| 0/0 |
2026/05/29 23:47 |
flow |
assessment-security |
46m
Results: map[DenialOfService:false Explanation:Based on the analysis of the bug report and the kernel source code, here is the
assessment:
The bug report indicates a soft lockup on CPU 0: `watchdog: BUG: soft lockup -
CPU#0 stuck for 70s!`.
However, looking at the register dump for CPU 0:
```
pstate: 60001000 (nZCv daif -PAN -UAO -TCO -DIT +SSBS BTYPE=--)
pc : 0000ffff8f64c32c
lr : 0000ffff8f667f08
sp : 0000fffff8c58310
```
The `pstate` value `60001000` has its lowest 4 bits (`M` field) set to `0b0000`,
which corresponds to `EL0t` (64-bit userspace mode). The program counter (`pc`)
is also clearly a userspace address.
A soft lockup is detected when the per-CPU watchdog timer interrupt fires and
observes that the watchdog timestamp hasn't been updated by the high-priority
watchdog stopper thread (`migration/N`) for over 20 seconds.
If a CPU is genuinely executing in userspace, preemption is fully enabled. The
timer interrupt would fire every 4 seconds, queue the watchdog update work, and
upon returning to userspace, the kernel would schedule the high-priority stopper
thread to update the timestamp. Thus, a soft lockup **cannot** legitimately
occur while a CPU is continuously running in userspace.
The only scenario where a soft lockup is reported with a userspace PC is when
the virtual CPU (vCPU) is preempted or paused by the host hypervisor for an
extended period (in this case, >70 seconds).
1. The vCPU is executing in userspace.
2. The host hypervisor preempts the vCPU (due to high host load, overcommit, or
being explicitly paused).
3. 70 seconds of physical time pass.
4. The host resumes the vCPU.
5. The guest's timer interrupt fires immediately because it is long overdue.
6. The timer interrupt handler checks the current time, sees that 70 seconds
have passed since the last watchdog touch, and immediately reports a soft
lockup.
7. Because the timer interrupt interrupted userspace execution, the register
dump shows the userspace PC and `EL0t` state.
On some architectures (like x86), KVM provides a paravirtualized feature
(`kvm_check_and_clear_guest_paused()`) to inform the guest that it was paused,
allowing the guest to suppress this false positive. However, on ARM64, this
feature is currently not implemented (it returns a hardcoded `false`), making
ARM64 guests susceptible to these false positive soft lockup reports when
running in constrained or overcommitted VM environments (like the syzbot GCE
instances).
**Conclusion:**
This is not a kernel bug, but a known false positive artifact of the virtualized
testing environment. It has no security impact, cannot be exploited, and does
not cause a denial of service. Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
|
| 1/1 |
2026/05/29 23:47 |
action |
syz-repro-to-c-repro |
0m
Results:
|
| 2/1 |
2026/05/29 23:47 |
action |
kernel-checkouter |
2m
Results: map[KernelSrc:/app/workdir/cache/src/116cd263b902234cb5408e237e95296533c171cd]
|
| 3/1 |
2026/05/29 23:49 |
action |
kernel-builder |
20m
Results: map[KernelObj:/app/workdir/cache/build/6eaa6de93b4317aa7a1f048aa0efd60f32e41638]
|
| 4/1 |
2026/05/30 00:10 |
action |
codesearch-prepare |
19m
Results: map[Index:codesearch-index]
|
| 5/1 |
2026/05/30 00:29 |
agent |
expert |
3m
Model:
Results: map[DenialOfService:false 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 or elevated privileges.
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.
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:
watchdog: BUG: soft lockup - CPU#0 stuck for 70s! [syz.7.1244:10838]
Modules linked in:
irq event stamp: 2180
hardirqs last enabled at (2179): [<ffff8000867a1b28>] __el0_irq_handler_common+0x18/0x24 arch/arm64/kernel/entry-common.c:840
hardirqs last disabled at (2180): [<ffff8000867a1b28>] __el0_irq_handler_common+0x18/0x24 arch/arm64/kernel/entry-common.c:840
softirqs last enabled at (2178): [<ffff800080309080>] softirq_handle_end kernel/softirq.c:468 [inline]
softirqs last enabled at (2178): [<ffff800080309080>] handle_softirqs+0xbc4/0xd34 kernel/softirq.c:650
softirqs last disabled at (2173): [<ffff8000800204b0>] __do_softirq+0x14/0x20 kernel/softirq.c:656
CPU: 0 UID: 0 PID: 10838 Comm: syz.7.1244 Tainted: G L syzkaller #0 PREEMPT
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
pstate: 60001000 (nZCv daif -PAN -UAO -TCO -DIT +SSBS BTYPE=--)
pc : 0000ffff8f64c32c
lr : 0000ffff8f667f08
sp : 0000fffff8c58310
x29: 0000fffff8c58310 x28: 0000ffff8f9e0440 x27: ffff80008192fde8
x26: 0000ffff8f9f6448 x25: 000000000004356f x24: 0000ffff8f1ff008
x23: 0000ffff8f9e03b8 x22: 0000000000000001 x21: 0000000000000000
x20: 0000ffff8f9e0000 x19: ffff80008192fb38 x18: 0000ffff8f7d78c8
x17: 0000ffff8f767e80 x16: 0000ffff8f9c0010 x15: 000000233d3b39a3
x14: 0000000000006ed0 x13: 000000007fffffff x12: 0000ffff90715000
x11: 000000003b9ac9ff x10: 0000ffff8f9e2000 x9 : 0000ffff8f9e0000
x8 : 000000008192fb3c x7 : 0000ffff8f9e0530 x6 : 0000ffff8f9e2530
x5 : 0000000000001b38 x4 : 0000000000001b38 x3 : ffff80008192fb38
x2 : 000000008192fb39 x1 : ffff80008192fb38 x0 : 0000000000000004
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 UID: 0 PID: 4693 Comm: syz-executor Tainted: G L syzkaller #0 PREEMPT
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
pstate: 63400005 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--)
pc : __sanitizer_cov_trace_pc+0x8/0x5c kernel/kcov.c:210
lr : local_lock_release include/linux/local_lock_internal.h:60 [inline]
lr : __folio_batch_add_and_move+0x5c0/0xac0 mm/swap.c:201
sp : ffff8000985272f0
x29: ffff800098527320 x28: 1fffe00035c0e9b6 x27: 0000000000000000
x26: ffff80008e635000 x25: dfff800000000000 x24: ffff80008e635ce0
x23: fffffdffc3d659c8 x22: 0000000000000000 x21: ffff80008892edb0
x20: ffff0001ae0749a8 x19: ffff0001ae074980 x18: 0000000000000000
x17: 0000000000000002 x16: 0000000000000000 x15: 0000000000000000
x14: ffff0001fea577c0 x13: 0000000000000001 x12: 0000000000000004
x11: ffff700011cd19b0 x10: 0000000000ff0100 x9 : ffff0000dc5a8000
x8 : ffff0000dc5a8000 x7 : ffff800080938bd8 x6 : 0000000000000000
x5 : 0000000000000000 x4 : 0000000000000001 x3 : ffff800080938f94
x2 : 0000000000000000 x1 : ffff0000dc5a8000 x0 : 0000000000000000
Call trace:
__sanitizer_cov_trace_pc+0x8/0x5c kernel/kcov.c:210 (P)
folio_activate+0xf8/0x124 mm/swap.c:339
folio_mark_accessed+0x210/0x820 mm/swap.c:486
zap_present_folio_ptes mm/memory.c:1641 [inline]
zap_present_ptes mm/memory.c:1714 [inline]
do_zap_pte_range mm/memory.c:1816 [inline]
zap_pte_range mm/memory.c:1918 [inline]
zap_pmd_range mm/memory.c:2004 [inline]
zap_pud_range mm/memory.c:2032 [inline]
zap_p4d_range mm/memory.c:2053 [inline]
__zap_vma_range+0x168c/0x404c mm/memory.c:2093
unmap_vmas+0x2d4/0x438 mm/memory.c:2162
exit_mmap+0x1e8/0xaf8 mm/mmap.c:1300
__mmput+0xe4/0x2f0 kernel/fork.c:1178
mmput+0x70/0xa8 kernel/fork.c:1201
exit_mm+0x190/0x26c kernel/exit.c:582
do_exit+0x518/0x1a6c kernel/exit.c:964
do_group_exit+0x194/0x22c kernel/exit.c:1119
__do_sys_exit_group kernel/exit.c:1130 [inline]
__se_sys_exit_group kernel/exit.c:1128 [inline]
pid_child_should_wake+0x0/0x110 kernel/exit.c:1128
__invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
invoke_syscall+0x98/0x244 arch/arm64/kernel/syscall.c:49
el0_svc_common+0xe8/0x23c arch/arm64/kernel/syscall.c:121
do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:140
el0_svc+0x64/0x260 arch/arm64/kernel/entry-common.c:740
el0t_64_sync_handler+0x48/0x148 arch/arm64/kernel/entry-common.c:759
el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:594
Reply: Based on the analysis of the bug report and the kernel source code, here is the assessment:
The bug report indicates a soft lockup on CPU 0: `watchdog: BUG: soft lockup - CPU#0 stuck for 70s!`.
However, looking at the register dump for CPU 0:
```
pstate: 60001000 (nZCv daif -PAN -UAO -TCO -DIT +SSBS BTYPE=--)
pc : 0000ffff8f64c32c
lr : 0000ffff8f667f08
sp : 0000fffff8c58310
```
The `pstate` value `60001000` has its lowest 4 bits (`M` field) set to `0b0000`, which corresponds to `EL0t` (64-bit userspace mode). The program counter (`pc`) is also clearly a userspace address.
A soft lockup is detected when the per-CPU watchdog timer interrupt fires and observes that the watchdog timestamp hasn't been updated by the high-priority watchdog stopper thread (`migration/N`) for over 20 seconds.
If a CPU is genuinely executing in userspace, preemption is fully enabled. The timer interrupt would fire every 4 seconds, queue the watchdog update work, and upon returning to userspace, the kernel would schedule the high-priority stopper thread to update the timestamp. Thus, a soft lockup **cannot** legitimately occur while a CPU is continuously running in userspace.
