[PATCH] tracing: Fix soft lockup when lseeking trace file

[PATCH] tracing: Fix soft lockup when lseeking trace file

[Tengda Wu]

A soft lockup may occur when accessing trace file via lseek while
tracing is active and a large offset is provided. The call trace
is shown below:

watchdog: BUG: soft lockup - CPU#3 stuck for 26s! [poc:141]
CPU: 3 UID: 0 PID: 141 Comm: poc Not tainted 6.19.0 #1 PREEMPT(none)
Call Trace:
  ring_buffer_iter_peek
  peek_next_entry
  __find_next_entry
  trace_find_next_entry_inc
  s_next
  traverse.part.0
  seq_lseek
  tracing_lseek
  __x64_sys_lseek
  do_syscall_64
  entry_SYSCALL_64_after_hwframe

The root cause is that the lseek implementation for trace files
is based on seq_lseek, which contains a loop that repeatedly calls
show() and next() functions until the position reaches the target
offset. Since no scheduling point is set within this loop, a large
offset can cause the CPU to be stuck in the loop for an extended
period, triggering the soft lockup detector.

Fixed by adding cond_resched() in s_next().

Fixes: bc0c38d139ec ("ftrace: latency tracer infrastructure")
Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>
---
 kernel/trace/trace.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 8bd4ec08fb36..3afe148ef683 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -3928,6 +3928,8 @@ static void *s_next(struct seq_file *m, void *v, loff_t *pos)
 
 	iter->pos = *pos;
 
+	cond_resched();
+
 	return ent;
 }
 
-- 
2.34.1

Re: [PATCH] tracing: Fix soft lockup when lseeking trace file

[Steven Rostedt]

Peter,

When is PREEMPT_NONE going to be removed? If that's going into 7.0, I want
to stop accepting these scattered cond_resched() additions.

-- Steve


On Mon,  9 Feb 2026 17:31:01 +0800
Tengda Wu <wutengda@huaweicloud.com> wrote:

> A soft lockup may occur when accessing trace file via lseek while
> tracing is active and a large offset is provided. The call trace
> is shown below:
> 
> watchdog: BUG: soft lockup - CPU#3 stuck for 26s! [poc:141]
> CPU: 3 UID: 0 PID: 141 Comm: poc Not tainted 6.19.0 #1 PREEMPT(none)
> Call Trace:
>   ring_buffer_iter_peek
>   peek_next_entry
>   __find_next_entry
>   trace_find_next_entry_inc
>   s_next
>   traverse.part.0
>   seq_lseek
>   tracing_lseek
>   __x64_sys_lseek
>   do_syscall_64
>   entry_SYSCALL_64_after_hwframe
> 
> The root cause is that the lseek implementation for trace files
> is based on seq_lseek, which contains a loop that repeatedly calls
> show() and next() functions until the position reaches the target
> offset. Since no scheduling point is set within this loop, a large
> offset can cause the CPU to be stuck in the loop for an extended
> period, triggering the soft lockup detector.
> 
> Fixed by adding cond_resched() in s_next().
> 
> Fixes: bc0c38d139ec ("ftrace: latency tracer infrastructure")
> Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>
> ---
>  kernel/trace/trace.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 8bd4ec08fb36..3afe148ef683 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -3928,6 +3928,8 @@ static void *s_next(struct seq_file *m, void *v, loff_t *pos)
>  
>  	iter->pos = *pos;
>  
> +	cond_resched();
> +
>  	return ent;
>  }
>