Specifically, I wondered if `rq_lock` might acquire *all* the necessary core locks under certain conditions, particularly if `sched_core_enabled` is true. This would provide the missing link, explaining how the locks are secured before `pick_next_task` is called. Therefore, my next logical step was to delve into the implementation of `rq_lock` to understand its behavior and confirm or deny this hypothesis.
|
| 55/2 |
2026/10/13 08:28 |
tool |
grepper |
0m
Args: map[Expression:rq_lock PathPrefixes:[kernel/sched/]]
Results: map[Output:
Full output is too long, showing 200 out of 1476 lines.
Use more precise expression or PathPrefixes if possible.
Number of matching lines per file (18 files in total):
kernel/sched/core.c:110
kernel/sched/core_sched.c:1
kernel/sched/cpuacct.c:2
kernel/sched/cputime.c:1
kernel/sched/deadline.c:10
kernel/sched/debug.c:2
kernel/sched/ext/ext.c:54
kernel/sched/ext/idle.c:1
kernel/sched/ext/internal.h:2
kernel/sched/ext/sub.c:10
kernel/sched/fair.c:24
kernel/sched/idle.c:1
kernel/sched/psi.c:7
kernel/sched/rt.c:4
kernel/sched/sched.h:119
kernel/sched/stats.h:1
kernel/sched/syscalls.c:5
kernel/sched/topology.c:1
kernel/sched/core.c=372=static struct task_struct *sched_core_find(struct rq *rq, unsigned long cookie)
--
kernel/sched/core.c-390- *
kernel/sched/core.c:391: * raw_spin_rq_lock(rq);
kernel/sched/core.c-392- * ...
--
kernel/sched/core.c=429=static void __sched_core_flip(bool enabled)
--
kernel/sched/core.c-447- * released by a lock-dropping ->pick_task(). Flipping would
kernel/sched/core.c:448: * rebind rq_lockp() under it. Wait it out.
kernel/sched/core.c-449- */
--
kernel/sched/core.c=561=int task_llc(const struct task_struct *p)
--
kernel/sched/core.c-594- *
kernel/sched/core.c:595: * System-calls and anything external will use task_rq_lock() which acquires
kernel/sched/core.c-596- * both p->pi_lock and rq->lock. As a consequence the state they change is
--
kernel/sched/core.c-620- * ON_RQ_MIGRATING state is used for migration without holding both
kernel/sched/core.c:621: * rq->locks. It indicates task_cpu() is not stable, see task_rq_lock().
kernel/sched/core.c-622- *
--
kernel/sched/core.c-654- * - for migration called under rq->lock:
kernel/sched/core.c:655: * [ see task_on_rq_migrating() in task_rq_lock() ]
kernel/sched/core.c-656- *
--
kernel/sched/core.c-659- *
kernel/sched/core.c:660: * - for migration called under double_rq_lock():
kernel/sched/core.c-661- *
--
kernel/sched/core.c-668-
kernel/sched/core.c:669:void raw_spin_rq_lock_nested(struct rq *rq, int subclass)
kernel/sched/core.c-670- __context_unsafe()
--
kernel/sched/core.c-683- for (;;) {
kernel/sched/core.c:684: lock = __rq_lockp(rq);
kernel/sched/core.c-685- raw_spin_lock_nested(lock, subclass);
kernel/sched/core.c:686: if (likely(lock == __rq_lockp(rq))) {
kernel/sched/core.c-687- /* preempt_count *MUST* be > 1 */
--
kernel/sched/core.c=695=bool raw_spin_rq_trylock(struct rq *rq)
--
kernel/sched/core.c-709- for (;;) {
kernel/sched/core.c:710: lock = __rq_lockp(rq);
kernel/sched/core.c-711- ret = raw_spin_trylock(lock);
kernel/sched/core.c:712: if (!ret || (likely(lock == __rq_lockp(rq)))) {
kernel/sched/core.c-713- preempt_enable();
--
kernel/sched/core.c-720-/*
kernel/sched/core.c:721: * double_rq_lock - safely lock two runqueues
kernel/sched/core.c-722- */
kernel/sched/core.c:723:void double_rq_lock(struct rq *rq1, struct rq *rq2)
kernel/sched/core.c-724-{
--
kernel/sched/core.c-729-
kernel/sched/core.c:730: raw_spin_rq_lock(rq1);
kernel/sched/core.c:731: if (__rq_lockp(rq1) != __rq_lockp(rq2))
kernel/sched/core.c:732: raw_spin_rq_lock_nested(rq2, SINGLE_DEPTH_NESTING);
kernel/sched/core.c-733- else
kernel/sched/core.c:734: __acquire_ctx_lock(__rq_lockp(rq2)); /* fake acquire */
kernel/sched/core.c-735-
--
kernel/sched/core.c-739-/*
kernel/sched/core.c:740: * ___task_rq_lock - lock the rq @p resides on.
kernel/sched/core.c-741- */
kernel/sched/core.c:742:struct rq *___task_rq_lock(struct task_struct *p, struct rq_flags *rf)
kernel/sched/core.c-743-{
--
kernel/sched/core.c-749- rq = task_rq(p);
kernel/sched/core.c:750: raw_spin_rq_lock(rq);
kernel/sched/core.c-751- if (likely(rq == task_rq(p) && !task_on_rq_migrating(p))) {
--
kernel/sched/core.c-762-/*
kernel/sched/core.c:763: * task_rq_lock - lock p->pi_lock and lock the rq @p resides on.
kernel/sched/core.c-764- */
kernel/sched/core.c:765:struct rq *_task_rq_lock(struct task_struct *p, struct rq_flags *rf)
kernel/sched/core.c-766-{
--
kernel/sched/core.c-771- rq = task_rq(p);
kernel/sched/core.c:772: raw_spin_rq_lock(rq);
kernel/sched/core.c-773- /*
kernel/sched/core.c:774: * move_queued_task() task_rq_lock()
kernel/sched/core.c-775- *
--
kernel/sched/core.c-782- *
kernel/sched/core.c:783: * If we observe the old CPU in task_rq_lock(), the acquire of
kernel/sched/core.c-784- * the old rq->lock will fully serialize against the stores.
kernel/sched/core.c-785- *
kernel/sched/core.c:786: * If we observe the new CPU in task_rq_lock(), the address
kernel/sched/core.c-787- * dependency headed by '[L] rq = task_rq()' and the acquire
--
kernel/sched/core.c=917=static enum hrtimer_restart hrtick(struct hrtimer *timer)
--
kernel/sched/core.c-923-
kernel/sched/core.c:924: rq_lock(rq, &rf);
kernel/sched/core.c-925- update_rq_clock(rq);
--
kernel/sched/core.c=955=static void __hrtick_start(void *arg)
--
kernel/sched/core.c-959-
kernel/sched/core.c:960: rq_lock(rq, &rf);
kernel/sched/core.c-961- hrtick_cond_restart(rq);
--
kernel/sched/core.c=1272=void resched_cpu(int cpu)
--
kernel/sched/core.c-1276-
kernel/sched/core.c:1277: raw_spin_rq_lock_irqsave(rq, flags);
kernel/sched/core.c-1278- if (cpu_online(cpu) || cpu == smp_processor_id())
--
kernel/sched/core.c=1681=static void uclamp_update_util_min_rt_default(struct task_struct *p)
--
kernel/sched/core.c-1686- /* Protect updates to p->uclamp_* */
kernel/sched/core.c:1687: guard(task_rq_lock)(p);
kernel/sched/core.c-1688- __uclamp_update_util_min_rt_default(p);
--
kernel/sched/core.c=1935=uclamp_update_active(struct task_struct *p)
--
kernel/sched/core.c-1948- */
kernel/sched/core.c:1949: rq = task_rq_lock(p, &rf);
kernel/sched/core.c-1950-
kernel/sched/core.c-1951- /*
kernel/sched/core.c:1952: * Setting the clamp bucket is serialized by task_rq_lock().
kernel/sched/core.c-1953- * If the task is not yet RUNNABLE and its task_struct is not
--
kernel/sched/core.c=2086=static void uclamp_fork(struct task_struct *p)
--
kernel/sched/core.c-2090- /*
kernel/sched/core.c:2091: * We don't need to hold task_rq_lock() when updating p->uclamp_* here
kernel/sched/core.c-2092- * as the task is still at its early fork stages.
--
kernel/sched/core.c=2353=unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state)
--
kernel/sched/core.c-2390- */
kernel/sched/core.c:2391: rq = task_rq_lock(p, &rf);
kernel/sched/core.c-2392- /*
--
kernel/sched/core.c=2461=static void migrate_disable_switch(struct rq *rq, struct task_struct *p)
--
kernel/sched/core.c-2473-
kernel/sched/core.c:2474: scoped_guard (task_rq_lock, p)
kernel/sched/core.c-2475- do_set_cpus_allowed(p, &ac);
--
kernel/sched/core.c=2556=static struct rq *move_queued_task(struct rq *rq, struct rq_flags *rf,
kernel/sched/core.c-2557- struct task_struct *p, int new_cpu)
kernel/sched/core.c:2558: __must_hold(__rq_lockp(rq))
kernel/sched/core.c-2559-{
--
kernel/sched/core.c-2567-
kernel/sched/core.c:2568: rq_lock(rq, rf);
kernel/sched/core.c-2569- WARN_ON_ONCE(task_cpu(p) != new_cpu);
--
kernel/sched/core.c=2603=static struct rq *__migrate_task(struct rq *rq, struct rq_flags *rf,
kernel/sched/core.c-2604- struct task_struct *p, int dest_cpu)
kernel/sched/core.c:2605: __must_hold(__rq_lockp(rq))
kernel/sched/core.c-2606-{
--
kernel/sched/core.c=2621=static int migration_cpu_stop(void *data)
--
kernel/sched/core.c-2648- raw_spin_lock(&p->pi_lock);
kernel/sched/core.c:2649: rq_lock(rq, &rf);
kernel/sched/core.c-2650-
--
kernel/sched/core.c=2735=int push_cpu_stop(void *arg)
--
kernel/sched/core.c-2740- raw_spin_lock_irq(&p->pi_lock);
kernel/sched/core.c:2741: raw_spin_rq_lock(rq);
kernel/sched/core.c-2742-
--
kernel/sched/core.c=2813=void set_cpus_allowed_force(struct task_struct *p, const struct cpumask *new_mask)
--
kernel/sched/core.c-2824-
kernel/sched/core.c:2825: scoped_guard (__task_rq_lock, p)
kernel/sched/core.c-2826- do_set_cpus_allowed(p, &ac);
--
kernel/sched/core.c=2969=static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flags *rf,
kernel/sched/core.c-2970- int dest_cpu, unsigned int flags)
kernel/sched/core.c:2971: __releases(__rq_lockp(rq), &p->pi_lock)
kernel/sched/core.c-2972-{
--
kernel/sched/core.c=3122=static int __set_cpus_allowed_ptr_locked(struct task_struct *p,
--
kernel/sched/core.c-3125- struct rq_flags *rf)
kernel/sched/core.c:3126: __releases(__rq_lockp(rq), &p->pi_lock)
kernel/sched/core.c-3127-{
--
kernel/sched/core.c=3207=int __set_cpus_allowed_ptr(struct task_struct *p, struct affinity_context *ctx)
--
kernel/sched/core.c-3211-
kernel/sched/core.c:3212: rq = task_rq_lock(p, &rf);
kernel/sched/core.c-3213- /*
--
kernel/sched/core.c=3245=static int restrict_cpus_allowed_ptr(struct task_struct *p,
--
kernel/sched/core.c-3256-
kernel/sched/core.c:3257: rq = task_rq_lock(p, &rf);
kernel/sched/core.c-3258-
--
kernel/sched/core.c=3351=void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
--
kernel/sched/core.c-3380- * Furthermore, all task_rq users should acquire both locks, see
]
|
| 56/2 |
2026/10/13 08:28 |
llm |
expert |
0m
Model:
Tokens:
input: 42193
output: 31
thoughts: 36
Thoughts: **My Thought Process Unraveling `__schedule`'s Locking Mechanism**
Okay, so as I delve into the `__schedule` function, my attention is immediately drawn to a critical line: `rq_lock(rq, &rf);`. This is where the core of my immediate inquiry lies. I recognize `rq_lock` as a fundamental locking primitive within the scheduler's domain. My mental index immediately points me to its definition residing in `kernel/sched/sched.h`. This header file is essentially the canonical source for the scheduler's low-level data structures and synchronization mechanisms. Therefore, understanding how `rq_lock` operates, what it protects, and its intended behavior is paramount to grasping the thread safety and correctness of the `__schedule` function itself. I'm essentially tracing the execution flow and identifying the critical section being entered and the mechanism used to guard it. My next step would be to examine the implementation of `rq_lock` within `sched.h` to understand its specific locking strategy (e.g., spinlock, mutex, etc.) and the implications for concurrency.
|
| 57/2 |
2026/10/13 08:28 |
tool |
grepper |
0m
Args: map[Expression:rq_lock PathPrefixes:[kernel/sched/sched.h]]
Results: map[Output:
Full output is too long, showing 200 out of 335 lines.