The only scenario where a soft lockup is reported with a userspace PC is when the virtual CPU (vCPU) is preempted or paused by the host hypervisor for an extended period (in this case, >70 seconds).
1. The vCPU is executing in userspace.
2. The host hypervisor preempts the vCPU (due to high host load, overcommit, or being explicitly paused).
3. 70 seconds of physical time pass.
4. The host resumes the vCPU.
5. The guest's timer interrupt fires immediately because it is long overdue.
6. The timer interrupt handler checks the current time, sees that 70 seconds have passed since the last watchdog touch, and immediately reports a soft lockup.
7. Because the timer interrupt interrupted userspace execution, the register dump shows the userspace PC and `EL0t` state.
On some architectures (like x86), KVM provides a paravirtualized feature (`kvm_check_and_clear_guest_paused()`) to inform the guest that it was paused, allowing the guest to suppress this false positive. However, on ARM64, this feature is currently not implemented (it returns a hardcoded `false`), making ARM64 guests susceptible to these false positive soft lockup reports when running in constrained or overcommitted VM environments (like the syzbot GCE instances).
**Conclusion:**
This is not a kernel bug, but a known false positive artifact of the virtualized testing environment. It has no security impact, cannot be exploited, and does not cause a denial of service.
|
| 6/2 |
2026/05/30 00:29 |
llm |
expert |
0m
Model:
Tokens:
input: 8065
output: 28
thoughts: 3368
Thoughts: **Analyzing a Soft Lockup on Arm64: A Deep Dive**
Okay, I'm looking at a soft lockup report from syzbot on an arm64 system. The critical detail is that CPU 0 is stuck for 70 seconds. The initial backtrace, though, seems a bit misleading, because it looks like it's a mix of CPU 0 and CPU 1. Initially, it suggests a problem potentially on CPU 1, deep within memory management β specifically, unmapping a VMA and related functions like `folio_activate`. However, it's CPU 0 that's actually the stuck one.
The initial confusion stems from the backtrace of CPU 0. The PC and LR values point to addresses that *look* like userspace, which is immediately suspect. A soft lockup, by definition, occurs when a CPU is stuck in kernel space without yielding. If it were in userspace, it should be preemptible. That address, `0000ffff8f64c32c`, *is* a valid userspace address on arm64 with a 48-bit VA. It's not a kernel address, not an issue of 32-bit vs. 64-bit confusion.
The key observation is `pstate: 60001000`. The important part here is the `daif` field, indicating interrupts are enabled. A soft lockup is impossible with preemption or interrupts enabled! However, on arm64 with interrupts enabled, this means it *shouldn't* be a soft lockup.
I need to understand *why* the CPU is flagged as soft-locked if it's in userspace. Could this be a priority issue with a real-time (SCHED_FIFO) userspace process hogging the CPU and preventing the soft lockup watchdog thread from running? Probably not, since the soft lockup detection is now handled by a timer interrupt, not a watchdog thread.
I then think this is a hypervisor issue. This is a Google Compute Engine VM, so the host might be pausing the VM or overcommitted. With this in mind, the guest sees the time go by. If the guest doesn't handle the steal time well, then it might flag a soft lockup when it's just waiting for the host.
I then consider the CPU 1 trace to see what is *actually* happening. It's deep in `unmap_vmas`, which involves calls to `zap_pte_range` and other memory management routines. `zap_pte_range` is a known potential bottleneck, but it does yield via `cond_resched()`. So CPU 1 *should* be fine. Is there a race or lock issue in `folio_activate`?
Finally, I'm thinking, this is probably just an artifact of the syzbot environment, where host overcommit can cause these kinds of transient issues. It is highly unlikely it is a real kernel bug.
|
| 7/2 |
2026/05/30 00:30 |
tool |
grepper |
0m
Args: map[Expression:folio_activate PathPrefix:mm/swap.c]
Results: map[Output:mm/swap.c=307=static void lru_activate(struct lruvec *lruvec, struct folio *folio)
--
mm/swap.c-324-#ifdef CONFIG_SMP
mm/swap.c:325:static void folio_activate_drain(int cpu)
mm/swap.c-326-{
--
mm/swap.c-332-
mm/swap.c:333:void folio_activate(struct folio *folio)
mm/swap.c-334-{
--
mm/swap.c-342-#else
mm/swap.c:343:static inline void folio_activate_drain(int cpu)
mm/swap.c-344-{
--
mm/swap.c-346-
mm/swap.c:347:void folio_activate(struct folio *folio)
mm/swap.c-348-{
--
mm/swap.c=461=void folio_mark_accessed(struct folio *folio)
--
mm/swap.c-485- if (folio_test_lru(folio))
mm/swap.c:486: folio_activate(folio);
mm/swap.c-487- else
--
mm/swap.c=648=void lru_add_drain_cpu(int cpu)
--
mm/swap.c-678-
mm/swap.c:679: folio_activate_drain(cpu);
mm/swap.c-680-}
]
|
| 8/2 |
2026/05/30 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 12482
output: 32
thoughts: 12
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience:
**Diving into `folio_activate`**
Alright, let's get down to brass tacks. I need to dissect the `folio_activate` function, understand its core logic, and perhaps identify any potential performance bottlenecks or design flaws. I'll need to examine the source code directly. This isn't just a casual glance; I'm looking for the nuances of its implementation, the assumptions it makes, and how it interacts with the broader system. I'll be particularly interested in:
* The overall flow of control within the function.
* Any error handling or edge-case management.
* Data structures it utilizes and how efficiently they're handled.
* Potential for race conditions or concurrency issues.
* Any dependencies it has on external libraries or services.
I'm expecting to quickly grasp the high-level architecture, then drill down into the details of specific code sections that catch my attention. Basically, I will have the source code opened and then look into it deeply.
|
| 9/2 |
2026/05/30 00:30 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/swap.c Name:folio_activate]
Results: map[SourceCode: 333: void folio_activate(struct folio *folio)
334: {
335: if (folio_test_active(folio) || folio_test_unevictable(folio) ||
336: !folio_test_lru(folio))
337: return;
338:
339: folio_batch_add_and_move(folio, lru_activate);
340: }
SourceFile:mm/swap.c]
|
| 10/2 |
2026/05/30 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 12905
output: 39
thoughts: 94
Thoughts: **Analyzing a Suspected Swap Bottleneck**
Okay, the backtrace is pointing me right at `__folio_batch_add_and_move` within `mm/swap.c`. Interesting. The stack trace indicates that `local_lock_release` is involved, and that's usually a good indicator of potential contention, especially within a core memory management function like this. I need to dive into this function to see what's happening. I'm going to carefully review the code at `mm/swap.c:201` and its surrounding lines to understand the context of this call and, more importantly, how the function is interacting with locks and shared resources. I want to see how these folios are being handled and moved, and whether there might be any potential for bottlenecks related to lock contention or inefficient batching in this process. Time to put on my memory management hat.
|
| 11/2 |
2026/05/30 00:30 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/swap.c Name:__folio_batch_add_and_move]
Results: map[SourceCode: 182: static void __folio_batch_add_and_move(struct folio_batch __percpu *fbatch,
183: struct folio *folio, move_fn_t move_fn, bool disable_irq)
184: {
185: unsigned long flags;
186:
187: folio_get(folio);
188:
189: if (disable_irq)
190: local_lock_irqsave(&cpu_fbatches.lock_irq, flags);
191: else
192: local_lock(&cpu_fbatches.lock);
193:
194: if (!folio_batch_add(this_cpu_ptr(fbatch), folio) ||
195: !folio_may_be_lru_cached(folio) || lru_cache_disabled())
196: folio_batch_move_lru(this_cpu_ptr(fbatch), move_fn);
197:
198: if (disable_irq)
199: local_unlock_irqrestore(&cpu_fbatches.lock_irq, flags);
200: else
201: local_unlock(&cpu_fbatches.lock);
202: }
SourceFile:mm/swap.c]
|
| 12/2 |
2026/05/30 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 13597
output: 38
thoughts: 52
Thoughts: Here's my analysis of that snippet:
**Diving Deep into `folio_batch_add_and_move`**
Okay, first things first, my initial thought is: hang on, `folio_batch_add_and_move` actually *calls* `__folio_batch_add_and_move`...and crucially, it passes a `disable_irq` value of `false`. That's a key observation.
Now I need to understand the implications of that call. I need to refresh my memory on what `folio_batch_add_and_move` does in the first place, and, critically, why it's potentially important that interrupts aren't disabled at this point, or at the least, what the function's responsibility is related to the interrupts. Time to go back and double-check the function definition and how it operates in the broader memory management context.
|
| 13/2 |
2026/05/30 00:30 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:mm/swap.c Name:folio_batch_add_and_move]
Results: map[SourceCode: SourceFile:]
|
| 14/2 |
2026/05/30 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 13899
output: 34
thoughts: 18
Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience, and presented as if it were my own:
**Macro or Inline? Let's Investigate.**
Right, I'm looking at this code and immediately I see something that's behaving like a function, but I have a strong suspicion it's either a macro or an inline function. The syntax, the way it's used... it all points that way. Given my experience, I'm pretty certain. To confirm and understand its expansion, and potential impact on performance or code size, I need to get to the source. The best way to do that, and quickly trace where it's defined, is to fire up `grep`. Let me look for it. A quick `grep` will pinpoint where that particular "function" is defined. That'll give me the context I need to figure out which mechanism it is, and where it is used.
