Re: [PATCH] tracing: Mitigate possible softlockup in __tracing_open()

[Steven Rostedt]

On Sat, 24 Aug 2024 11:03:43 +0800
Zheng Yejian <zhengyej...@huaweicloud.com> wrote:

> In __tracing_open(), when max latency tracers took place on the cpu,
> the time start of its buffer would be updated, then event entries with
> timestamps being earlier than start of the buffer would be skipped
> (see tracing_iter_reset()).
> 
> Softlockup will occur if the kernel is non-preemptible and too many
> entries were skipped in the loop that reset every cpu buffer, so add
> cond_resched() to avoid it.
> 
> Signed-off-by: Zheng Yejian <zhengyej...@huaweicloud.com>
> ---
>  kernel/trace/trace.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index ebe7ce2f5f4a..88faa95b457b 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -4706,6 +4706,15 @@ __tracing_open(struct inode *inode, struct file *file, 
> bool snapshot)
>               for_each_tracing_cpu(cpu) {
>                       ring_buffer_read_start(iter->buffer_iter[cpu]);
>                       tracing_iter_reset(iter, cpu);
> +                     /*
> +                      * When max latency tracers took place on the cpu, the 
> time start
> +                      * of its buffer would be updated, then event entries 
> with timestamps
> +                      * being earlier than start of the buffer would be 
> skipped
> +                      * (see tracing_iter_reset()). Softlockup will occur if 
> the kernel
> +                      * is non-preemptible and too many entries were skipped 
> in the loop,
> +                      * so add cond_resched() to mitigate it.
> +                      */
> +                     cond_resched();

This is the wrong place to put this. If the problem is with
tracing_iter_reset(), then add it there.

        while (ring_buffer_iter_peek(buf_iter, &ts)) {
                if (ts >= iter->array_buffer->time_start)
                        break;
                entries++;
                ring_buffer_iter_advance(buf_iter);
                /* This could be a big loop */
                cond_resched();
        }

-- Steve



>               }
>       } else {
>               cpu = iter->cpu_file;