Use more precise expression or PathPrefixes if possible.
Number of matching lines per file (1 files in total):
kernel/sched/sched.h:119
kernel/sched/sched.h=1158=struct rq {
--
kernel/sched/sched.h-1386-
kernel/sched/sched.h:1387: /* Scratch cpumask to be temporarily used under rq_lock */
kernel/sched/sched.h-1388- cpumask_var_t scratch_mask;
--
kernel/sched/sched.h=1489=static inline bool sched_core_disabled(void)
--
kernel/sched/sched.h-1497- */
kernel/sched/sched.h:1498:static inline raw_spinlock_t *rq_lockp(struct rq *rq)
kernel/sched/sched.h-1499-{
--
kernel/sched/sched.h-1505-
kernel/sched/sched.h:1506:static inline raw_spinlock_t *__rq_lockp(struct rq *rq)
kernel/sched/sched.h:1507: __returns_ctx_lock(rq_lockp(rq)) /* alias them */
kernel/sched/sched.h-1508-{
--
kernel/sched/sched.h=1604=static inline bool sched_core_disabled(void)
--
kernel/sched/sched.h-1608-
kernel/sched/sched.h:1609:static inline raw_spinlock_t *rq_lockp(struct rq *rq)
kernel/sched/sched.h-1610-{
--
kernel/sched/sched.h-1613-
kernel/sched/sched.h:1614:static inline raw_spinlock_t *__rq_lockp(struct rq *rq)
kernel/sched/sched.h:1615: __returns_ctx_lock(rq_lockp(rq)) /* alias them */
kernel/sched/sched.h-1616-{
--
kernel/sched/sched.h=1662=static inline void lockdep_assert_rq_held(struct rq *rq)
kernel/sched/sched.h:1663: __assumes_ctx_lock(__rq_lockp(rq))
kernel/sched/sched.h-1664-{
kernel/sched/sched.h:1665: lockdep_assert_held(__rq_lockp(rq));
kernel/sched/sched.h-1666-}
kernel/sched/sched.h-1667-
kernel/sched/sched.h:1668:extern void raw_spin_rq_lock_nested(struct rq *rq, int subclass)
kernel/sched/sched.h:1669: __acquires(__rq_lockp(rq));
kernel/sched/sched.h-1670-
kernel/sched/sched.h=1671=extern bool raw_spin_rq_trylock(struct rq *rq)
kernel/sched/sched.h:1672: __cond_acquires(true, __rq_lockp(rq));
kernel/sched/sched.h-1673-
kernel/sched/sched.h:1674:static inline void raw_spin_rq_lock(struct rq *rq)
kernel/sched/sched.h:1675: __acquires(__rq_lockp(rq))
kernel/sched/sched.h-1676-{
kernel/sched/sched.h:1677: raw_spin_rq_lock_nested(rq, 0);
kernel/sched/sched.h-1678-}
--
kernel/sched/sched.h=1680=static inline void raw_spin_rq_unlock(struct rq *rq)
kernel/sched/sched.h:1681: __releases(__rq_lockp(rq))
kernel/sched/sched.h-1682-{
kernel/sched/sched.h:1683: raw_spin_unlock(rq_lockp(rq));
kernel/sched/sched.h-1684-}
kernel/sched/sched.h-1685-
kernel/sched/sched.h:1686:static inline void raw_spin_rq_lock_irq(struct rq *rq)
kernel/sched/sched.h:1687: __acquires(__rq_lockp(rq))
kernel/sched/sched.h-1688-{
kernel/sched/sched.h-1689- local_irq_disable();
kernel/sched/sched.h:1690: raw_spin_rq_lock(rq);
kernel/sched/sched.h-1691-}
--
kernel/sched/sched.h=1693=static inline void raw_spin_rq_unlock_irq(struct rq *rq)
kernel/sched/sched.h:1694: __releases(__rq_lockp(rq))
kernel/sched/sched.h-1695-{
--
kernel/sched/sched.h-1699-
kernel/sched/sched.h:1700:static inline unsigned long _raw_spin_rq_lock_irqsave(struct rq *rq)
kernel/sched/sched.h:1701: __acquires(__rq_lockp(rq))
kernel/sched/sched.h-1702-{
--
kernel/sched/sched.h-1705- local_irq_save(flags);
kernel/sched/sched.h:1706: raw_spin_rq_lock(rq);
kernel/sched/sched.h-1707-
--
kernel/sched/sched.h=1711=static inline void raw_spin_rq_unlock_irqrestore(struct rq *rq, unsigned long flags)
kernel/sched/sched.h:1712: __releases(__rq_lockp(rq))
kernel/sched/sched.h-1713-{
--
kernel/sched/sched.h-1717-
kernel/sched/sched.h:1718:#define raw_spin_rq_lock_irqsave(rq, flags) \
kernel/sched/sched.h-1719-do { \
kernel/sched/sched.h:1720: flags = _raw_spin_rq_lock_irqsave(rq); \
kernel/sched/sched.h-1721-} while (0)
--
kernel/sched/sched.h=1935=static inline void rq_pin_lock(struct rq *rq, struct rq_flags *rf)
kernel/sched/sched.h-1936-{
kernel/sched/sched.h:1937: rf->cookie = lockdep_pin_lock(__rq_lockp(rq));
kernel/sched/sched.h-1938-
--
kernel/sched/sched.h=1944=static inline void rq_unpin_lock(struct rq *rq, struct rq_flags *rf)
--
kernel/sched/sched.h-1949- scx_rq_clock_invalidate(rq);
kernel/sched/sched.h:1950: lockdep_unpin_lock(__rq_lockp(rq), rf->cookie);
kernel/sched/sched.h-1951-}
--
kernel/sched/sched.h=1953=static inline void rq_repin_lock(struct rq *rq, struct rq_flags *rf)
kernel/sched/sched.h-1954-{
kernel/sched/sched.h:1955: lockdep_repin_lock(__rq_lockp(rq), rf->cookie);
kernel/sched/sched.h-1956-
--
kernel/sched/sched.h-1962-
kernel/sched/sched.h:1963:#define __task_rq_lock(...) __acquire_ret(___task_rq_lock(__VA_ARGS__), __rq_lockp(__ret))
kernel/sched/sched.h:1964:extern struct rq *___task_rq_lock(struct task_struct *p, struct rq_flags *rf) __acquires_ret;
kernel/sched/sched.h-1965-
kernel/sched/sched.h:1966:#define task_rq_lock(...) __acquire_ret(_task_rq_lock(__VA_ARGS__), __rq_lockp(__ret))
kernel/sched/sched.h:1967:extern struct rq *_task_rq_lock(struct task_struct *p, struct rq_flags *rf)
kernel/sched/sched.h-1968- __acquires(&p->pi_lock) __acquires_ret;
--
kernel/sched/sched.h=1971=__task_rq_unlock(struct rq *rq, struct task_struct *p, struct rq_flags *rf)
kernel/sched/sched.h:1972: __releases(__rq_lockp(rq))
kernel/sched/sched.h-1973-{
--
kernel/sched/sched.h=1979=task_rq_unlock(struct rq *rq, struct task_struct *p, struct rq_flags *rf)
kernel/sched/sched.h:1980: __releases(__rq_lockp(rq), &p->pi_lock)
kernel/sched/sched.h-1981-{
--
kernel/sched/sched.h-1985-
kernel/sched/sched.h:1986:DEFINE_LOCK_GUARD_1(task_rq_lock, struct task_struct,
kernel/sched/sched.h:1987: _T->rq = task_rq_lock(_T->lock, &_T->rf),
kernel/sched/sched.h-1988- task_rq_unlock(_T->rq, _T->lock, &_T->rf),
kernel/sched/sched.h-1989- struct rq *rq; struct rq_flags rf)
kernel/sched/sched.h:1990:DECLARE_LOCK_GUARD_1_ATTRS(task_rq_lock, __acquires(_T->pi_lock), __releases((*(struct task_struct **)_T)->pi_lock))
kernel/sched/sched.h:1991:#define class_task_rq_lock_constructor(_T) WITH_LOCK_GUARD_1_ATTRS(task_rq_lock, _T)
kernel/sched/sched.h-1992-
kernel/sched/sched.h:1993:DEFINE_LOCK_GUARD_1(__task_rq_lock, struct task_struct,
kernel/sched/sched.h:1994: _T->rq = __task_rq_lock(_T->lock, &_T->rf),
kernel/sched/sched.h-1995- __task_rq_unlock(_T->rq, _T->lock, &_T->rf),
--
kernel/sched/sched.h-1997-
kernel/sched/sched.h:1998:static inline void rq_lock_irqsave(struct rq *rq, struct rq_flags *rf)
kernel/sched/sched.h:1999: __acquires(__rq_lockp(rq))
kernel/sched/sched.h-2000-{
kernel/sched/sched.h:2001: raw_spin_rq_lock_irqsave(rq, rf->flags);
kernel/sched/sched.h-2002- rq_pin_lock(rq, rf);
--
kernel/sched/sched.h-2004-
kernel/sched/sched.h:2005:static inline void rq_lock_irq(struct rq *rq, struct rq_flags *rf)
kernel/sched/sched.h:2006: __acquires(__rq_lockp(rq))
kernel/sched/sched.h-2007-{
kernel/sched/sched.h:2008: raw_spin_rq_lock_irq(rq);
kernel/sched/sched.h-2009- rq_pin_lock(rq, rf);
--
kernel/sched/sched.h-2011-
kernel/sched/sched.h:2012:static inline void rq_lock(struct rq *rq, struct rq_flags *rf)
kernel/sched/sched.h:2013: __acquires(__rq_lockp(rq))
kernel/sched/sched.h-2014-{
kernel/sched/sched.h:2015: raw_spin_rq_lock(rq);
kernel/sched/sched.h-2016- rq_pin_lock(rq, rf);
--
kernel/sched/sched.h=2019=static inline void rq_unlock_irqrestore(struct rq *rq, struct rq_flags *rf)
kernel/sched/sched.h:2020: __releases(__rq_lockp(rq))
kernel/sched/sched.h-2021-{
--
kernel/sched/sched.h=2026=static inline void rq_unlock_irq(struct rq *rq, struct rq_flags *rf)
kernel/sched/sched.h:2027: __releases(__rq_lockp(rq))
kernel/sched/sched.h-2028-{
--
kernel/sched/sched.h=2033=static inline void rq_unlock(struct rq *rq, struct rq_flags *rf)
kernel/sched/sched.h:2034: __releases(__rq_lockp(rq))
kernel/sched/sched.h-2035-{
--
kernel/sched/sched.h-2039-
kernel/sched/sched.h:2040:DEFINE_LOCK_GUARD_1(rq_lock, struct rq,
kernel/sched/sched.h:2041: rq_lock(_T->lock, &_T->rf),
kernel/sched/sched.h-2042- rq_unlock(_T->lock, &_T->rf),
--
kernel/sched/sched.h-2044-
kernel/sched/sched.h:2045:DECLARE_LOCK_GUARD_1_ATTRS(rq_lock, __acquires(__rq_lockp(_T)),
kernel/sched/sched.h:2046: __releases(__rq_lockp(*(struct rq **)_T)));
kernel/sched/sched.h:2047:#define class_rq_lock_constructor(_T) WITH_LOCK_GUARD_1_ATTRS(rq_lock, _T)
kernel/sched/sched.h-2048-
kernel/sched/sched.h:2049:DEFINE_LOCK_GUARD_1(rq_lock_irq, struct rq,
kernel/sched/sched.h:2050: rq_lock_irq(_T->lock, &_T->rf),
kernel/sched/sched.h-2051- rq_unlock_irq(_T->lock, &_T->rf),
--
kernel/sched/sched.h-2053-
kernel/sched/sched.h:2054:DECLARE_LOCK_GUARD_1_ATTRS(rq_lock_irq, __acquires(__rq_lockp(_T)),
kernel/sched/sched.h:2055: __releases(__rq_lockp(*(struct rq **)_T)));
kernel/sched/sched.h:2056:#define class_rq_lock_irq_constructor(_T) WITH_LOCK_GUARD_1_ATTRS(rq_lock_irq, _T)
kernel/sched/sched.h-2057-
kernel/sched/sched.h:2058:DEFINE_LOCK_GUARD_1(rq_lock_irqsave, struct rq,
kernel/sched/sched.h:2059: rq_lock_irqsave(_T->lock, &_T->rf),
kernel/sched/sched.h-2060- rq_unlock_irqrestore(_T->lock, &_T->rf),
--
kernel/sched/sched.h-2062-
kernel/sched/sched.h:2063:DECLARE_LOCK_GUARD_1_ATTRS(rq_lock_irqsave, __acquires(__rq_lockp(_T)),
kernel/sched/sched.h:2064: __releases(__rq_lockp(*(struct rq **)_T)));
kernel/sched/sched.h:2065:#define class_rq_lock_irqsave_constructor(_T) WITH_LOCK_GUARD_1_ATTRS(rq_lock_irqsave, _T)
kernel/sched/sched.h-2066-