|
| 15/2 |
2026/05/30 00:30 |
tool |
grepper |
0m
Args: map[Expression:folio_batch_add_and_move PathPrefix:mm/swap.c]
Results: map[Output:mm/swap.c=158=static void folio_batch_move_lru(struct folio_batch *fbatch, move_fn_t move_fn)
--
mm/swap.c-181-
mm/swap.c:182:static void __folio_batch_add_and_move(struct folio_batch __percpu *fbatch,
mm/swap.c-183- struct folio *folio, move_fn_t move_fn, bool disable_irq)
--
mm/swap.c-203-
mm/swap.c:204:#define folio_batch_add_and_move(folio, op) \
mm/swap.c:205: __folio_batch_add_and_move( \
mm/swap.c-206- &cpu_fbatches.op, \
--
mm/swap.c=231=void folio_rotate_reclaimable(struct folio *folio)
--
mm/swap.c-236-
mm/swap.c:237: folio_batch_add_and_move(folio, lru_move_tail);
mm/swap.c-238-}
--
mm/swap.c=333=void folio_activate(struct folio *folio)
--
mm/swap.c-338-
mm/swap.c:339: folio_batch_add_and_move(folio, lru_activate);
mm/swap.c-340-}
--
mm/swap.c=506=void folio_add_lru(struct folio *folio)
--
mm/swap.c-516-
mm/swap.c:517: folio_batch_add_and_move(folio, lru_add);
mm/swap.c-518-}
--
mm/swap.c=692=void deactivate_file_folio(struct folio *folio)
--
mm/swap.c-700-
mm/swap.c:701: folio_batch_add_and_move(folio, lru_deactivate_file);
mm/swap.c-702-}
--
mm/swap.c=712=void folio_deactivate(struct folio *folio)
--
mm/swap.c-719-
mm/swap.c:720: folio_batch_add_and_move(folio, lru_deactivate);
mm/swap.c-721-}
--
mm/swap.c=730=void folio_mark_lazyfree(struct folio *folio)
--
mm/swap.c-736-
mm/swap.c:737: folio_batch_add_and_move(folio, lru_lazyfree);
mm/swap.c-738-}
]
|
| 16/2 |
2026/05/30 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 14844
output: 31
thoughts: 444
Thoughts: **Analyzing a Lock Contention Issue**
Okay, let's break this down. I see that `folio_batch_add_and_move` is calling `__folio_batch_add_and_move`, and crucially, it's passing `disable_irq = false`. Right, so interrupts aren't disabled. Looking at the locking logic, that means it's using `local_lock(&cpu_fbatches.lock)`. Not `local_lock_irqsave`, just `local_lock`. And I'm pretty sure `local_lock` only disables preemption, not interrupts.
Now, the backtrace is showing the problem at `local_unlock(&cpu_fbatches.lock);` inside `__folio_batch_add_and_move`. Specifically, it's calling `local_lock_release`. That suggests it's getting hung up *releasing* the lock, which is odd. It is likely a lock contention. Why is this release taking so long?
Could CPU 1 be stuck somewhere? My mind jumps to `unmap_vmas`. This function iterates through all pages in a VMA. If the VMA is large, then it makes sense for it to hang. However, I think `zap_pte_range` has `cond_resched()`, which could give another CPU a chance to run.
Okay, I need to investigate what's happening within `zap_pmd_range` and `zap_pte_range` functions in `mm/memory.c`. I need to understand if there is a long loop or some other contention that is causing the slow `local_unlock`.
|
| 17/2 |
2026/05/30 00:30 |
tool |
grepper |
0m
Args: map[Expression:zap_pmd_range PathPrefix:mm/memory.c]
Results: map[Output:mm/memory.c=1884=static unsigned long zap_pte_range(struct mmu_gather *tlb,
--
mm/memory.c-1975-
mm/memory.c:1976:static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
mm/memory.c-1977- struct vm_area_struct *vma, pud_t *pud,
--
mm/memory.c=2012=static inline unsigned long zap_pud_range(struct mmu_gather *tlb,
--
mm/memory.c-2031- continue;
mm/memory.c:2032: next = zap_pmd_range(tlb, vma, pud, addr, next, details);
mm/memory.c-2033-next:
]
|
| 18/2 |
2026/05/30 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 15890
output: 35
thoughts: 13
Thoughts: Okay, let's dive into this. Here's my take on those thoughts, as if they're my own:
**Analyzing the `zap_pmd_range` Function**
Alright, time to get into this `zap_pmd_range` function. Hmm, based on the name, I'm expecting this to be related to, well, zapping or clearing a range of page map entries (PMDs). Given the context of my current work, I'm likely dealing with memory management or page table manipulation, so this is critical. I'm immediately thinking about efficiency; how is this function implemented? Is it optimized for common cases? Does it handle potential edge cases gracefully? I'm already anticipating questions about atomic operations, cache coherency, and the interplay with multi-threading if any. My initial assessment is that this is going to need a close look. Let's see how it works!
|
| 19/2 |
2026/05/30 00:30 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/memory.c Name:zap_pmd_range]
Results: map[SourceCode:1976: static inline unsigned long zap_pmd_range(struct mmu_gather *tlb,
1977: struct vm_area_struct *vma, pud_t *pud,
1978: unsigned long addr, unsigned long end,
1979: struct zap_details *details)
1980: {
1981: pmd_t *pmd;
1982: unsigned long next;
1983:
1984: pmd = pmd_offset(pud, addr);
1985: do {
1986: next = pmd_addr_end(addr, end);
1987: if (pmd_is_huge(*pmd)) {
1988: if (next - addr != HPAGE_PMD_SIZE)
1989: __split_huge_pmd(vma, pmd, addr, false);
1990: else if (zap_huge_pmd(tlb, vma, pmd, addr)) {
1991: addr = next;
1992: continue;
1993: }
1994: /* fall through */
1995: } else if (details && details->single_folio &&
1996: folio_test_pmd_mappable(details->single_folio) &&
1997: next - addr == HPAGE_PMD_SIZE && pmd_none(*pmd)) {
1998: sync_with_folio_pmd_zap(tlb->mm, pmd);
1999: }
2000: if (pmd_none(*pmd)) {
2001: addr = next;
2002: continue;
2003: }
2004: addr = zap_pte_range(tlb, vma, pmd, addr, next, details);
2005: if (addr != next)
2006: pmd--;
2007: } while (pmd++, cond_resched(), addr != end);
2008:
2009: return addr;
2010: }
SourceFile:mm/memory.c]
|
| 20/2 |
2026/05/30 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 16716
output: 34
thoughts: 271
Thoughts: **Analyzing the Potential Soft Lockup in `zap_pmd_range`**
Okay, so I'm looking at this `zap_pmd_range` function and immediately, my spider sense starts tingling. I see that `cond_resched()` call *right* in the loop after every PMD, which is 2MB of memory. My first thought is, "Wow, that's yielding the CPU pretty frequently!" So, I ask, how could this possibly trigger a soft lockup? A soft lockup, by definition, requires a task to run for over 20 seconds without yielding.
But then I remember that `cond_resched()` only yields if `TIF_NEED_RESCHED` is set. So, if there's nothing else to run on the CPU, `cond_resched()` effectively does nothing, right? And my mind jumps, like, wait a minute, even if my task is the only task, the watchdog or timer interrupts will *still* fire, won't they? And those interrupts can update timestamps and potentially trigger a reschedule!
So, even if `cond_resched()` is a no-op, the task would continue running, but the timer interrupt would *still* fire and update the softlockup timestamp. In theory, that means the softlockup detector would never actually trip! Unless... unless interrupts are disabled. Aha! The softlockup detection, while seemingly defeated, could still be a problem if interrupts are disabled at a higher level!
But I don't see any explicit disabling of interrupts in `zap_pmd_range`. So, *that* can't be it. Wait, now I need to check `zap_pte_range` as *that* is called by `zap_pmd_range` and I know that it can and does disable interrupts in certain circumstances. So, let's go there and see what is going on.
|
| 21/2 |
2026/05/30 00:30 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/memory.c Name:zap_pte_range]
Results: map[SourceCode:1884: static unsigned long zap_pte_range(struct mmu_gather *tlb,
1885: struct vm_area_struct *vma, pmd_t *pmd,
1886: unsigned long addr, unsigned long end,
1887: struct zap_details *details)
1888: {
1889: bool can_reclaim_pt = pte_table_reclaim_possible(addr, end, details);
1890: bool force_flush = false, force_break = false;
1891: struct mm_struct *mm = tlb->mm;
1892: int rss[NR_MM_COUNTERS];
1893: spinlock_t *ptl;
1894: pte_t *start_pte;
1895: pte_t *pte;
1896: pmd_t pmdval;
1897: unsigned long start = addr;
1898: bool direct_reclaim = true;
1899: int nr;
1900:
1901: retry:
1902: tlb_change_page_size(tlb, PAGE_SIZE);
1903: init_rss_vec(rss);
1904: start_pte = pte = pte_offset_map_lock(mm, pmd, addr, &ptl);
1905: if (!pte)
1906: return addr;
1907:
1908: flush_tlb_batched_pending(mm);
1909: lazy_mmu_mode_enable();
1910: do {
1911: bool any_skipped = false;
1912:
1913: if (need_resched()) {
1914: direct_reclaim = false;
1915: break;
1916: }
1917:
1918: nr = do_zap_pte_range(tlb, vma, pte, addr, end, details, rss,
1919: &force_flush, &force_break, &any_skipped);
1920: if (any_skipped)
1921: can_reclaim_pt = false;
1922: if (unlikely(force_break)) {
1923: addr += nr * PAGE_SIZE;
1924: direct_reclaim = false;
1925: break;
1926: }
1927: } while (pte += nr, addr += PAGE_SIZE * nr, addr != end);
1928:
1929: /*
1930: * Fast path: try to hold the pmd lock and unmap the PTE page.
1931: *
1932: * If the pte lock was released midway (retry case), or if the attempt
1933: * to hold the pmd lock failed, then we need to recheck all pte entries
1934: * to ensure they are still none, thereby preventing the pte entries
1935: * from being repopulated by another thread.
1936: */
1937: if (can_reclaim_pt && direct_reclaim && addr == end)
1938: direct_reclaim = zap_empty_pte_table(mm, pmd, ptl, &pmdval);
1939:
1940: add_mm_rss_vec(mm, rss);
1941: lazy_mmu_mode_disable();
1942:
1943: /* Do the actual TLB flush before dropping ptl */
1944: if (force_flush) {
1945: tlb_flush_mmu_tlbonly(tlb);
1946: tlb_flush_rmaps(tlb, vma);
1947: }
1948: pte_unmap_unlock(start_pte, ptl);
1949:
1950: /*
1951: * If we forced a TLB flush (either due to running out of
1952: * batch buffers or because we needed to flush dirty TLB
1953: * entries before releasing the ptl), free the batched
1954: * memory too. Come back again if we didn't do everything.