kernel/sched/sched.h:2067:DEFINE_LOCK_GUARD_1(raw_spin_rq_lock_irqsave, struct rq,
kernel/sched/sched.h:2068: raw_spin_rq_lock_irqsave(_T->lock, _T->flags),
kernel/sched/sched.h-2069- raw_spin_rq_unlock_irqrestore(_T->lock, _T->flags),
--
kernel/sched/sched.h-2071-
kernel/sched/sched.h:2072:DECLARE_LOCK_GUARD_1_ATTRS(raw_spin_rq_lock_irqsave, __acquires(__rq_lockp(_T)),
kernel/sched/sched.h:2073: __releases(__rq_lockp(*(struct rq **)_T)));
kernel/sched/sched.h:2074:#define class_raw_spin_rq_lock_irqsave_constructor(_T) \
kernel/sched/sched.h:2075: WITH_LOCK_GUARD_1_ATTRS(raw_spin_rq_lock_irqsave, _T)
kernel/sched/sched.h-2076-
kernel/sched/sched.h:2077:#define this_rq_lock_irq(...) __acquire_ret(_this_rq_lock_irq(__VA_ARGS__), __rq_lockp(__ret))
kernel/sched/sched.h:2078:static inline struct rq *_this_rq_lock_irq(struct rq_flags *rf) __acquires_ret
kernel/sched/sched.h-2079-{
]
|
| 58/2 |
2026/10/13 08:28 |
llm |
expert |
0m
Model:
Tokens:
input: 46761
output: 36
thoughts: 454
Thoughts: **My Journey Through `rq_lock` and the Scheduler's Underpinnings**
My initial thought process as I encountered `rq_lock` in `kernel/sched/sched.h` was that it was a straightforward wrapper, simply acquiring `rq->lock`. The definition itself, a call to `raw_spin_rq_lock` followed by `rq_pin_lock`, reinforced this initial impression.
However, diving deeper, my attention was immediately drawn to `raw_spin_rq_lock`. It's not a direct spinlock acquisition but rather a call to `raw_spin_rq_lock_nested` with a subclass of 0. This immediately signals a more nuanced locking mechanism at play, hinting at potential reentrancy or specific locking strategies.
The real intrigue, though, began when I examined the implementation of `raw_spin_rq_lock_nested` in `kernel/sched/core.c`. The `preempt_disable()` at the outset, coupled with the conditional logic involving `sched_core_disabled()`, indicated a critical section that needs to protect against preemption. The handling within the `if (sched_core_disabled())` block, where it directly acquires `&rq->__lock` and then `preempt_enable_no_resched()`, struck me as a fallback or an alternative path when core scheduling functionalities are temporarily unavailable. The comment about `preempt_count *MUST* be > 1` is a critical invariant I'm noting here, underscoring the careful management of preemption context.
The `for (;;)` loop in the main path is what really piqued my curiosity. The pattern of acquiring a lock (`raw_spin_lock_nested(lock, subclass)`), checking if the lock pointer (`lock`) itself has changed, and then potentially releasing and retrying (`raw_spin_unlock(lock)`) is a classic deadlock avoidance or highly concurrent lock acquisition strategy. This is where my understanding shifted from a simple lock acquisition to a more dynamic, potentially contended locking scenario. The `likely(lock == __rq_lockp(rq))` condition is the key; it suggests that the underlying lock pointer itself might change under certain scheduler conditions (perhaps related to core enabling/disabling or migration), and this loop ensures I'm always operating on the *current* valid lock. Once the condition is met and the lock acquired, `preempt_enable_no_resched()` is called, again highlighting the meticulous control over preemption state during these critical scheduler operations.
My immediate next step, naturally, was to investigate `__rq_lockp(rq)`. The fact that it returns either `rq->core->__lock` or `rq->__lock` depending on `rq->core`'s existence is crucial. This implies that the actual spinlock being acquired can be in one of two locations, and its location can change. This reinforces the necessity of the retry loop within `raw_spin_rq_lock_nested`. If the `core` structure or its associated lock is being managed dynamically, the lock pointer could indeed change, necessitating the re-acquisition strategy I observed. It's this dynamic nature of the lock's location that makes the `rq_lock` implementation more sophisticated than a simple, static spinlock.
|
| 59/2 |
2026/10/13 08:29 |
tool |
read-file |
0m
Args: map[File:kernel/sched/sched.h FirstLine:1498 LineCount:20]
Results: map[Contents:1498: static inline raw_spinlock_t *rq_lockp(struct rq *rq)
1499: {
1500: if (sched_core_enabled(rq))
1501: return &rq->core->__lock;
1502:
1503: return &rq->__lock;
1504: }
1505:
1506: static inline raw_spinlock_t *__rq_lockp(struct rq *rq)
1507: __returns_ctx_lock(rq_lockp(rq)) /* alias them */
1508: {
1509: if (rq->core_enabled)
1510: return &rq->core->__lock;
1511:
1512: return &rq->__lock;
1513: }
1514:
1515: extern bool
1516: cfs_prio_less(const struct task_struct *a, const struct task_struct *b, bool fi);
1517:
]
|
| 60/2 |
2026/10/13 08:29 |
llm |
expert |
0m
Model:
Tokens:
input: 47522
output: 42
thoughts: 392
Thoughts: **My Thought Process Unfolding: Core Locks, Scheduling, and the Peril of Hotplug**
Right, so I'm digging into this scheduler code, specifically around the `rq_lock` mechanism. It's immediately apparent that `__rq_lockp(rq)` returns `&rq->core->__lock` if `rq->core_enabled`. This is a crucial point, because it means that when `rq_lock(rq, &rf)` is invoked, it's not just locking *my* runqueue (`rq`) in isolation, it's actually acquiring the lock for the *entire core* that `rq` belongs to, through `rq->core->__lock`.
This implies a significant contention point: all CPUs within a given core share this *single* lock. Consequently, when `__schedule` calls `rq_lock(rq, &rf)`, it's effectively locking the *entire core*. What's more, the `pick_next_task` function is called while this core lock is held. This has a direct implication for concurrency: no other CPU within that core can be concurrently executing `pick_next_task`, nor can they be concurrently modifying `rq_i->core_pick` β which is a variable that `pick_next_task` likely relies on. This makes sense for ensuring atomicity in the core's task selection process.
However, this then triggers a follow-up question in my mind: if the core lock is held, what else might be able to change concurrently? My immediate thought turns to CPU hotplug. CPU hotplug operations, I know, typically acquire the `cpu_hotplug_lock`. The question is, does `pick_next_task` hold this `cpu_hotplug_lock`? The initial analysis suggests it doesn't.
But then I consider the nature of CPU hotplug threads. They are, after all, tasks themselves, managed by the scheduler. So, for a CPU hotplug thread (like `smpboot_thread_fn`) to execute, it needs to be scheduled. This brings up a potential race.
Let's trace further. I recall that `remove_siblinginfo` is called from `takedown_cpu`. And `takedown_cpu` itself is executed on the very CPU that is being taken down, with interrupts disabled, which is a critical state. The `takedown_cpu` function is ultimately invoked from `cpuhp_invoke_callback`.
Now, hereβs where the potential conflict emerges vividly: imagine CPU A is running `takedown_cpu`. During this process, it might be modifying topology information, specifically `topology_sibling_cpumask(CPU B)`. At the same time, CPU B could be concurrently running `pick_next_task`. This is a scenario where CPU B, holding the core lock (as established earlier), is in the midst of its scheduling decision, while CPU A, on a sibling core, is actively altering the sibling mask that CPU B's scheduler might be relying upon.
To get to the bottom of this, I need to directly examine the implementation of `remove_siblinginfo` within `arch/x86/kernel/smpboot.c`. That's the next logical step to ascertain if the `cpu_hotplug_lock` is indeed involved or if there's another mechanism in place to prevent such a race condition between `takedown_cpu` and `pick_next_task`.
|
| 61/2 |
2026/10/13 08:29 |
tool |
read-file |
0m
Args: map[File:arch/x86/kernel/smpboot.c FirstLine:1320 LineCount:20]
Results: map[Contents:1320: */
1321: if (cpumask_weight(topology_sibling_cpumask(cpu)) == 1)
1322: cpu_data(sibling).booted_cores--;
1323: }
1324:
1325: for_each_cpu(sibling, topology_die_cpumask(cpu))
1326: cpumask_clear_cpu(cpu, topology_die_cpumask(sibling));
1327:
1328: for_each_cpu(sibling, topology_sibling_cpumask(cpu)) {
1329: cpumask_clear_cpu(cpu, topology_sibling_cpumask(sibling));
1330: if (cpumask_weight(topology_sibling_cpumask(sibling)) == 1)
1331: cpu_data(sibling).smt_active = false;
1332: }
1333:
1334: for_each_cpu(sibling, cpu_llc_shared_mask(cpu))
1335: cpumask_clear_cpu(cpu, cpu_llc_shared_mask(sibling));
1336: for_each_cpu(sibling, cpu_l2c_shared_mask(cpu))
1337: cpumask_clear_cpu(cpu, cpu_l2c_shared_mask(sibling));
1338: cpumask_clear(cpu_llc_shared_mask(cpu));
1339: cpumask_clear(cpu_l2c_shared_mask(cpu));
]
|
| 62/2 |
2026/10/13 08:29 |
llm |
expert |
0m
Model:
Tokens:
input: 48357
output: 41
thoughts: 223
Thoughts: **A Deep Dive into CPU Topology and Task Picking Logic**
My current line of inquiry is focused on understanding the intricate interplay between CPU topology changes and the task scheduling mechanism, specifically within the context of `pick_next_task`. I've been tracing the implications of a CPU going offline, and how that event propagates through the system's view of CPU relationships.
Here's my thought process unfolding:
First, I'm considering the immediate consequence of a CPU, let's call it CPU A, going offline. The system's topology management correctly reflects this by removing CPU A from the `topology_sibling_cpumask` of its sibling, CPU B. This is a fundamental aspect of maintaining an accurate representation of which CPUs share physical resources.
Now, imagine CPU B is actively engaged in the `pick_next_task` routine. In this scenario, the `smt_mask` within `pick_next_task` is dynamically set to point to CPU B's `topology_sibling_cpumask`. The crucial implication here is that as soon as CPU A is removed from CPU B's `topology_sibling_cpumask`, it is also effectively removed from CPU B's `smt_mask`.
The timing of this change is critical. If this removal from the `smt_mask` occurs *between* the `for_each_cpu_wrap` loop and the subsequent `for_each_cpu` loop within `pick_next_task`, the behavior of the latter loop will be affected. The `for_each_cpu` loop, which iterates over CPUs based on the `smt_mask`, will simply iterate over a smaller set of CPUs. Specifically, it will *not* visit CPU A because it's no longer present in the relevant mask.
However, I'm wrestling with a specific condition: the problem arises only when `rq_i->core_pick` is `NULL`, *and* this `NULL` state is encountered for a CPU that was *not* visited during the initial `for_each_cpu_wrap` phase. If `for_each_cpu` correctly skips CPU A due to its offline status, and `rq_i->core_pick` is `NULL` for a CPU that *was* visited by `for_each_cpu_wrap`, then no issue arises. The problematic scenario is when `for_each_cpu` *would have* visited CPU A (if it were online), but now doesn't, and `rq_i->core_pick` is `NULL` for that specific, now-unvisited, CPU.
This leads me to question the nature of the `remove_siblinginfo` operation. My understanding is that `remove_siblinginfo` is designed solely to *clear* bits, representing the removal of a CPU's relationship. It cannot, by its design, *add* a CPU back into any topology mask.
This naturally prompts me to consider the counterpoint: what about when a CPU comes *online*? The function responsible for this is `set_cpu_sibling_map`. My next step is to delve into the implementation of `set_cpu_sibling_map` to understand how it establishes these sibling relationships and how that might interact with the scenario I'm analyzing.
|
| 63/2 |
2026/10/13 08:29 |
tool |
read-file |
0m
Args: map[File:arch/x86/kernel/smpboot.c FirstLine:715 LineCount:20]
Results: map[Contents: 715: o = &cpu_data(i);
716:
717: if (match_pkg(c, o) && !topology_same_node(c, o))
718: WARN_ON_ONCE(topology_num_nodes_per_package() == 1);
719:
720: if ((i == cpu) || (has_smt && match_smt(c, o)))
721: link_mask(topology_sibling_cpumask, cpu, i);
722:
723: if ((i == cpu) || (has_mp && match_llc(c, o)))
724: link_mask(cpu_llc_shared_mask, cpu, i);
725:
726: if ((i == cpu) || (has_mp && match_l2c(c, o)))
727: link_mask(cpu_l2c_shared_mask, cpu, i);
728:
729: if ((i == cpu) || (has_mp && match_die(c, o)))
730: link_mask(topology_die_cpumask, cpu, i);
731: }
732:
733: threads = cpumask_weight(topology_sibling_cpumask(cpu));
734: if (threads > __max_smt_threads)
]
|
| 64/2 |
2026/10/13 08:29 |
llm |
expert |
0m
Model:
Tokens:
input: 48990
output: 41
thoughts: 330
Thoughts: **The Interplay of CPU Online Events and Task Scheduling**
My thought process here is centered on a potential race condition I've identified within the kernel's CPU management and task scheduling mechanisms. The core of the issue revolves around how CPU affinity masks are updated when a new CPU comes online, specifically focusing on the `link_mask` operation.
I understand that `link_mask` is designed to establish a reciprocal link in the affinity masks between two CPUs. If CPU A is coming online and CPU B is already active, `link_mask` will set the bit corresponding to CPU A in CPU B's mask, and vice-versa. This is intended to inform each CPU about its sibling cores.
Now, consider the scenario where CPU A is in the process of coming online. A separate CPU, let's call it CPU B, is actively running the `pick_next_task` function, which is responsible for selecting the next task to schedule. The critical moment I'm examining is what happens if CPU B's iteration through the available CPUs (using `for_each_cpu_wrap` and then `for_each_cpu`) encompasses CPU A *after* CPU A's mask has been updated by `link_mask`, but *before* CPU A has fully initialized its own task-related structures.
The concern is that if CPU A is still coming online, its run queue's `core_pick` field might be `NULL`. If CPU B then proceeds to evaluate CPU A (because it now appears in its mask) and attempts to dereference this `NULL` pointer β specifically when checking `cookie_equals(p, cookie)` β a kernel crash is imminent.
This leads me to question the concurrency: Is it even possible for CPU B to be executing `pick_next_task` while CPU A is simultaneously in the midst of `set_cpu_sibling_map`? I recall that `set_cpu_sibling_map` is typically invoked from `smp_callin` or `start_secondary`. `start_secondary`, being the initial execution path on a newly booted CPU (our CPU A), operates with interrupts disabled. However, CPU B, having been online already, is free to schedule and run its processes, including `pick_next_task`.
Therefore, my conclusion is a definitive *yes*. CPU B can indeed be running `pick_next_task` concurrently with CPU A executing `set_cpu_sibling_map` within the context of `smp_callin`. This concurrent execution is where the potential `NULL` pointer dereference originates. I'll need to verify this by examining the specific code paths, particularly in `arch/x86/kernel/smpboot.c`, where `start_secondary` leads to `smp_callin`, which in turn calls `set_cpu_sibling_map`. This confirmation will solidify my understanding of the race condition.
|
| 65/2 |
2026/10/13 08:29 |
tool |
read-file |
0m
Args: map[File:arch/x86/kernel/smpboot.c FirstLine:240 LineCount:20]
Results: map[Contents: 240: * table established. Yet another historical divergence.
241: */
242: if (IS_ENABLED(CONFIG_X86_32)) {
243: /* switch away from the initial page table */
244: load_cr3(swapper_pg_dir);
245: __flush_tlb_all();
246: }
247:
248: cpu_init_exception_handling(false);
249:
250: /*
251: * Load the microcode before reaching the AP alive synchronization
252: * point below so it is not part of the full per CPU serialized
253: * bringup part when "parallel" bringup is enabled.
254: *
255: * That's even safe when hyperthreading is enabled in the CPU as
256: * the core code starts the primary threads first and leaves the
257: * secondary threads waiting for SIPI. Loading microcode on
258: * physical cores concurrently is a safe operation.
259: *
]
|
| 66/2 |
2026/10/13 08:29 |
llm |
expert |
0m
Model:
Tokens:
input: 49653
output: 41
thoughts: 21
Thoughts: **My Thought Process on Locating `set_cpu_sibling_map`**
Alright, so my objective here is to pinpoint the `set_cpu_sibling_map` function within the `smpboot.c` file. I'm going to start by opening up the source code for `smpboot.c` and then performing a targeted search for the string "set_cpu_sibling_map". This is a straightforward string search, and given my familiarity with kernel code structure, I expect this function to be directly involved in the setup and configuration of CPU siblings, likely during the boot process or as part of CPU hotplugging mechanisms. I'll be looking for its definition, paying close attention to its parameters and return values, to understand how it's intended to be used and what information it manipulates. My expectation is that it will likely involve data structures related to the processor topology, such as `cpuinfo_x86` or similar, and will be called from a core boot or initialization routine.
|
| 67/2 |
2026/10/13 08:30 |
tool |
grepper |
0m
Args: map[Expression:set_cpu_sibling_map PathPrefixes:[arch/x86/kernel/smpboot.c]]
Results: map[Output:arch/x86/kernel/smpboot.c=170=static void ap_starting(void)
--
arch/x86/kernel/smpboot.c-195- */
arch/x86/kernel/smpboot.c:196: set_cpu_sibling_map(cpuid);
arch/x86/kernel/smpboot.c-197-
--
arch/x86/kernel/smpboot.c=674=int arch_sched_node_distance(int from, int to)
--
arch/x86/kernel/smpboot.c-693-
arch/x86/kernel/smpboot.c:694:void set_cpu_sibling_map(int cpu)
arch/x86/kernel/smpboot.c-695-{
--
arch/x86/kernel/smpboot.c=1172=void __init smp_prepare_cpus_common(void)
--
arch/x86/kernel/smpboot.c-1191-
arch/x86/kernel/smpboot.c:1192: set_cpu_sibling_map(0);
arch/x86/kernel/smpboot.c-1193-}
]
|
| 68/2 |
2026/10/13 08:30 |
llm |
expert |
0m
Model:
Tokens:
input: 50028
output: 37
thoughts: 29
Thoughts: **My Thought Process Regarding AP Initialization**
Okay, so when I'm looking at the boot process for secondary processors (APs), the first thing that catches my eye is the `ap_starting` function. It seems to be the central hub for getting an AP up and running. My understanding is that `ap_starting` is actually invoked from `start_secondary`. This makes sense; `start_secondary` is likely responsible for the initial awakening and setup of the AP, and then it hands off the more detailed configuration to `ap_starting`.
Within `ap_starting` itself, a crucial step is the call to `set_cpu_sibling_map`. This tells me that establishing the relationship between cores and their hyperthreads β the "sibling" relationship β is a fundamental part of the AP's initialization. It's not just about getting the core active; it's about making sure the system understands its topology, particularly how it fits into the larger NUMA architecture and how cache coherency will be managed. This mapping is vital for efficient scheduling and resource utilization down the line. So, in essence, `ap_starting` orchestrates the initial activation of an AP and then immediately focuses on defining its place within the system's hardware hierarchy by setting up its sibling map.
|
| 69/2 |
2026/10/13 08:30 |
tool |
grepper |
0m
Args: map[Expression:ap_starting PathPrefixes:[arch/x86/kernel/smpboot.c]]
Results: map[Output:arch/x86/kernel/smpboot.c=152=static inline void smpboot_restore_warm_reset_vector(void)
--
arch/x86/kernel/smpboot.c-169-/* Run the next set of setup steps for the upcoming CPU */
arch/x86/kernel/smpboot.c:170:static void ap_starting(void)
arch/x86/kernel/smpboot.c-171-{
--
arch/x86/kernel/smpboot.c=229=static void notrace __noendbr start_secondary(void *unused)
--
arch/x86/kernel/smpboot.c-279-
arch/x86/kernel/smpboot.c:280: ap_starting();
arch/x86/kernel/smpboot.c-281-
]
|
| 70/2 |
2026/10/13 08:30 |
llm |
expert |
0m
Model:
Tokens:
input: 50307
output: 36
thoughts: 482
Thoughts: **The Perils of AP Startup: A Race Condition in Task Selection**
My thought process here is grappling with a potential race condition during the bring-up of an Application Processor (AP). It all starts with `start_secondary`, the entry point for this AP, operating with interrupts disabled initially. This function proceeds to call `ap_starting`, which in turn invokes `set_cpu_sibling_map`.