1955: */
1956: if (force_flush)
1957: tlb_flush_mmu(tlb);
1958:
1959: if (addr != end) {
1960: cond_resched();
1961: force_flush = false;
1962: force_break = false;
1963: goto retry;
1964: }
1965:
1966: if (can_reclaim_pt) {
1967: if (direct_reclaim || zap_pte_table_if_empty(mm, pmd, start, &pmdval)) {
1968: pte_free_tlb(tlb, pmd_pgtable(pmdval), addr);
1969: mm_dec_nr_ptes(mm);
1970: }
1971: }
1972:
1973: return addr;
1974: }
SourceFile:mm/memory.c]
|
| 22/2 |
2026/05/30 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 18853
output: 37
thoughts: 411
Thoughts: **Analyzing a Potential Performance Bottleneck**
Okay, so I'm looking at this `pte_offset_map_lock` function, and it's acquiring the page table lock, the `ptl`. The initial thought was that it might be holding the lock for an excessive amount of time, especially given the context of a CPU seemingly stalled.
I see this loop: it checks `need_resched()`. If that's true, it breaks and yields, meaning it releases the lock, calls `cond_resched()`, and then retries the operation. So, it *shouldn't* hold the lock indefinitely if it needs to reschedule. Excellent.
But then I thought, what if `need_resched()` is *false*? It proceeds to process up to 512 PTEs (that's 2MB of memory). But processing 2MB shouldn't take anywhere near 70 seconds. So, the original premise doesn't hold: CPU 1 isn't getting stuck in `zap_pte_range` without yielding.
But...wait. The backtrace from CPU 1 is the critical clue. It shows `__folio_batch_add_and_move` being the culprit. Now, I know that `__folio_batch_add_and_move` is called from `folio_activate`, which in turn is called from `folio_mark_accessed`. And finally, `folio_mark_accessed` is called from `zap_present_folio_ptes`. Hmm... This suggests that `zap_present_folio_ptes` is the critical function, and that it potentially calls `folio_mark_accessed` for *every* PTE. Okay, time to dive deeper and examine `zap_present_folio_ptes` in more detail. That's where the focus needs to be.
|
| 23/2 |
2026/05/30 00:30 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/memory.c Name:zap_present_folio_ptes]
Results: map[SourceCode:1622: static __always_inline void zap_present_folio_ptes(struct mmu_gather *tlb,
1623: struct vm_area_struct *vma, struct folio *folio,
1624: struct page *page, pte_t *pte, pte_t ptent, unsigned int nr,
1625: unsigned long addr, struct zap_details *details, int *rss,
1626: bool *force_flush, bool *force_break, bool *any_skipped)
1627: {
1628: struct mm_struct *mm = tlb->mm;
1629: bool delay_rmap = false;
1630:
1631: if (!folio_test_anon(folio)) {
1632: ptent = get_and_clear_full_ptes(mm, addr, pte, nr, tlb->fullmm);
1633: if (pte_dirty(ptent)) {
1634: folio_mark_dirty(folio);
1635: if (tlb_delay_rmap(tlb)) {
1636: delay_rmap = true;
1637: *force_flush = true;
1638: }
1639: }
1640: if (pte_young(ptent) && likely(vma_has_recency(vma)))
1641: folio_mark_accessed(folio);
1642: rss[mm_counter(folio)] -= nr;
1643: } else {
1644: /* We don't need up-to-date accessed/dirty bits. */
1645: clear_full_ptes(mm, addr, pte, nr, tlb->fullmm);
1646: rss[MM_ANONPAGES] -= nr;
1647: }
1648: /* Checking a single PTE in a batch is sufficient. */
1649: arch_check_zapped_pte(vma, ptent);
1650: tlb_remove_tlb_entries(tlb, pte, nr, addr);
1651: if (unlikely(userfaultfd_pte_wp(vma, ptent)))
1652: *any_skipped = zap_install_uffd_wp_if_needed(vma, addr, pte,
1653: nr, details, ptent);
1654:
1655: if (!delay_rmap) {
1656: folio_remove_rmap_ptes(folio, page, nr, vma);
1657:
1658: if (unlikely(folio_mapcount(folio) < 0))
1659: print_bad_pte(vma, addr, ptent, page);
1660: }
1661: if (unlikely(__tlb_remove_folio_pages(tlb, page, nr, delay_rmap))) {
1662: *force_flush = true;
1663: *force_break = true;
1664: }
1665: }
SourceFile:mm/memory.c]
|
| 24/2 |
2026/05/30 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 20528
output: 47
thoughts: 1059
Thoughts: **Analyzing a Soft Lockup on CPU 0**
Okay, so the initial clue is the `folio_mark_accessed` function being called when `pte_young(ptent)` returns true. That makes sense; marking the page as accessed. Then I see `folio_mark_accessed` calling `folio_activate` if the folio is on the Least Recently Used (LRU) list. `folio_activate`, in turn, is calling `folio_batch_add_and_move`. `folio_batch_add_and_move` takes a `local_lock` using `cpu_fbatches.lock`.
Aha, wait a minute... on PREEMPT_RT, that `local_lock` would be a real lock, but on standard PREEMPT it's just disabling preemption. I need to keep that distinction in mind. Looking at the backtrace on CPU 1, I see `folio_activate`, and more importantly I see `local_lock_release` in the trace, meaning that it's just releasing the local lock and reenabling preemption. That explains why the NMI backtrace includes it.
So, why is CPU 1 in the NMI backtrace anyway? Because CPU 0 sent an NMI to it! CPU 1 is doing its normal thing and not stuck, so it's *CPU 0* that's the problem. The watchdog is reporting a soft lockup on CPU 0: "soft lockup - CPU#0 stuck for 70s! [syz.7.1244:10838]". Got it.
Let's look at CPU 0's state again: CPU 0 is in userspace, and the PC is pointing to `0000ffff8f64c32c`. This seems strange... If CPU 0 is in userspace, how can it be a soft lockup?
Okay, let's dive into CPU 0's softirq trace. I see that hardirqs were disabled and then immediately enabled. The specific function here is `__el0_irq_handler_common`, which is the IRQ handler from EL0 (userspace).
So, the key here is that the problem appears related to userspace IRQ handling. I should examine the code in `arch/arm64/kernel/entry-common.c` to see what is going on.
|
| 25/2 |
2026/05/30 00:30 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:arch/arm64/kernel/entry-common.c Name:__el0_irq_handler_common]
Results: map[SourceCode: SourceFile:]
|
| 26/2 |
2026/05/30 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 22173
output: 43
thoughts: 17
Thoughts: Okay, here's my take on those thoughts, tailored for an expert audience, presented in the first person:
**Analyzing the Interrupt Handler**
Right, let's dive in. I'm thinking I need to find the core of the interrupt handling logic. Given my experience, the `__el0_irq_handler_common` symbol is a likely candidate, and a good place to start. A quick `grep` will allow me to narrow down the relevant code paths. It's a standard approach for this kind of investigation β quickly identifying the key function or function group for any handler is vital. Itβll give me a jumping-off point to understand the overall interrupt flow and where specific actions are taken within that flow. This is probably going to involve looking at the assembly in the final analysis, but this grep will give me the C level calls as a start.
|
| 27/2 |
2026/05/30 00:30 |
tool |
grepper |
0m
Args: map[Expression:__el0_irq_handler_common PathPrefix:arch/arm64/kernel/entry-common.c]
Results: map[Output:arch/arm64/kernel/entry-common.c=821=static void noinstr el0_interrupt(struct pt_regs *regs,
--
arch/arm64/kernel/entry-common.c-837-
arch/arm64/kernel/entry-common.c:838:static void noinstr __el0_irq_handler_common(struct pt_regs *regs)
arch/arm64/kernel/entry-common.c-839-{
--
arch/arm64/kernel/entry-common.c=843=asmlinkage void noinstr el0t_64_irq_handler(struct pt_regs *regs)
arch/arm64/kernel/entry-common.c-844-{
arch/arm64/kernel/entry-common.c:845: __el0_irq_handler_common(regs);
arch/arm64/kernel/entry-common.c-846-}
--
arch/arm64/kernel/entry-common.c=953=asmlinkage void noinstr el0t_32_irq_handler(struct pt_regs *regs)
arch/arm64/kernel/entry-common.c-954-{
arch/arm64/kernel/entry-common.c:955: __el0_irq_handler_common(regs);
arch/arm64/kernel/entry-common.c-956-}
]
|
| 28/2 |
2026/05/30 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 22780
output: 47
thoughts: 16
Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience, and written in the first person:
**Diving into the Interrupt Handler**
Alright, let's take a look at `__el0_irq_handler_common`. I'm immediately curious about this specific entry point, likely for EL0 interrupts. Given the name, it seems like a common handler, which implies that it's going to be a key piece of infrastructure. I expect to see a carefully crafted sequence of actions here, because performance in an interrupt context is critical. It'll probably involve context saving, dispatching to the appropriate interrupt service routine (ISR), and then restoring context. My mind's already racing, wondering about the specifics: What kind of preemption is this set up for? Are there any optimizations for fast-path interrupt handling? Are they utilizing any advanced features of the architecture like the Generic Interrupt Controller (GIC)? I'm anticipating some interesting code! Time to dig in and see how they've structured this.
|
| 29/2 |
2026/05/30 00:30 |
tool |
codesearch-definition-source |
0m
Error:
Args: map[ContextFile:arch/arm64/kernel/entry-common.c Name:__el0_irq_handler_common]
Results: map[SourceCode: SourceFile:]
|
| 30/2 |
2026/05/30 00:30 |
llm |
expert |
0m
Model:
Tokens:
input: 23082
output: 45
thoughts: 34
Thoughts: Okay, here's my interpretation of that thought process, assuming I'm deep in the weeds of kernel debugging:
**Diving into the IRQ Handler**
Right, so we're looking at `static void noinstr __el0_irq_handler_common`. This signature is immediately familiar; it's the common interrupt handler for EL0 (Exception Level 0) on this architecture. The `noinstr` attribute is also a good indicator - meaning the compiler isn't doing any instruction scheduling tricks here which is usually good for debugging. Based on that, I'll need to go to line 838 of whatever file this is defined in. This is likely in `arch/arm64/kernel/entry.S` or a similar assembly file for the architecture. I'm hoping to get the stack pointer set up and see which registers are being saved, what the code is doing to determine the interrupt source, and how it's dispatching to the appropriate interrupt service routine (ISR). From there, I can walk through the ISR itself. This could be where the issue lies...
|
| 31/2 |
2026/05/30 00:31 |
tool |
read-file |
0m
Error:
Args: map[File:arch/arm64/kernel/entry-common.c FirstLine:830 LineCount:30]
|
| 32/2 |
2026/05/30 00:31 |
llm |
expert |
0m
Model:
Tokens:
input: 23420
output: 41
thoughts: 8
Thoughts: Okay, here's the summary, rendered as your internal thought process:
**Integer Constraint Imposed**
Ah, *FirstLine* must be an integer. It's crucial for the subsequent calculations or operations I have in mind. I need to be absolutely certain of this constraint before proceeding any further. Otherwise, the foundation of the logic I'm building will be fundamentally flawed. This is a very basic check, but an essential one, that needs to be done. Let me just be sure to enforce that now.
|
| 33/2 |
2026/05/30 00:31 |
tool |
read-file |
0m
Args: map[File:arch/arm64/kernel/entry-common.c FirstLine:830 LineCount:30]
Results: map[Contents: 830:
831: irq_enter_rcu();
832: do_interrupt_handler(regs, handler);
833: irq_exit_rcu();
834:
835: arm64_exit_to_user_mode(regs);
836: }
837:
838: static void noinstr __el0_irq_handler_common(struct pt_regs *regs)
839: {
840: el0_interrupt(regs, handle_arch_irq);
841: }
842:
843: asmlinkage void noinstr el0t_64_irq_handler(struct pt_regs *regs)
844: {
845: __el0_irq_handler_common(regs);
846: }
847:
848: static void noinstr __el0_fiq_handler_common(struct pt_regs *regs)
849: {
850: el0_interrupt(regs, handle_arch_fiq);
851: }
852:
853: asmlinkage void noinstr el0t_64_fiq_handler(struct pt_regs *regs)
854: {
855: __el0_fiq_handler_common(regs);
856: }
857:
858: static void noinstr __el0_error_handler_common(struct pt_regs *regs)
859: {
]
|
| 34/2 |
2026/05/30 00:31 |
llm |
expert |
0m
Model:
Tokens:
input: 24021
output: 41
thoughts: 32
Thoughts: Okay, here's my interpretation of that thought process, tailored for an expert audience, and written in the first person:
**Diving into the EL0 Interrupt Handling**
Right, let me see. I'm following the interrupt path, and I've hit `__el0_irq_handler_common`. Okay, good. Now, this one calls `el0_interrupt`. Excellent. So, `__el0_irq_handler_common` is effectively the entry point to handle IRQs originating from EL0, and it delegates further processing to `el0_interrupt`. Makes perfect sense. Now, let's drill down into `el0_interrupt`. I need to understand what this function is *actually* doing. I want to see how it dispatches to the appropriate interrupt handler, how it preserves context, what the overall flow is. Is there any architecture-specific code that I should know about? What level of abstraction is it? Does it call into the generic interrupt controller code or is this the interrupt handling itself?
|
| 35/2 |
2026/05/30 00:31 |
tool |
read-file |
0m
Args: map[File:arch/arm64/kernel/entry-common.c FirstLine:815 LineCount:25]
Results: map[Contents: 815: break;
816: default:
817: el0_inv(regs, esr);
818: }
819: }
820:
821: static void noinstr el0_interrupt(struct pt_regs *regs,
822: void (*handler)(struct pt_regs *))
823: {
824: arm64_enter_from_user_mode(regs);
825:
826: write_sysreg(DAIF_PROCCTX_NOIRQ, daif);
827:
828: if (regs->pc & BIT(55))
829: arm64_apply_bp_hardening();
830:
831: irq_enter_rcu();
832: do_interrupt_handler(regs, handler);
833: irq_exit_rcu();
834:
835: arm64_exit_to_user_mode(regs);
836: }
837:
838: static void noinstr __el0_irq_handler_common(struct pt_regs *regs)
839: {
]
|
| 36/2 |
2026/05/30 00:31 |
llm |
expert |
0m
Model:
Tokens:
input: 24661
output: 35
thoughts: 443
Thoughts: **Analyzing a Soft Lockup During Userpace IRQ Handling**
Okay, so I'm looking at this trace, and it's pointing to `__el0_irq_handler_common` as the entry point for IRQs coming from userspace. The lockdep trace is highlighting a strange sequence: hardirqs are *enabled* and then immediately *disabled* at the exact same point within `__el0_irq_handler_common`. Specifically, that offset of `0x18` leads me to `el0_interrupt`. And that's where things get interesting because `el0_interrupt` calls `arm64_enter_from_user_mode`, which, theoretically, could be enabling or disabling hardirqs.
The timing is bizarre; the hardirqs are immediately re-enabled. That strongly suggests the CPU is handling an interrupt *from* userspace. The IRQ count confirms this is happening: it's at 2180. The implications of this are, the CPU must be running in user space, and then it is being interrupted.
But here's the kicker, the mystery is if it's running in userspace, why am I seeing a soft lockup? The soft lockup happens when the watchdog timer triggers and sees that `watchdog_touch_ts` hasn't been updated for 20 seconds. I immediately think that `watchdog_timer_fn` is responsible for updating the timestamp, but wait no. That is just the timer interrupt that *checks* the timestamp.
So, if the CPU is in userspace, shouldn't the scheduler tick be updating the timestamp? This is my next line of inquiry. Time to dig into `kernel/watchdog.c` and figure out exactly how this soft lockup detection is supposed to work. Something isn't adding up.
|
| 37/2 |
2026/05/30 00:31 |
tool |
read-file |
0m
Args: map[File:kernel/watchdog.c FirstLine:350 LineCount:100]
Results: map[Contents: 350: *
351: * Caller needs to make sure that the hard watchdogs are off, so this
352: * can't race with watchdog_hardlockup_disable().
353: */
354: static void lockup_detector_update_enable(void)
355: {
356: watchdog_enabled = 0;
357: if (!watchdog_user_enabled)
358: return;
359: if (watchdog_hardlockup_available && watchdog_hardlockup_user_enabled)
360: watchdog_enabled |= WATCHDOG_HARDLOCKUP_ENABLED;
361: if (watchdog_softlockup_user_enabled)
362: watchdog_enabled |= WATCHDOG_SOFTOCKUP_ENABLED;
363: }
364:
365: #ifdef CONFIG_SOFTLOCKUP_DETECTOR
366:
367: /*
368: * Delay the soflockup report when running a known slow code.
369: * It does _not_ affect the timestamp of the last successdul reschedule.
370: */
371: #define SOFTLOCKUP_DELAY_REPORT ULONG_MAX
372:
373: #ifdef CONFIG_SMP
374: int __read_mostly sysctl_softlockup_all_cpu_backtrace;
375: #endif
376:
377: /*
378: * bitmasks to control what kinds of system info to be printed when
379: * soft lockup is detected, it could be task, memory, lock etc.
380: * Refer include/linux/sys_info.h for detailed bit definition.
381: */
382: static unsigned long softlockup_si_mask;
383:
384: static struct cpumask watchdog_allowed_mask __read_mostly;
385:
386: /* Global variables, exported for sysctl */
387: unsigned int __read_mostly softlockup_panic =
388: CONFIG_BOOTPARAM_SOFTLOCKUP_PANIC;
389:
390: static bool softlockup_initialized __read_mostly;
391: static u64 __read_mostly sample_period;
392:
393: #ifdef CONFIG_SYSFS
394:
395: static unsigned int softlockup_count;
396:
397: static ssize_t softlockup_count_show(struct kobject *kobj, struct kobj_attribute *attr,
398: char *page)
399: {
400: return sysfs_emit(page, "%u\n", softlockup_count);
401: }
402:
403: static struct kobj_attribute softlockup_count_attr = __ATTR_RO(softlockup_count);
404:
405: static __init int kernel_softlockup_sysfs_init(void)
406: {
407: sysfs_add_file_to_group(kernel_kobj, &softlockup_count_attr.attr, NULL);
408: return 0;
409: }
410:
411: late_initcall(kernel_softlockup_sysfs_init);
412:
413: #endif // CONFIG_SYSFS
414:
415: /* Timestamp taken after the last successful reschedule. */
416: static DEFINE_PER_CPU(unsigned long, watchdog_touch_ts);
417: /* Timestamp of the last softlockup report. */
418: static DEFINE_PER_CPU(unsigned long, watchdog_report_ts);
419: static DEFINE_PER_CPU(struct hrtimer, watchdog_hrtimer);
420: static DEFINE_PER_CPU(bool, softlockup_touch_sync);
421: static unsigned long soft_lockup_nmi_warn;
422:
423: static int __init softlockup_panic_setup(char *str)
424: {
425: softlockup_panic = simple_strtoul(str, NULL, 0);
426: return 1;
427: }
428: __setup("softlockup_panic=", softlockup_panic_setup);
429:
430: static int __init nowatchdog_setup(char *str)
431: {
432: watchdog_user_enabled = 0;
433: return 1;
434: }
435: __setup("nowatchdog", nowatchdog_setup);
436:
437: static int __init nosoftlockup_setup(char *str)
438: {
439: watchdog_softlockup_user_enabled = 0;
440: return 1;
441: }
442: __setup("nosoftlockup", nosoftlockup_setup);
443:
444: static int __init watchdog_thresh_setup(char *str)
445: {
446: get_option(&str, &watchdog_thresh);
447: return 1;
448: }
449: __setup("watchdog_thresh=", watchdog_thresh_setup);
]
|
| 38/2 |
2026/05/30 00:31 |
llm |
expert |
0m
Model:
Tokens:
input: 27061
output: 32
thoughts: 12
Thoughts: Okay, let's dive into this. Here's my take on those thoughts, as if they're my own:
**Analyzing the `watchdog_timer_fn` Function**
Alright, let's take a look at this `watchdog_timer_fn`. My initial thought is to figure out its purpose within the system. Is it a core function, a supporting routine, or something more specialized? I'm already mentally running through common watchdog implementations: is it a hardware watchdog, a software watchdog, or perhaps a hybrid approach? What kind of system are we dealing with here β embedded, server, something else? The context is crucial.