Now, the critical point is that at this stage, the AP (let's call it CPU A) isn't fully online; its `cpu_online_mask` hasn't been set yet. However, it's already modifying the `topology_sibling_cpumask` of other CPUs residing on the same core.
Meanwhile, a CPU that's already online (let's call it CPU B) might be concurrently executing `pick_next_task`. CPU B relies on its SMT mask, which is derived from `topology_sibling_cpumask(CPU B)`.
Here's where the problem arises: If CPU B reads its `smt_mask` within `pick_next_task` *before* CPU A has finished updating the sibling mask, it might only see itself in that mask. It then proceeds to iterate over this (incomplete) mask.
Subsequently, CPU A completes its update, setting the bit for itself in CPU B's `topology_sibling_cpumask`.
If CPU B then re-evaluates or continues its iteration over the `smt_mask` (which has now been updated to include CPU A), it will now visit CPU A. The code will fetch the runqueue for CPU A (`rq_i = cpu_rq(CPU A)`) and then access `p = rq_i->core_pick`.
The crucial question then becomes: what is the value of `rq_i->core_pick` for CPU A at this exact moment? Given that CPU A is just coming online and wasn't part of the first iteration on CPU B, its `rq->core_pick` might still be `NULL`. This is likely because `sched_core_cpu_starting` (the function that *does* set this value) is called when the CPU is transitioning to the online state, and the race might be occurring *before* that. My intuition is that `rq->core_pick` would be `NULL` by default or explicitly set to `NULL` in `sched_core_cpu_starting`. I need to investigate the exact timing and content of `sched_core_cpu_starting` to confirm this.
|
| 71/2 |
2026/10/13 08:30 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:6609 LineCount:20]
Results: map[Contents:6609: static void sched_core_cpu_starting(unsigned int cpu)
6610: {
6611: const struct cpumask *smt_mask = cpu_smt_mask(cpu);
6612: struct rq *rq = cpu_rq(cpu), *core_rq = NULL;
6613: int t;
6614:
6615: guard(core_lock)(&cpu);
6616:
6617: WARN_ON_ONCE(rq->core != rq);
6618:
6619: /* if we're the first, we'll be our own leader */
6620: if (cpumask_weight(smt_mask) == 1)
6621: return;
6622:
6623: /* find the leader */
6624: for_each_cpu(t, smt_mask) {
6625: if (t == cpu)
6626: continue;
6627: rq = cpu_rq(t);
6628: if (rq->core == rq) {
]
|
| 72/2 |
2026/10/13 08:30 |
llm |
expert |
0m
Model:
Tokens:
input: 51115
output: 41
thoughts: 107
Thoughts: **My Thought Process Regarding CPU Hotplug and Scheduling Initialization**
Okay, let me trace this through. My current focus is on understanding the intricate dance of CPU hotplugging and how it interacts with the scheduler's initialization on newly brought-up cores. I'm trying to pinpoint the exact sequence of events.
My immediate observation is that `sched_core_cpu_starting` β which, I understand, is a crucial hook for initializing scheduling-related data structures on a core β is invoked *from* `sched_cpu_starting`. This tells me there's a higher-level scheduling initialization routine that `sched_core_cpu_starting` is a part of.
Now, where does `sched_cpu_starting` itself get called from? My investigation reveals it's typically invoked within the context of the CPU hotplug state machine. Specifically, functions like `smpboot_setup_warm_reset_vector` or similar entry points are responsible for orchestrating this. This is a significant clue: the CPU hotplug state machine is where the lifecycle of an AP (Application Processor) core is managed.
This naturally leads me to consider *where* this state machine executes. It's well-established that the CPU hotplug state machine runs on the AP itself. This is logical, as it's managing the state of *other* APs coming online.
However, a potential point of confusion or a critical detail I need to reconcile is this: I recall seeing `set_cpu_sibling_map` being called *before* `sched_core_cpu_starting`. This sequence seems counterintuitive if `sched_core_cpu_starting` is meant to initialize the core's scheduling environment. If the sibling map, which defines the core's relationships within a CPU group, is set up *after* the core's scheduling is already being initialized, that could lead to issues or at least requires careful consideration of dependencies.
To resolve this, my next logical step is to delve into the actual code execution flow within `start_secondary`. This function is the primary entry point for bringing up a secondary CPU, and examining its structure will be key to verifying the precise order of operations, especially the relationship between the call to `set_cpu_sibling_map` and `sched_core_cpu_starting`. I need to see definitively if `set_cpu_sibling_map` precedes `sched_core_cpu_starting` and, if so, understand the rationale behind this ordering.
|
| 73/2 |
2026/10/13 08:30 |
tool |
read-file |
0m
Args: map[File:arch/x86/kernel/smpboot.c FirstLine:270 LineCount:30]
Results: map[Contents: 270: * synchronization state to ALIVE and spin-waits for the control CPU to
271: * release this CPU for further bringup.
272: */
273: cpuhp_ap_sync_alive();
274:
275: cpu_init();
276: fpu__init_cpu();
277: rcutree_report_cpu_starting(raw_smp_processor_id());
278: x86_cpuinit.early_percpu_clock_init();
279:
280: ap_starting();
281:
282: /* Check TSC synchronization with the control CPU. */
283: check_tsc_sync_target();
284:
285: /*
286: * Calibrate the delay loop after the TSC synchronization check.
287: * This allows to skip the calibration when TSC is synchronized
288: * across sockets.
289: */
290: ap_calibrate_delay();
291:
292: speculative_store_bypass_ht_init();
293:
294: /*
295: * Lock vector_lock, set CPU online and bring the vector
296: * allocator online. Online must be set with vector_lock held
297: * to prevent a concurrent irq setup/teardown from seeing a
298: * half valid vector space.
299: */
]
|
| 74/2 |
2026/10/13 08:30 |
llm |
expert |
0m
Model:
Tokens:
input: 51669
output: 41
thoughts: 65
Thoughts: **My Thought Process Regarding `sched_cpu_starting`**
My immediate question is, where exactly is `sched_cpu_starting` being invoked within the system? I recall it's associated with CPU hotplugging, so I'm looking for its role within that mechanism. My initial hypothesis is that it's a hotplug callback, and my next step is to confirm this.
Knowing it's a hotplug callback, I'm now tracing the execution path. I know that the specific hotplug callbacks for the `CPUHP_AP_SCHED_STARTING` stage are triggered by a function named `notify_cpu_starting`. Therefore, my focus shifts to identifying all the call sites for `notify_cpu_starting`. This will help me pinpoint the precise moment and context in which `sched_cpu_starting` is brought into play.
|
| 75/2 |
2026/10/13 08:30 |
tool |
read-file |
0m
Args: map[File:arch/x86/kernel/smpboot.c FirstLine:300 LineCount:20]
Results: map[Contents: 300: lock_vector_lock();
301: set_cpu_online(smp_processor_id(), true);
302: lapic_online();
303: unlock_vector_lock();
304: x86_platform.nmi_init();
305:
306: /* enable local interrupts */
307: local_irq_enable();
308:
309: x86_cpuinit.setup_percpu_clockev();
310:
311: wmb();
312: cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
313: }
314: ANNOTATE_NOENDBR_SYM(start_secondary);
315:
316: static bool
317: topology_same_node(struct cpuinfo_x86 *c, struct cpuinfo_x86 *o)
318: {
319: int cpu1 = c->cpu_index, cpu2 = o->cpu_index;
]
|
| 76/2 |
2026/10/13 08:30 |
llm |
expert |
0m
Model:
Tokens:
input: 52064
output: 31
thoughts: 54
Thoughts: **My Thought Process Regarding CPU Startup Notifications**
Okay, so I'm looking at the code and I see this function, `notify_cpu_starting`. My immediate thought is, "Wait a minute, where is that actually being called from?" My internal hypothesis is that it's being invoked from `cpu_startup_entry`. But then, a little voice in my head, based on my understanding of the system's architecture, says, "Hold on, isn't `cpu_startup_entry` actually the idle loop for a CPU once it's up and running?" This creates a bit of cognitive dissonance. If it's the idle loop, it seems unlikely to be the place where we'd *initiate* the notification that a CPU is *starting*.
To resolve this, I need to verify. My next step is to go directly to the source. I'm going to open up the `kernel/cpu.c` file and find the definition of `notify_cpu_starting`. By examining its call sites within that file, I'll be able to confirm or refute my initial assumption about where it's being invoked from and understand the actual control flow for CPU startup notifications. This is a critical step for ensuring I have a correct grasp of the system's initialization sequence.
|
| 77/2 |
2026/10/13 08:30 |
tool |
grepper |
0m
Args: map[Expression:notify_cpu_starting PathPrefixes:[kernel/cpu.c]]
Results: map[Output:kernel/cpu.c=798=static int bringup_wait_for_ap_online(unsigned int cpu)
--
kernel/cpu.c-812- * BIOS 'wait for SIPI' state in order to set the CR4.MCE bit. The
kernel/cpu.c:813: * CPU marked itself as booted_once in notify_cpu_starting() so the
kernel/cpu.c-814- * cpu_bootable() check will now return false if this is not the
--
kernel/cpu.c=1523=void smp_shutdown_nonboot_cpus(unsigned int primary_cpu)
--
kernel/cpu.c-1569-/**
kernel/cpu.c:1570: * notify_cpu_starting(cpu) - Invoke the callbacks on the starting CPU
kernel/cpu.c-1571- * @cpu: cpu that just started
--
kernel/cpu.c-1575- */
kernel/cpu.c:1576:void notify_cpu_starting(unsigned int cpu)
kernel/cpu.c-1577-{
]
|
| 78/2 |
2026/10/13 08:30 |
llm |
expert |
0m
Model:
Tokens:
input: 52413
output: 32
thoughts: 16
Thoughts: **My Thought Process Regarding `notify_cpu_starting` in x86**
I'm trying to pinpoint the origin of the `notify_cpu_starting` function within the x86 architecture. My immediate goal is to understand the execution flow and context surrounding this notification mechanism. Specifically, I'm looking for the precise call sites that trigger this function. My expectation is that it will be related to the initialization or bringing online of a CPU core, likely within the kernel's boot process or as part of an SMP (Symmetric Multiprocessing) bring-up sequence. I'll need to delve into the relevant x86 architecture-specific code, probably in files related to CPU initialization, interrupt handling, or SMP setup, to trace the execution path and identify where this function is invoked.
|
| 79/2 |
2026/10/13 08:30 |
tool |
grepper |
0m
Args: map[Expression:notify_cpu_starting PathPrefixes:[arch/x86/]]
Results: map[Output:arch/x86/kernel/smpboot.c=170=static void ap_starting(void)
--
arch/x86/kernel/smpboot.c-193- * The topology information must be up to date before
arch/x86/kernel/smpboot.c:194: * notify_cpu_starting().
arch/x86/kernel/smpboot.c-195- */
--
arch/x86/kernel/smpboot.c-207- */
arch/x86/kernel/smpboot.c:208: notify_cpu_starting(cpuid);
arch/x86/kernel/smpboot.c-209-}
--
arch/x86/xen/smp_pv.c=57=static void cpu_bringup(void)
--
arch/x86/xen/smp_pv.c-78-
arch/x86/xen/smp_pv.c:79: notify_cpu_starting(cpu);
arch/x86/xen/smp_pv.c-80-
]
|
| 80/2 |
2026/10/13 08:30 |
llm |
expert |
0m
Model:
Tokens:
input: 52733
output: 36
thoughts: 284
Thoughts: **My Thought Process Regarding AP Startup and CPU Topology**
It's striking how the `ap_starting` function, executed when an Application Processor (AP) begins its life, immediately prioritizes establishing the CPU topology *before* signaling that the CPU is actually starting up. The sequence is quite deliberate: `set_cpu_sibling_map(cpuid);` is called, and *only then* is `notify_cpu_starting(cpuid);` invoked.