My expert eye immediately begins scanning for key indicators: the function's arguments, return values, any calls it makes to other functions. I'm looking for clues about the timer's behavior. How is the timer set up? What's the timeout value? Is it a periodic timer, or does it require explicit kicking/rearming? Does it perform any diagnostics before resetting the system, or does it immediately trigger a reboot?
Next, I'm analyzing the reset behavior. Is the reset immediate, or does the function attempt graceful shutdown operations, such as saving logs or powering down non-critical processes? What error conditions trigger the timer to kick in, and are these conditions handled in a way that minimizes damage? I am also considering any race conditions that might exist.
Furthermore, how does this function integrate into the overall system architecture? Where is this watchdog timer enabled and disabled? Are there configuration options, and if so, how are those settings managed? Is there an ability to monitor or disable the watchdog? My expert mind will immediately begin thinking through the system's fault-tolerant design and how the watchdog fits into it. Does the function have any dependencies or assumptions that could be a potential weakness?
Finally, what's the test plan like? Is it well tested and documented? This may dictate how much confidence I have in the function.
In short, my primary goals here are to understand the watchdog timer's function, its impact on the system's stability, and to assess its potential vulnerabilities. That's the baseline, and then I'll delve into the details as I continue my analysis.
|
| 39/2 |
2026/05/30 00:31 |
tool |
grepper |
0m
Args: map[Expression:watchdog_timer_fn PathPrefix:kernel/watchdog.c]
Results: map[Output:kernel/watchdog.c=775=static DEFINE_PER_CPU(struct cpu_stop_work, softlockup_stop_work);
--
kernel/watchdog.c-782- * for more than 2*watchdog_thresh seconds then the debug-printout
kernel/watchdog.c:783: * triggers in watchdog_timer_fn().
kernel/watchdog.c-784- */
kernel/watchdog.c=785=static int softlockup_fn(void *data)
--
kernel/watchdog.c-794-/* watchdog kicker functions */
kernel/watchdog.c:795:static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
kernel/watchdog.c-796-{
--
kernel/watchdog.c=913=static void watchdog_enable(unsigned int cpu)
--
kernel/watchdog.c-926- */
kernel/watchdog.c:927: hrtimer_setup(hrtimer, watchdog_timer_fn, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
kernel/watchdog.c-928- hrtimer_start(hrtimer, ns_to_ktime(sample_period),
--
kernel/watchdog.c=1002=static void __lockup_detector_reconfigure(bool thresh_changed)
--
kernel/watchdog.c-1008- /*
kernel/watchdog.c:1009: * To prevent watchdog_timer_fn from using the old interval and
kernel/watchdog.c-1010- * the new watchdog_thresh at the same time, which could lead to
]
|
| 40/2 |
2026/05/30 00:31 |
llm |
expert |
0m
Model:
Tokens:
input: 27996
output: 36
thoughts: 12
Thoughts: Okay, here's my interpretation of what's going through my mind when I encounter the phrase `watchdog_timer_fn`, keeping in mind my expertise:
**Diving into Watchdog Timer Functionality**
Alright, `watchdog_timer_fn`. Immediately, my brain kicks into gear. This function is likely the heart of a system's resilience β the code that keeps the entire thing from locking up. I'm already formulating some hypotheses: Is this a hardware-specific implementation, or is it abstracted? Is it a single function, or a pointer to a function call in a structure? Where does it get called from? Is it periodic, or event-driven? Does it reset the timer itself, or is that handled elsewhere in the code? And importantly, what constitutes a "kick" or "pet" of the watchdog? Is it just a write to a register, or something more complex? The architecture will inform all of this: is this an embedded system with tight resource constraints, or something larger and more complex? I want to know about the register access mechanism. Memory mapped? I2C? SPI? That will give me immediate insight into the level of abstraction involved. I'm eager to understand the exact mechanisms at play. This is where I start drilling down β what are the edge cases? What happens if the timer expires? Is there a graceful shutdown, or a hard reset? And most critically, what's the *reason* for this watchdog in the first place? Is it safety-critical, dealing with potentially dangerous outputs? Or is it simply a means to maintain uptime in a less critical application? This initial glance at the function name should serve to start the process of quickly determining answers to these questions.
|
| 41/2 |
2026/05/30 00:31 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/watchdog.c Name:watchdog_timer_fn]
Results: map[SourceCode: 794: /* watchdog kicker functions */
795: static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
796: {
797: unsigned long touch_ts, period_ts, now;
798: struct pt_regs *regs = get_irq_regs();
799: int softlockup_all_cpu_backtrace;
800: int duration, thresh_count;
801: unsigned long flags;
802:
803: if (!watchdog_enabled)
804: return HRTIMER_NORESTART;
805:
806: /*
807: * pass the buddy check if a panic is in process
808: */
809: if (panic_in_progress())
810: return HRTIMER_NORESTART;
811:
812: softlockup_all_cpu_backtrace = (softlockup_si_mask & SYS_INFO_ALL_BT) ?
813: 1 : sysctl_softlockup_all_cpu_backtrace;
814:
815: watchdog_hardlockup_kick();
816:
817: /* kick the softlockup detector */
818: if (completion_done(this_cpu_ptr(&softlockup_completion))) {
819: reinit_completion(this_cpu_ptr(&softlockup_completion));
820: stop_one_cpu_nowait(smp_processor_id(),
821: softlockup_fn, NULL,
822: this_cpu_ptr(&softlockup_stop_work));
823: }
824:
825: /* .. and repeat */
826: hrtimer_forward_now(hrtimer, ns_to_ktime(sample_period));
827:
828: /*
829: * Read the current timestamp first. It might become invalid anytime
830: * when a virtual machine is stopped by the host or when the watchog
831: * is touched from NMI.
832: */
833: now = get_timestamp();
834: /*
835: * If a virtual machine is stopped by the host it can look to
836: * the watchdog like a soft lockup. This function touches the watchdog.
837: */
838: kvm_check_and_clear_guest_paused();
839: /*
840: * The stored timestamp is comparable with @now only when not touched.
841: * It might get touched anytime from NMI. Make sure that is_softlockup()
842: * uses the same (valid) value.
843: */
844: period_ts = READ_ONCE(*this_cpu_ptr(&watchdog_report_ts));
845:
846: update_cpustat();
847:
848: /* Reset the interval when touched by known problematic code. */
849: if (period_ts == SOFTLOCKUP_DELAY_REPORT) {
850: if (unlikely(__this_cpu_read(softlockup_touch_sync))) {
851: /*
852: * If the time stamp was touched atomically
853: * make sure the scheduler tick is up to date.
854: */
855: __this_cpu_write(softlockup_touch_sync, false);
856: sched_clock_tick();
857: }
858:
859: update_report_ts();
860: return HRTIMER_RESTART;
861: }
862:
863: /* Check for a softlockup. */
864: touch_ts = __this_cpu_read(watchdog_touch_ts);
865: duration = is_softlockup(touch_ts, period_ts, now);
866: if (unlikely(duration)) {
867: #ifdef CONFIG_SYSFS
868: ++softlockup_count;
869: #endif
870:
871: /*
872: * Prevent multiple soft-lockup reports if one cpu is already
873: * engaged in dumping all cpu back traces.
874: */
875: if (softlockup_all_cpu_backtrace) {
876: if (test_and_set_bit_lock(0, &soft_lockup_nmi_warn))
877: return HRTIMER_RESTART;
878: }
879:
880: /* Start period for the next softlockup warning. */
881: update_report_ts();
882:
883: printk_cpu_sync_get_irqsave(flags);
884: pr_emerg("BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
885: smp_processor_id(), duration,
886: current->comm, task_pid_nr(current));
887: report_cpu_status();
888: print_modules();
889: print_irqtrace_events(current);
890: if (regs)
891: show_regs(regs);
892: else
893: dump_stack();
894: printk_cpu_sync_put_irqrestore(flags);
895:
896: if (softlockup_all_cpu_backtrace) {
897: trigger_allbutcpu_cpu_backtrace(smp_processor_id());
898: if (!softlockup_panic)
899: clear_bit_unlock(0, &soft_lockup_nmi_warn);
900: }
901:
902: add_taint(TAINT_SOFTLOCKUP, LOCKDEP_STILL_OK);
903: sys_info(softlockup_si_mask & ~SYS_INFO_ALL_BT);
904: thresh_count = duration / get_softlockup_thresh();
905:
906: if (softlockup_panic && thresh_count >= softlockup_panic)
907: panic("softlockup: hung tasks");
908: }
909:
910: return HRTIMER_RESTART;
911: }
SourceFile:kernel/watchdog.c]
|
| 42/2 |
2026/05/30 00:31 |
llm |
expert |
0m
Model:
Tokens:
input: 30219
output: 35
thoughts: 206
Thoughts: **Analyzing the Watchdog Timer Update**
Okay, so `watchdog_touch_ts` is the crucial timestamp that tells us the watchdog is still alive, and `watchdog_touch_ts()` is the function responsible for updating it. Naturally, I need to understand the call chain. Hmm, typically, `touch_softlockup_watchdog()` would be the caller, no surprises there. But wait, it looks like the softlockup detector works by scheduling a high-priority `stop_machine` work. Let me check the code snippet⦠right, it's kicking the detector with that `stop_one_cpu_nowait` call. The important part here is the target of the call: `softlockup_fn`. That's where the timestamp is getting updated! I need to dig into `softlockup_fn` to confirm and see how it fits into the bigger picture. This is key to understanding the watchdog behavior, I need to see how the softlockup function fits in.
|
| 43/2 |
2026/05/30 00:31 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:kernel/watchdog.c Name:softlockup_fn]
Results: map[SourceCode: 777: /*
778: * The watchdog feed function - touches the timestamp.