My immediate instinct is to probe deeper into what `set_cpu_sibling_map` actually accomplishes. The comment within `ap_starting` itself is a key indicator: "The topology information must be up to date before `notify_cpu_starting()`." This strongly suggests that `set_cpu_sibling_map` is responsible for populating or updating the `topology_sibling_cpumask` for *other* CPUs that share the same physical core. This makes perfect sense β before a CPU can fully participate, it needs to know its neighbors, its siblings within the core, to avoid contention and ensure proper scheduling.
Following this, I'm drawn to `notify_cpu_starting`. The comment here is equally illuminating: " `notify_cpu_starting()` will call into the sched callbacks which set the `irq_work_capacity` to 1." This tells me that `notify_cpu_starting` is the gateway for the scheduler to become aware of the newly active CPU. It's the point where the scheduler begins to integrate this CPU into its operational framework.
My next step in unraveling this is to trace the execution path from `notify_cpu_starting`. The text indicates that it calls `sched_core_cpu_starting`. This is where the detailed mechanics of the scheduler's core logic come into play. Now, I'm particularly focused on the synchronization primitives used within `sched_core_cpu_starting`. I see the mention of `core_lock` and the `guard(core_lock)(&cpu);` construct. My immediate thought is: is this `core_lock` a standard mutex, or something more specialized given the context of CPU core operations? The `guard` syntax hints at a lock acquisition and release pattern, but the underlying type of lock is critical. I need to confirm if `core_lock` is indeed a `raw_spinlock` to understand the precise nature of the contention and locking mechanisms employed during this critical phase of CPU initialization. My next action would be to dive into the definition of `core_lock` within `kernel/sched/core.c` to solidify this understanding.
|
| 81/2 |
2026/10/13 08:31 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core.c FirstLine:6600 LineCount:20]
Results: map[Contents:6600:
6601: queue_balance_callback(rq, &per_cpu(core_balance_head, rq->cpu), sched_core_balance);
6602: }
6603:
6604: DEFINE_LOCK_GUARD_1(core_lock, int,
6605: sched_core_lock(*_T->lock, &_T->flags),
6606: sched_core_unlock(*_T->lock, &_T->flags),
6607: unsigned long flags)
6608:
6609: static void sched_core_cpu_starting(unsigned int cpu)
6610: {
6611: const struct cpumask *smt_mask = cpu_smt_mask(cpu);
6612: struct rq *rq = cpu_rq(cpu), *core_rq = NULL;
6613: int t;
6614:
6615: guard(core_lock)(&cpu);
6616:
6617: WARN_ON_ONCE(rq->core != rq);
6618:
6619: /* if we're the first, we'll be our own leader */
]
|
| 82/2 |
2026/10/13 08:31 |
llm |
expert |
1m
Model:
Tokens:
input: 53364
output: 42
thoughts: 5771
Thoughts: **My Thoughts on a Critical Race Condition in the Linux Scheduler**
I've been diving deep into a particularly thorny issue within the Linux kernel's scheduler, and I think I've finally pinpointed the root cause. Itβs a race condition, as many subtle bugs are, but this one is quite interesting because it involves CPU hotplugging and the `CONFIG_SCHED_CORE` feature.
My initial realization was about `sched_core_lock`. I saw that this function locks all the `rq` (runqueue) locks within a CPU core. This is a significant locking mechanism, ensuring that operations affecting an entire core are serialized. Then, I looked at `sched_core_cpu_starting`, which indeed acquires the `rq` locks for all CPUs in a core.
However, a crucial detail immediately jumped out at me: `set_cpu_sibling_map` is called *before* `sched_core_cpu_starting` during the CPU startup process. This is where the alarm bells started ringing. `set_cpu_sibling_map` modifies the `topology_sibling_cpumask`βwhich dictates the SMT (Simultaneous Multi-Threading) siblings for a CPUβ*without* holding the `rq` locks for the affected CPUs.
This immediately creates a window of vulnerability. Imagine CPU B is busy executing `pick_next_task`. This function, as I understand it, holds the `rq` locks for all CPUs in its current SMT mask. Now, if CPU A is booting up and calls `set_cpu_sibling_map(CPU A)`, it can modify the `topology_sibling_cpumask` for CPU B. The problem is that CPU B might be concurrently iterating over this very mask within `pick_next_task`.
Let me walk through the scenario:
1. **CPU B is running `pick_next_task`:** It acquires the `rq` locks for all CPUs in its current `smt_mask`. It then proceeds to iterate through this mask.
2. **CPU A starts booting:** It calls `ap_starting` and subsequently `set_cpu_sibling_map(CPU A)`.
3. **The mask changes:** `set_cpu_sibling_map` modifies the `topology_sibling_cpumask` for CPU B to include CPU A. Crucially, this happens *without* the `rq` locks being held for CPU B's SMT siblings.
4. **CPU A proceeds:** It calls `notify_cpu_starting(CPU A)`, which in turn calls `sched_core_cpu_starting(CPU A)`.
5. **`sched_core_cpu_starting` attempts to lock:** It calls `sched_core_lock(CPU A)`. This function tries to acquire the `rq` locks for all CPUs in CPU A's `smt_mask`.
6. **Deadlock or worse:** `sched_core_lock` will likely block on CPU B's `rq` lock because CPU B is holding it in `pick_next_task`. However, the critical damage might have already been done.
Let's look at what happens inside `pick_next_task` on CPU B:
The function first reads the `smt_mask`:
```c
smt_mask = cpu_smt_mask(cpu); // Points to topology_sibling_cpumask(CPU B)
```
CPU B then iterates over this mask in the first loop:
```c
for_each_cpu_wrap(i, smt_mask, cpu) {
rq_i = cpu_rq(i);
p = pick_task(rq_i, rf);
// ...
rq_i->core_pick = p; // Sets the core pick for CPU i
}
```
Suppose, initially, `smt_mask` for CPU B only contained CPU B itself. CPU B would set `rq_i->core_pick` for CPU B.
Then, as described, CPU A executes `set_cpu_sibling_map(CPU A)`, which adds CPU A to CPU B's `smt_mask`.
Now, CPU B enters the *second* loop in `pick_next_task`:
```c
for_each_cpu(i, smt_mask) {
rq_i = cpu_rq(i);
p = rq_i->core_pick; // Reads the core_pick for CPU i
if (!cookie_equals(p, cookie)) {
// ...
}
}
```
When CPU B reads the `smt_mask` again, it now sees both CPU B *and* CPU A. It proceeds to iterate. When it gets to CPU A (`i = CPU A`), it reads `rq_i->core_pick` for CPU A.
Here's the critical point: Since CPU A was just starting up and `set_cpu_sibling_map` was called *before* `sched_core_cpu_starting` finished its setup, CPU A's `rq->core_pick` was likely never properly initialized. In a typical scenario, an uninitialized `rq->core_pick` for a new CPU would be `NULL`.
So, `p` becomes `NULL`. The code then calls `cookie_equals(p, cookie)`, which translates to `cookie_equals(NULL, cookie)`. This function, in turn, calls `is_task_rq_idle(NULL)`, which then calls `task_rq(NULL)`, and finally `task_cpu(NULL)`. Dereferencing a `NULL` pointer here leads to a catastrophic **NULL pointer dereference**. Boom!
This scenario perfectly explains the observed bug. It's a classic race condition:
* **The Race:** `set_cpu_sibling_map` (part of CPU hotplug) modifies the `topology_sibling_cpumask` concurrently with `pick_next_task` iterating over it.
* **The Consequence:** Because `pick_next_task` iterates twice, and the mask can change between iterations, a CPU (CPU A in my example) that wasn't present in the mask during the first loop might be present in the second. If its `core_pick` field was not initialized (because it wasn't iterated over in the first loop), reading it will result in a `NULL` value.
* **The Exploitability:** This `NULL` value is then used in subsequent checks (`cookie_equals`, `is_task_rq_idle`), eventually leading to a `NULL` pointer dereference, typically causing a kernel Oops or panic.
Now, regarding the exploitability and accessibility of this bug:
* **Exploitable?** No. While it's a NULL pointer dereference, the kernel's memory protection mechanisms (specifically `mmap_min_addr`) prevent unprivileged users from mapping page 0. This means the attacker cannot control the address being dereferenced to manipulate the system. It leads to a crash, but not to privilege escalation or code execution.
* **Denial of Service?** Absolutely. It causes a kernel panic, bringing the system down.
* **Accessible from Unprivileged Processes?** No. The core of this race condition is triggered by CPU hotplug operations (bringing a CPU online). These operations are privileged and require `CAP_SYS_ADMIN`.
* **Accessible from User Namespaces?** No. CPU hotplug requires global `CAP_SYS_ADMIN`, not just namespace-specific capabilities.
* **Can a Guest VM Trigger This?** A guest VM cannot directly trigger this in the host kernel. However, if a hypervisor hotplugs a vCPU into a guest, the guest kernel will perform its CPU startup sequence, and this race could occur within the guest. But this is not a security boundary violation, as the hypervisor already controls the guest.
It's also worth noting that the `prctl(PR_SCHED_CORE)` system call, which enables core scheduling for unprivileged processes, *is* accessible to unprivileged users. However, the bug itself hinges on the *CPU hotplug* mechanism, which is the privileged part, and not directly on the unprivileged use of `PR_SCHED_CORE`. The fact that `CONFIG_SCHED_CORE` is involved simply means that `pick_next_task` is being called in a context where core scheduling is active.
In summary, I've identified a critical race condition between CPU hotplug and the core scheduler's task selection logic. It's a robust Denial of Service vulnerability, but due to its reliance on privileged CPU hotplug operations, it is not exploitable by unprivileged users.
|
| 83/2 |
2026/10/13 08:32 |
tool |
grepper |
0m
Args: map[Expression:PR_SCHED_CORE PathPrefixes:[kernel/sys.c kernel/sched/core.c]]
Results: map[Output:kernel/sys.c=2534=SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
--
kernel/sys.c-2806-#ifdef CONFIG_SCHED_CORE
kernel/sys.c:2807: case PR_SCHED_CORE:
kernel/sys.c-2808- error = sched_core_share_pid(arg2, arg3, arg4, arg5);
]
|
| 84/2 |
2026/10/13 08:32 |
llm |
expert |
2m
Model:
Tokens:
input: 59285
output: 37
thoughts: 13
Thoughts: **My Thought Process**
Right, so the first thing that comes to my mind, as I'm diving into this problem, is to get a handle on how scheduling decisions are being influenced by core sharing. My immediate go-to for that is `sched_core_share_pid`. I need to see what information that particular kernel symbol is exposing, or potentially controlling, in relation to process scheduling and how it interacts with core allocation. Itβs the key to understanding the granular behavior I'm looking for.
|
| 85/2 |
2026/10/13 08:34 |
tool |
read-file |
0m
Error:
Args: map[File:kernel/sched/core_sched.c FirstLine:330 LineCount:40]
Results:
|
| 86/2 |
2026/10/13 08:34 |
llm |
expert |
0m
Model:
Tokens:
input: 59359
output: 32
thoughts: 35
Thoughts: **My Thought Process**
My initial impulse is to delve into the kernel code, specifically the scheduler's core logic. I'm thinking about the function `sched_core_share_pid`. My immediate instinct is to locate this function within the kernel source tree, likely in `kernel/sched/core.c` or perhaps `kernel/sched/core_sched.c` if it's in a more specialized file. I'm trying to pinpoint where this particular scheduling mechanism is defined and implemented.
|
| 87/2 |
2026/10/13 08:34 |
tool |
grepper |
0m
Args: map[Expression:sched_core_share_pid PathPrefixes:[kernel/sched/]]
Results: map[Output:kernel/sched/core_sched.c=123=static void __sched_core_set(struct task_struct *p, unsigned long cookie)
--
kernel/sched/core_sched.c-130-/* Called from prctl interface: PR_SCHED_CORE */
kernel/sched/core_sched.c:131:int sched_core_share_pid(unsigned int cmd, pid_t pid, enum pid_type type,
kernel/sched/core_sched.c-132- unsigned long uaddr)
]
|
| 88/2 |
2026/10/13 08:34 |
llm |
expert |
0m
Model:
Tokens:
input: 59548
output: 37
thoughts: 13
Thoughts: **My Thought Process Regarding `sched_core_share_pid`**
Alright, I need to dig into this `sched_core_share_pid`. My first instinct is to understand its fundamental purpose. What exactly does this function *do*? It sounds like it's related to scheduling, specifically concerning "core shares" and PIDs. This immediately triggers thoughts about how the Linux kernel manages processes, how it assigns them to CPU cores, and how it might enforce fairness or resource allocation among them.