779: *
780: * It only runs once every sample_period seconds (4 seconds by
781: * default) to reset the softlockup timestamp. If this gets delayed
782: * for more than 2*watchdog_thresh seconds then the debug-printout
783: * triggers in watchdog_timer_fn().
784: */
785: static int softlockup_fn(void *data)
786: {
787: update_touch_ts();
788: stop_counting_irqs();
789: complete(this_cpu_ptr(&softlockup_completion));
790:
791: return 0;
792: }
SourceFile:kernel/watchdog.c]
|
| 44/2 |
2026/05/30 00:31 |
llm |
expert |
0m
Model:
Tokens:
input: 30927
output: 38
thoughts: 1636
Thoughts: **Analyzing a Softlockup on an ARM64 System**
Okay, so I'm diving into this softlockup issue. My initial thought is the softlockup detector uses a `stop_one_cpu_nowait` work, which is scheduled in the `migration/N` thread. This thread is a SCHED_FIFO thread with the highest priority (99). The core idea is that if the `migration/N` thread doesn't run for 20 seconds, the `watchdog_timer_fn` detects it, and triggers the softlockup. So, the question is what prevents that `migration/N` from running?
The obvious suspects are: interrupts disabled (but that's a hardlockup), preemption disabled (spinlocks, `preempt_disable()`), or a higher priority task is hogging the CPU (which is unlikely given `migration/N`'s priority). And wait, on a userland CPU, preemption should ALWAYS work, the `migration/N` should be able to preempt a userland CPU, the CPU is free of interrupt disabling! So it *should* run!
I'm thinking, could it be a situation where the processor *thinks* it's in userspace, but somehow is stuck in a kernel-land loop? I check the processor state using the `pstate` register, and the `M` field is 0, which means the processor is indeed in EL0, or userspace. So preemption *should* be in effect.
Why wouldn't `migration/N` preempt? Let's consider the interrupt handling process. The timer interrupt fires, queues the stop work, the timer interrupt returns. On return, there's `TIF_NEED_RESCHED`, which should trigger `schedule()`. The scheduler should select `migration/N`, which should run, update a timestamp, and then go back to sleep. So this mechanism *should* prevent the softlockup from happening!
Then, how? I see in the lockdep trace that the interrupt "disables" and "enables" hardirqs inside `__el0_irq_handler_common`. `__el0_irq_handler_common` is called when an IRQ is taken from EL0. It calls `el0_interrupt`, which calls `irq_enter_rcu()`, `do_interrupt_handler()`, `irq_exit_rcu()`. `hardirqs last disabled at` is inside `__el0_irq_handler_common+0x18`. Looking at the assembly of `el0_interrupt` shows `arm64_enter_from_user_mode` and `write_sysreg(DAIF_PROCCTX_NOIRQ, daif)`. Wait, it's not masking interrupts here. It calls `trace_hardirqs_off()`, and I know that the `do_interrupt_handler` may not always enable interrupts either.
My attention shifts to softirqs because I see softirqs processed, so interrupt handling must have happened and returned. This still means that the timer fires, queues the work, processes softirqs, and returns. If it returns to EL0, it should schedule! What's blocking the scheduling?
Then I see the "Hardware name: Google Compute Engine, BIOS Google..."! Could the VM have been paused by the host? If the VM was paused for, say, 70 seconds, the guest's timer wouldn't fire during the pause. When resumed, the guest's timer fires. It reads the current time, calls `kvm_check_and_clear_guest_paused()`. If the host reported a pause, it sets `watchdog_report_ts` to `SOFTLOCKUP_DELAY_REPORT`, and the softlockup is *ignored*! This seems very plausible. But I need to confirm the KVM interaction and how the guest is informed. Is `kvm_check_and_clear_guest_paused()` implemented? I think so on arm64. I think this might be it.
|
| 45/2 |
2026/05/30 00:31 |
tool |
grepper |
0m
Error:
Args: map[Expression:kvm_check_and_clear_guest_paused PathPrefix:arch/arm64/]
Results:
|
| 46/2 |
2026/05/30 00:31 |
llm |
expert |
0m
Model:
Tokens:
input: 33494
output: 34
thoughts: 52
Thoughts: Here's my analysis of the situation:
**Investigating the Missing `kvm_check_and_clear_guest_paused()` Implementation**
Okay, so `kvm_check_and_clear_guest_paused()` isn't implemented on arm64. That's a critical piece of information. This isn't unexpected, these architecture-specific differences are common. I need to understand *why* it's missing and what the implications are. My immediate thought is to start with a direct comparison. Let's see how x86 handles this function. I'll need to look at `arch/x86/kernel/kvm.c` and then benchmark that against the code in the `arch/arm64/` directory. That comparative analysis should give me a good handle on how the pausing mechanism is handled in each architecture and what strategies are employed by x86, which will in turn, highlight the absence of a parallel implementation on arm64, and the direction for its resolution. The key here will be understanding if the underlying functionality is achieved in a completely different way on arm64, or if there's a gap that needs to be addressed.
|
| 47/2 |
2026/05/30 00:31 |
tool |
grepper |
0m
Args: map[Expression:kvm_check_and_clear_guest_paused PathPrefix:arch/]
Results: map[Output:arch/loongarch/include/asm/kvm_para.h=179=static inline unsigned int kvm_arch_para_hints(void)
--
arch/loongarch/include/asm/kvm_para.h-183-
arch/loongarch/include/asm/kvm_para.h:184:static inline bool kvm_check_and_clear_guest_paused(void)
arch/loongarch/include/asm/kvm_para.h-185-{
--
arch/powerpc/include/asm/kvm_para.h=33=static inline unsigned int kvm_arch_para_hints(void)
--
arch/powerpc/include/asm/kvm_para.h-37-
arch/powerpc/include/asm/kvm_para.h:38:static inline bool kvm_check_and_clear_guest_paused(void)
arch/powerpc/include/asm/kvm_para.h-39-{
--
arch/s390/include/asm/kvm_para.h=113=static inline unsigned int kvm_arch_para_hints(void)
--
arch/s390/include/asm/kvm_para.h-117-
arch/s390/include/asm/kvm_para.h:118:static inline bool kvm_check_and_clear_guest_paused(void)
arch/s390/include/asm/kvm_para.h-119-{
--
arch/x86/include/asm/kvm_para.h-12-#ifdef CONFIG_KVM_GUEST
arch/x86/include/asm/kvm_para.h:13:bool kvm_check_and_clear_guest_paused(void);
arch/x86/include/asm/kvm_para.h-14-#else
arch/x86/include/asm/kvm_para.h:15:static inline bool kvm_check_and_clear_guest_paused(void)
arch/x86/include/asm/kvm_para.h-16-{
--
arch/x86/kernel/kvmclock.c=124=static void __init kvm_get_preset_lpj(void)
--
arch/x86/kernel/kvmclock.c-135-
arch/x86/kernel/kvmclock.c:136:bool kvm_check_and_clear_guest_paused(void)
arch/x86/kernel/kvmclock.c-137-{
]
|
| 48/2 |
2026/05/30 00:31 |
llm |
expert |
0m
Model:
Tokens:
input: 34473
output: 42
thoughts: 41
Thoughts: Okay, here's my take on those thoughts, framed as an expert's inner monologue:
**Investigating `kvm_check_and_clear_guest_paused()` on ARM64**
Hmm, `kvm_check_and_clear_guest_paused()`β¦ right, that rings a bell. Wait a second, wasn't that specific function *not* implemented on ARM64? I seem to recall some odd architecture-specific behavior there. I need to verify that; it's critical to understanding how pausing and resuming works in this context. Let me just quickly double-check that by looking at `include/linux/kvm_para.h`. That's where all the kvm paravirt interface definitions are, so it's the logical place to start. I'll need to confirm that my memory is correct on its definition or implementation, as this could have broader implications for how guests handle VM states.
|
| 49/2 |
2026/05/30 00:32 |
tool |
grepper |
0m
Error:
Args: map[Expression:kvm_check_and_clear_guest_paused PathPrefix:include/linux/kvm_para.h]
Results:
|
| 50/2 |
2026/05/30 00:32 |
llm |
expert |
0m
Model:
Tokens:
input: 34769
output: 34
thoughts: 31
Thoughts: Here's my attempt at summarizing the thought process, tailored for an expert audience and written in the first person:
**Navigating the KVM Paravirtualization Landscape**
Okay, so we're dealing with guest pause management within KVM. My immediate instinct is to dive into the core header file for paravirtualization within the Linux kernel. Let's see, that would be `include/linux/kvm_para.h`. I'm specifically interested in the function `kvm_check_and_clear_guest_paused`. This likely handles the critical mechanics of detecting if a guest is paused and, crucially, clearing that paused state. I need to understand how this function operates in detail, as it plays a key part in the correct functioning of the system. I'll need to assess the calling conventions, locking, and synchronization primitives used in relation to this function. Understanding this will be crucial to grasping any related performance implications or potential race conditions. From this point, I will be able to start figuring out if there are any subtle bugs here.
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| 51/2 |
2026/05/30 00:32 |
tool |
grepper |
0m
Args: map[Expression:kvm_check_and_clear_guest_paused PathPrefix:include/]
Results: map[Output:include/asm-generic/kvm_para.h-11- */
include/asm-generic/kvm_para.h:12:static inline bool kvm_check_and_clear_guest_paused(void)
include/asm-generic/kvm_para.h-13-{
]
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| 52/2 |
2026/05/30 00:32 |
llm |
expert |
0m
Model:
Tokens:
input: 35137
output: 33
thoughts: 4371
Thoughts: **Analysis of a Soft Lockup Report in a KVM Guest Environment**
Okay, here's the deal: I've been digging into this soft lockup report, and it looks like a false positive due to how KVM handles guest pausing on arm64. It all boils down to the fact that `kvm_check_and_clear_guest_paused()` in `include/asm-generic/kvm_para.h` returns `false` all the time on arm64. This means, if the host pauses the virtual machine, there isn't a mechanism, like `KVM_FEATURE_PV_UNHALT`, to properly signal the guest when it resumes.