So, I'll begin by dissecting the name itself: `sched_core_share_pid`.
* `sched`: This clearly indicates it's part of the scheduler subsystem. My mind immediately goes to the scheduler's responsibilities β task selection, CPU allocation, context switching, and ensuring system responsiveness and throughput.
* `core_share`: This is the intriguing part. It suggests a mechanism for sharing resources at the core level, likely CPU cores. This could imply things like:
* **Affinity:** Is it about setting or managing CPU affinity for processes?
* **Load Balancing:** Could it be involved in distributing tasks across cores to prevent overload on any single core?
* **Resource Partitioning:** Is it a way to guarantee a certain level of CPU resource for specific groups of processes?
* **Fairness:** Perhaps it's about ensuring that a "share" of the core is given to a PID or a group of PIDs, implying some form of fair-share scheduling.
* `pid`: This is straightforward β it refers to a Process ID. So, the functionality is directly tied to individual processes.
Given this initial breakdown, I'm anticipating that `sched_core_share_pid` will likely involve:
1. **Identifying the target PID:** The function will need to know *which* process we're talking about.
2. **Determining its "share" of a core:** This is the core logic. How is this share quantified? Is it a percentage, a fixed number of cycles, a priority level?
3. **Applying this share:** How does the scheduler translate this "share" into actual CPU time or core assignment for the PID? This might involve modifying existing scheduling policies, creating new scheduling entities, or interacting with other scheduler components.
4. **Potential interactions with other kernel components:** Does it interact with cgroups? Does it use RT_SCHED or other scheduling classes? Does it rely on specific CPU schedulers (like CFS)?
My next step would be to look at the function's signature and its immediate surrounding code to get concrete details. What arguments does it take? What does it return? What data structures does it manipulate? Is it a public API, or an internal helper function? The context in which it's called will be crucial for understanding its role.
Essentially, I'm trying to build a mental model of *how* a PID gets its designated "share" of a CPU core, and the underlying mechanism the kernel employs to achieve this. The term "share" is key, suggesting a granular control over resource allocation beyond simple CPU assignment. I'm expecting to see logic that either directly assigns a PID to a core and limits its usage, or more likely, integrates this "share" concept into the existing scheduling algorithms to ensure the desired outcome.
|
| 89/2 |
2026/10/13 08:35 |
tool |
read-file |
0m
Args: map[File:kernel/sched/core_sched.c FirstLine:131 LineCount:40]
Results: map[Contents: 131: int sched_core_share_pid(unsigned int cmd, pid_t pid, enum pid_type type,
132: unsigned long uaddr)
133: {
134: unsigned long cookie = 0, id = 0;
135: struct task_struct *task, *p;
136: struct pid *grp;
137: int err = 0;
138:
139: if (!sched_smt_active())
140: return -ENODEV;
141:
142: BUILD_BUG_ON(PR_SCHED_CORE_SCOPE_THREAD != PIDTYPE_PID);
143: BUILD_BUG_ON(PR_SCHED_CORE_SCOPE_THREAD_GROUP != PIDTYPE_TGID);
144: BUILD_BUG_ON(PR_SCHED_CORE_SCOPE_PROCESS_GROUP != PIDTYPE_PGID);
145:
146: if (type > PIDTYPE_PGID || cmd >= PR_SCHED_CORE_MAX || pid < 0 ||
147: (cmd != PR_SCHED_CORE_GET && uaddr))
148: return -EINVAL;
149:
150: rcu_read_lock();
151: if (pid == 0) {
152: task = current;
153: } else {
154: task = find_task_by_vpid(pid);
155: if (!task) {
156: rcu_read_unlock();
157: return -ESRCH;
158: }
159: }
160: get_task_struct(task);
161: rcu_read_unlock();
162:
163: /*
164: * Check if this process has the right to modify the specified
165: * process. Use the regular "ptrace_may_access()" checks.
166: */
167: if (!ptrace_may_access(task, PTRACE_MODE_READ_REALCREDS)) {
168: err = -EPERM;
169: goto out;
170: }
]
|
| 90/2 |
2026/10/13 08:35 |
llm |
expert |
0m
Model:
Tokens:
input: 60205
output: 34
thoughts: 229
Thoughts: **My Thought Process Regarding the `prctl(PR_SCHED_CORE)` Vulnerability**
Alright, let's break down my current thinking on this `prctl(PR_SCHED_CORE)` issue, particularly concerning its exploitability by unprivileged users.
My initial thought, and what seems to be confirmed, is that the `prctl(PR_SCHED_CORE)` call itself is unprivileged. This means, assuming standard `ptrace` access controls are in place, an unprivileged user *can* indeed enable core scheduling. This is a significant point β the prerequisite feature is accessible.
However, my attention immediately shifts to the *trigger* for the bug. The documentation or analysis I'm looking at points to CPU hotplug being the necessary condition to induce the race condition. This is where my analysis hits a wall for unprivileged users.
Can an unprivileged user initiate CPU hotplug events? My understanding is a resounding "no." This action typically requires elevated privileges.
Therefore, the logical conclusion at this point is that while an unprivileged user can *enable* the problematic feature, they are fundamentally blocked from *triggering* the vulnerability because they cannot create the race condition itself through CPU hotplug. It's like having a key to a locked room, but no way to open the door.
But, being thorough, I always ask myself, "Is this the *only* path?" I start probing for alternative ways the critical state might change. I'm specifically wondering if `smt_mask` can be modified through avenues *other than* the explicit `prctl` call or the race condition.
My mind then goes to `sched_smt_active()`. I know this is a static key, implying it's designed for dynamic runtime modification. If `sched_smt_active()` changes, does that, in turn, alter `smt_mask`?
My immediate answer to that is "no," because `cpu_smt_mask(cpu)` appears to always return `topology_sibling_cpumask(cpu)`. The `sched_smt_active()` state itself doesn't seem to directly dictate `smt_mask`.
This leads me to a more granular question: what if `topology_sibling_cpumask` itself is modified by something else? This seems like a more plausible, albeit less direct, avenue. To investigate this, I'd be diving into the kernel source, specifically looking for any instances where `link_mask` (which I assume is related to forming these topology masks) or `cpumask_set_cpu` are called on `topology_sibling_cpumask`. This is my current line of inquiry to see if any other system event, potentially even one an unprivileged user could influence indirectly, could alter the sibling mask and thus indirectly affect the `smt_mask` in a way that might expose the bug.
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| 91/2 |
2026/10/13 08:35 |
tool |
grepper |
0m
Args: map[Expression:topology_sibling_cpumask PathPrefixes:[arch/x86/]]
Results: map[Output:arch/x86/events/amd/power.c=223=static int power_cpu_exit(unsigned int cpu)
--
arch/x86/events/amd/power.c-234- */
arch/x86/events/amd/power.c:235: target = cpumask_any_but(topology_sibling_cpumask(cpu), cpu);
arch/x86/events/amd/power.c-236- if (target < nr_cpumask_bits) {
--
arch/x86/events/amd/power.c=243=static int power_cpu_init(unsigned int cpu)
--
arch/x86/events/amd/power.c-255- */
arch/x86/events/amd/power.c:256: target = cpumask_any_but(topology_sibling_cpumask(cpu), cpu);
arch/x86/events/amd/power.c-257- if (target >= nr_cpumask_bits)
--
arch/x86/events/intel/core.c=6428=static void intel_pmu_cpu_starting(int cpu)
--
arch/x86/events/intel/core.c-6482- if (!(x86_pmu.flags & PMU_FL_NO_HT_SHARING)) {
arch/x86/events/intel/core.c:6483: for_each_cpu(i, topology_sibling_cpumask(cpu)) {
arch/x86/events/intel/core.c-6484- struct intel_shared_regs *pc;
--
arch/x86/events/intel/core.c-6500- if (x86_pmu.flags & PMU_FL_EXCL_CNTRS) {
arch/x86/events/intel/core.c:6501: for_each_cpu(i, topology_sibling_cpumask(cpu)) {
arch/x86/events/intel/core.c-6502- struct cpu_hw_events *sibling;
--
arch/x86/include/asm/topology.h=192=static inline int topology_get_logical_id(u32 apicid, enum x86_topology_domains at_level)
--
arch/x86/include/asm/topology.h-202-#define topology_core_cpumask(cpu) (per_cpu(cpu_core_map, cpu))
arch/x86/include/asm/topology.h:203:#define topology_sibling_cpumask(cpu) (per_cpu(cpu_sibling_map, cpu))
arch/x86/include/asm/topology.h-204-
--
arch/x86/kernel/cpu/microcode/core.c=438=static void __load_primary(unsigned int cpu)
arch/x86/kernel/cpu/microcode/core.c-439-{
arch/x86/kernel/cpu/microcode/core.c:440: struct cpumask *secondaries = topology_sibling_cpumask(cpu);
arch/x86/kernel/cpu/microcode/core.c-441- enum sibling_ctrl ctrl;
--
arch/x86/kernel/cpu/microcode/core.c=698=static bool setup_cpus(void)
--
arch/x86/kernel/cpu/microcode/core.c-736- */
arch/x86/kernel/cpu/microcode/core.c:737: ctrl.ctrl_cpu = cpumask_first(topology_sibling_cpumask(cpu));
arch/x86/kernel/cpu/microcode/core.c-738- per_cpu(ucode_ctrl, cpu) = ctrl;
--
arch/x86/kernel/process.c=528=void speculative_store_bypass_ht_init(void)
--
arch/x86/kernel/process.c-548- */
arch/x86/kernel/process.c:549: for_each_cpu(cpu, topology_sibling_cpumask(this_cpu)) {
arch/x86/kernel/process.c-550- if (cpu == this_cpu)
--
arch/x86/kernel/smpboot.c=694=void set_cpu_sibling_map(int cpu)
--
arch/x86/kernel/smpboot.c-704- if (!has_mp) {
arch/x86/kernel/smpboot.c:705: cpumask_set_cpu(cpu, topology_sibling_cpumask(cpu));
arch/x86/kernel/smpboot.c-706- cpumask_set_cpu(cpu, cpu_llc_shared_mask(cpu));
--
arch/x86/kernel/smpboot.c-720- if ((i == cpu) || (has_smt && match_smt(c, o)))
arch/x86/kernel/smpboot.c:721: link_mask(topology_sibling_cpumask, cpu, i);
arch/x86/kernel/smpboot.c-722-
--
arch/x86/kernel/smpboot.c-732-
arch/x86/kernel/smpboot.c:733: threads = cpumask_weight(topology_sibling_cpumask(cpu));
arch/x86/kernel/smpboot.c-734- if (threads > __max_smt_threads)
--
arch/x86/kernel/smpboot.c-736-
arch/x86/kernel/smpboot.c:737: for_each_cpu(i, topology_sibling_cpumask(cpu))
arch/x86/kernel/smpboot.c-738- cpu_data(i).smt_active = threads > 1;
--
arch/x86/kernel/smpboot.c-741- * This needs a separate iteration over the cpus because we rely on all
arch/x86/kernel/smpboot.c:742: * topology_sibling_cpumask links to be set-up.