Here's the scenario: Imagine the host decides to pause the VM for 70 seconds. When the vCPU thread resumes, the watchdog timer fires, sees a significant time jump (because `kvm_check_and_clear_guest_paused()` always says the VM *wasn't* paused), and flags a soft lockup. This makes the guest think there's a problem when there isn't one. The guest's virtual timer doesn't advance when it's paused.
I've checked other possibilities. If the vCPU was actually *running* in userspace continuously for 70 seconds, the timer would fire at regular intervals, updating the timestamp, and the soft lockup wouldn't happen. Itβs the gap in execution that's the problem. I've also checked for scenarios where the `migration/N` thread is blocked, or other weird lockups, but the code doesn't suggest that's a likely cause.
So, the timer interrupt fires, sees a 70-second gap, and immediately reports the soft lockup before the stopper thread has a chance to update the timestamp.
The PC being in userspace, and the lack of a known signal that the host paused the VM, is the key, and it all makes sense. The "fix" is to signal the guest that the VM was paused, but there doesn't seem to be a method for that.
I've ruled out other explanations, like corruption of `watchdog_touch_ts` or userspace disabling the timer. The lockdep trace even supports the fact that the timer interrupt handler is the current context when the event is triggered. This narrows it down to host preemption. Checking the CPU 1 backtrace, which is unmapping a VMA, is also irrelevant to the root cause. This is a false positive that happens in an environment when the VM is paused/overloaded.
And finally, to address security concerns, this isn't exploitable. This happens in testing, so there is no denial of service. It is a limitation of the environment, not a bug, and is not a security risk. I've analyzed potential deadlocks in CPU 1 and it doesn't seem likely. It is a false positive and likely caused by host preemption.
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| 53/2 |
2026/05/30 00:32 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/swap.c Name:lru_activate]
Results: map[SourceCode: 307: static void lru_activate(struct lruvec *lruvec, struct folio *folio)
308: {
309: long nr_pages = folio_nr_pages(folio);
310:
311: if (folio_test_active(folio) || folio_test_unevictable(folio))
312: return;
313:
314:
315: lruvec_del_folio(lruvec, folio);
316: folio_set_active(folio);
317: lruvec_add_folio(lruvec, folio);
318: trace_mm_lru_activate(folio);
319:
320: __count_vm_events(PGACTIVATE, nr_pages);
321: count_memcg_events(lruvec_memcg(lruvec), PGACTIVATE, nr_pages);
322: }
SourceFile:mm/swap.c]
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| 54/2 |
2026/05/30 00:32 |
llm |
expert |
0m
Model:
Tokens:
input: 40459
output: 37
thoughts: 82
Thoughts: Okay, let me break this down.
**Diving into the `folio_batch_move_lru` Conundrum**
Wait a second, my mental model of `folio_batch_move_lru` is clearly off. I thought it called `move_fn(folio)`. But that doesn't quite jive with how `lru_activate` is defined. `lru_activate` needs the `struct lruvec *lruvec` and the `struct folio *folio` parameters. This suggests I was misremembering how that function operates. I need to be absolutely certain I understand the call chain. I'm going to pull up the source code for `folio_batch_move_lru` and trace it again to confirm what arguments are actually passed. Gotta get my facts straight before I move forward.
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| 55/2 |
2026/05/30 00:32 |
tool |
codesearch-definition-source |
0m
Args: map[ContextFile:mm/swap.c Name:folio_batch_move_lru]
Results: map[SourceCode: 158: static void folio_batch_move_lru(struct folio_batch *fbatch, move_fn_t move_fn)
159: {
160: int i;
161: struct lruvec *lruvec = NULL;
162: unsigned long flags = 0;
163:
164: for (i = 0; i < folio_batch_count(fbatch); i++) {
165: struct folio *folio = fbatch->folios[i];
166:
167: /* block memcg migration while the folio moves between lru */
168: if (move_fn != lru_add && !folio_test_clear_lru(folio))
169: continue;
170:
171: folio_lruvec_relock_irqsave(folio, &lruvec, &flags);
172: move_fn(lruvec, folio);
173:
174: folio_set_lru(folio);
175: }
176:
177: if (lruvec)
178: lruvec_unlock_irqrestore(lruvec, flags);
179: folios_put(fbatch);
180: }
SourceFile:mm/swap.c]
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| 56/2 |
2026/05/30 00:32 |
llm |
expert |
0m
Model:
Tokens:
input: 41139
output: 66
thoughts: 2202
Thoughts: **My Analysis of the Soft Lockup Report**
Okay, here's my thought process on this soft lockup report. First, I see that the kernel reports a soft lockup on CPU 0. The context provided is crucial. CPU 1 is initially mentioned; I need to understand if the activity on CPU 1 is connected. I initially check CPU 1 and it's holding a spinlock. However, the backtrace for CPU 1 shows that it is *releasing* a lock, not waiting for one. The `__sanitizer_cov_trace_pc` presence also suggests normal code execution interrupted by an NMI. So, CPU 1 is not the problem.
Now, focusing on CPU 0. It's in userspace (`pc : 0000ffff8f64c32c`, a user-space address). The report indicates a "soft lockup" after 70 seconds. I've seen this before; it's likely a false positive, resulting from the host preempting the virtual CPU (vCPU). Let me verify this.
I start by systematically eliminating other possibilities. Is it a BPF program stuck in a loop? No, that would be in kernel space, not userspace. Is it a 32-bit compatibility issue? No, `pstate` indicates 64-bit userspace (EL0t). Could the kernel somehow have jumped to userspace due to a bug (e.g., ret2usr)? No, the `pstate` is consistent with genuine userspace execution.
Could the userspace task be in a tight loop and the kernel is not preempting it? The timer interrupt is still going to fire, regardless, and the stopper thread should always update the watchdog timestamp and thus prevent a soft lockup. The timer interrupt will fire and set TIF_NEED_RESCHED. Even a high priority userspace task is not going to stop the kernel from preempting and running the stopper thread. If the watchdog sees that 20 seconds have elapsed, then the system will report a soft lockup. That can't happen when a vCPU is truly running userspace tasks and is not itself preempted by the host.
So, the only explanation is host preemption. The physical CPU wasn't executing the vCPU for an extended period, which means that the VM has been paused or the vCPU was preempted. When the vCPU resumes, the timer immediately fires, detects the time jump, and reports the soft lockup. The interrupted context *was* userspace, which explains the PC and `pstate`. This is a classic false positive in virtualized environments, and is well known on ARM64 as `kvm_check_and_clear_guest_paused()` is not implemented to suppress the warning.
I checked again and can say there is no kernel bug here.
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| 57/2 |
2026/05/30 00:33 |
tool |
set-results |
0m
Args: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
Results: map[DenialOfService:false Exploitable:false FilesystemTrigger:false NetworkTrigger:false PeripheralTrigger:false RemoteTrigger:false Unprivileged:false UserNamespace:false VMGuestTrigger:false VMHostTrigger:false]
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| 58/2 |
2026/05/30 00:33 |
llm |
expert |
0m
Model:
Tokens:
input: 44078
output: 694
thoughts: 14
Thoughts: Okay, here's the summary, rendered as a first-person thought process, keeping in mind the user is an expert:
**Delivering the Verdict**
Alright, the processing is complete. The simulations, analyses, and whatever else was involved to get us to this point β they're locked in. Now, it's time to solidify everything and formulate the final response. There's no further tweaking, no more iterations; it's go time. I'm focusing on clarity and precision as I bring it all together. The goal now is to synthesize everything and present a clear, definitive answer, assuming the expert understands the inputs.
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| 59/1 |
2026/05/30 00:33 |
action |
format-explanation |
0m
Results: map[Explanation:Based on the analysis of the bug report and the kernel source code, here is the
assessment:
The bug report indicates a soft lockup on CPU 0: `watchdog: BUG: soft lockup -
CPU#0 stuck for 70s!`.
However, looking at the register dump for CPU 0:
```
pstate: 60001000 (nZCv daif -PAN -UAO -TCO -DIT +SSBS BTYPE=--)
pc : 0000ffff8f64c32c
lr : 0000ffff8f667f08
sp : 0000fffff8c58310
```
The `pstate` value `60001000` has its lowest 4 bits (`M` field) set to `0b0000`,
which corresponds to `EL0t` (64-bit userspace mode). The program counter (`pc`)
is also clearly a userspace address.
A soft lockup is detected when the per-CPU watchdog timer interrupt fires and
observes that the watchdog timestamp hasn't been updated by the high-priority
watchdog stopper thread (`migration/N`) for over 20 seconds.
If a CPU is genuinely executing in userspace, preemption is fully enabled. The
timer interrupt would fire every 4 seconds, queue the watchdog update work, and
upon returning to userspace, the kernel would schedule the high-priority stopper
thread to update the timestamp. Thus, a soft lockup **cannot** legitimately
occur while a CPU is continuously running in userspace.
The only scenario where a soft lockup is reported with a userspace PC is when
the virtual CPU (vCPU) is preempted or paused by the host hypervisor for an
extended period (in this case, >70 seconds).
1. The vCPU is executing in userspace.
2. The host hypervisor preempts the vCPU (due to high host load, overcommit, or
being explicitly paused).
3. 70 seconds of physical time pass.
4. The host resumes the vCPU.
5. The guest's timer interrupt fires immediately because it is long overdue.
6. The timer interrupt handler checks the current time, sees that 70 seconds
have passed since the last watchdog touch, and immediately reports a soft
lockup.
7. Because the timer interrupt interrupted userspace execution, the register
dump shows the userspace PC and `EL0t` state.
On some architectures (like x86), KVM provides a paravirtualized feature
(`kvm_check_and_clear_guest_paused()`) to inform the guest that it was paused,
allowing the guest to suppress this false positive. However, on ARM64, this
feature is currently not implemented (it returns a hardcoded `false`), making
ARM64 guests susceptible to these false positive soft lockup reports when
running in constrained or overcommitted VM environments (like the syzbot GCE
instances).
**Conclusion:**
This is not a kernel bug, but a known false positive artifact of the virtualized
testing environment. It has no security impact, cannot be exploited, and does
not cause a denial of service.]
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