arch/x86/kernel/smpboot.c-743- */
--
arch/x86/kernel/smpboot.c-758- if (cpumask_first(
arch/x86/kernel/smpboot.c:759: topology_sibling_cpumask(i)) == i)
arch/x86/kernel/smpboot.c-760- c->booted_cores++;
--
arch/x86/kernel/smpboot.c=1160=static __init void disable_smp(void)
--
arch/x86/kernel/smpboot.c-1166-
arch/x86/kernel/smpboot.c:1167: cpumask_set_cpu(0, topology_sibling_cpumask(0));
arch/x86/kernel/smpboot.c-1168- cpumask_set_cpu(0, topology_core_cpumask(0));
--
arch/x86/kernel/smpboot.c=1297=static void recompute_smt_state(void)
--
arch/x86/kernel/smpboot.c-1302- for_each_online_cpu (cpu) {
arch/x86/kernel/smpboot.c:1303: int threads = cpumask_weight(topology_sibling_cpumask(cpu));
arch/x86/kernel/smpboot.c-1304-
--
arch/x86/kernel/smpboot.c=1311=static void remove_siblinginfo(int cpu)
--
arch/x86/kernel/smpboot.c-1320- */
arch/x86/kernel/smpboot.c:1321: if (cpumask_weight(topology_sibling_cpumask(cpu)) == 1)
arch/x86/kernel/smpboot.c-1322- cpu_data(sibling).booted_cores--;
--
arch/x86/kernel/smpboot.c-1327-
arch/x86/kernel/smpboot.c:1328: for_each_cpu(sibling, topology_sibling_cpumask(cpu)) {
arch/x86/kernel/smpboot.c:1329: cpumask_clear_cpu(cpu, topology_sibling_cpumask(sibling));
arch/x86/kernel/smpboot.c:1330: if (cpumask_weight(topology_sibling_cpumask(sibling)) == 1)
arch/x86/kernel/smpboot.c-1331- cpu_data(sibling).smt_active = false;
--
arch/x86/kernel/smpboot.c-1339- cpumask_clear(cpu_l2c_shared_mask(cpu));
arch/x86/kernel/smpboot.c:1340: cpumask_clear(topology_sibling_cpumask(cpu));
arch/x86/kernel/smpboot.c-1341- cpumask_clear(topology_core_cpumask(cpu));
]
|
| 92/2 |
2026/10/13 08:35 |
llm |
expert |
0m
Model:
Tokens:
input: 62835
output: 66
thoughts: 167
Thoughts: **My Thought Process Regarding the CPU Sibling Map Race Condition**
I've been analyzing a specific race condition related to the CPU sibling map. My investigation has led me to conclude that the only functions that can modify this critical data structure are `set_cpu_sibling_map`, `disable_smp`, and `remove_siblinginfo`. Crucially, all of these operations are confined to specific, limited windows: either during the initial boot process or, more significantly for potential races, during CPU hotplug events. This observation leads me to a firm conclusion: the race condition I'm concerned with is strictly limited to the context of CPU hotplug.
Based on this understanding, I've assessed the potential exploitability and impact of this race. I am highly confident in my analysis.
* **Exploitable:** I've determined that this race is **False**. While a NULL pointer dereference might occur, it doesn't open the door to arbitrary code execution or the mapping of page 0, which are typical hallmarks of exploitable vulnerabilities.
* **Denial of Service:** This race condition is **True** for Denial of Service. The potential for a kernel oops or panic is a direct consequence, leading to system instability or a crash.
* **Unprivileged:** This vulnerability is **False** for unprivileged users. The prerequisite for triggering the race is CPU hotplug, an operation that inherently requires root privileges.
* **UserNamespace:** This is **False**. CPU hotplug operations are not something that can be confined within a user namespace. They necessitate global CAP\_SYS\_ADMIN capabilities.
* **VMGuestTrigger:** This is **False**. The vulnerability cannot be triggered from within a guest VM.
* **VMHostTrigger:** This is **False**. It's not possible to trigger this from the host itself in a way that would exploit the race condition.
* **NetworkTrigger:** This is **False**. There is no network vector for this vulnerability.
* **RemoteTrigger:** This is **False**. It cannot be triggered remotely.
* **PeripheralTrigger:** This is **False**. No peripheral device interaction is involved.
* **FilesystemTrigger:** This is **False**. The race condition is not related to filesystem operations.
In summary, my analysis points to a race condition primarily during CPU hotplug. While it can cause a denial of service due to potential kernel panics, it is not exploitable in the sense of arbitrary code execution and requires root privileges to even attempt to trigger.
|
| 93/2 |
2026/10/13 08:35 |
tool |
set-results |
0m
Args: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:true Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 94/2 |
2026/10/13 08:35 |
llm |
expert |
0m
Model:
Tokens:
input: 63088
output: 725
thoughts: 614
Thoughts: **My Thoughts on the Kernel Bug**
Alright, I've analyzed the results and I'm ready to articulate my understanding of this kernel bug.
So, what I've pinpointed is a classic NULL pointer dereference, and it's happening right in the heart of the core scheduling logic, specifically within `kernel/sched/core.c`. The root cause is a race condition that can occur between the `pick_next_task` function and CPU hotplug operations, particularly when a CPU is coming online.
Let me break down the sequence of events as I see it. Inside `pick_next_task`, when the `CONFIG_SCHED_CORE` option is enabled, the scheduler is designed to iterate over the SMT siblings of the current CPU, and it does this *twice*. The first pass, using `for_each_cpu_wrap(i, smt_mask, cpu)`, is where it calls `pick_task` for each sibling and stashes the result in `rq_i->core_pick`. Then, in the second pass, using `for_each_cpu(i, smt_mask)`, it revisits these siblings, retrieves that stored `core_pick` value, and performs a check using `cookie_equals(p, cookie)`.
Now, here's where the trouble brews. The `smt_mask` we're talking about here is a pointer to the `topology_sibling_cpumask` associated with a CPU. The critical vulnerability lies in the fact that when a CPU is brought online β during the `set_cpu_sibling_map` operation β this `cpumask` is modified to include the new CPU. And importantly, this modification happens *without* holding any runqueue locks.
So, imagine this scenario: a CPU is actively running `pick_next_task`. Concurrently, another CPU is going through the hotplug process and coming online. In its first loop, the scheduler on the running CPU might see the `smt_mask` and only perceive itself as a sibling. Then, just as it's about to proceed, the booting CPU modifies the mask. When the scheduler on the running CPU enters its second loop and re-reads the `smt_mask`, it now sees the newly added CPU. It then attempts to access `rq_i->core_pick` for this *new* CPU. Since this new CPU wasn't part of the first iteration (where `pick_task` would have initialized `core_pick`), its `core_pick` remains uninitialized β in essence, `NULL`. This `NULL` pointer is then passed to `cookie_equals`, which, in trying to access `p->cpu` via `task_cpu(p)` at offset `0x14`, triggers the dereference, leading to a kernel Oops.
Regarding exploitability, this is a straightforward NULL pointer dereference. The faulting address, `0xdffffc0000000002`, is indicative of trying to read `p->cpu` from a `NULL` pointer. Because unprivileged users can't map page zero due to `mmap_min_addr` restrictions, this particular bug isn't exploitable for privilege escalation or code execution. It's strictly a Denial of Service (DoS) vulnerability.
And concerning accessibility, triggering this requires a specific race condition: the scheduler operation coinciding with a CPU coming online. This CPU hotplug operation, specifically writing to `/sys/devices/system/cpu/cpuX/online`, demands global `CAP_SYS_ADMIN` privileges. Therefore, unprivileged users are completely out of luck. It's also not something that can be triggered from within a user namespace, nor is it remotely accessible or exploitable through unprivileged peripheral devices. It's a high-privilege, local race condition.
|
| 95/1 |
2026/10/13 08:36 |
action |
format-explanation |
0m
Results: map[Explanation:Based on the analysis of the bug report and the kernel source code, here is the
assessment of the vulnerability:
**Root Cause Analysis:**
The bug is a NULL pointer dereference in the core scheduling code
(`kernel/sched/core.c`). It occurs due to a race condition between the
scheduler's `pick_next_task()` function and CPU hotplug operations.
When `CONFIG_SCHED_CORE` is enabled, `pick_next_task()` performs a core-wide
task selection. It iterates over the SMT siblings of the current CPU using
`smt_mask = cpu_smt_mask(cpu)` (which points to the CPU's
`topology_sibling_cpumask`). It does this in two passes:
1. **First loop** (`for_each_cpu_wrap`): It calls `pick_task()` for each sibling
and stores the selected task in `rq_i->core_pick`.
2. **Second loop** (`for_each_cpu`): It reads `rq_i->core_pick` for each sibling
and checks if it matches the core cookie using `cookie_equals(p, cookie)`.
During CPU hotplug (specifically when a CPU is coming online),
`set_cpu_sibling_map()` modifies the `topology_sibling_cpumask` to add the new
CPU. Crucially, this modification happens *without* holding the runqueue locks.
If a CPU is running `pick_next_task()` concurrently with another CPU coming
online, it can read the `smt_mask` in the first loop and see only itself. Then,
the booting CPU modifies the mask. In the second loop, the scheduling CPU reads
the mask again and sees the newly added CPU. It then accesses `rq_i->core_pick`
for the new CPU. Since the new CPU was not visited in the first loop, its
`core_pick` is uninitialized (i.e., `NULL`). This `NULL` pointer is passed to
`cookie_equals()`, which dereferences it when calling `task_cpu(p)`, resulting
in a kernel Oops.
**Security Impact:**
* **Exploitable:** No. The faulting address is `0xdffffc0000000002` (KASAN
shadow for `0x10-0x17`), which corresponds to reading `p->cpu` at offset `0x14`
from a `NULL` pointer. Because `mmap_min_addr` prevents mapping page 0 in modern
Linux systems, an attacker cannot place controlled data at this address.
Therefore, it cannot be exploited for privilege escalation or arbitrary code
execution.
* **Denial of Service:** Yes. Dereferencing the NULL pointer will cause a kernel
Oops/panic, crashing the system or deadlocking the runqueues.
* **Accessible From Unprivileged Processes / User Namespaces:** No. Triggering
this race condition strictly requires performing CPU hotplug operations (e.g.,
writing to `/sys/devices/system/cpu/cpuX/online`). CPU hotplug is a highly
privileged operation that requires global `CAP_SYS_ADMIN`. It cannot be
triggered by unprivileged users or from within a user namespace.
* **Remote / Peripheral / Filesystem Triggers:** No. The bug is tied to local
CPU topology management and cannot be triggered over the network, via untrusted
filesystems, or by plugging in standard peripheral devices.]
|