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

[Steven Rostedt]

On Sat, 24 Aug 2024 11:03:43 +0800
Zheng Yejian <zhengyejian@xxxxxxxxxxxxxxx> 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 <zhengyejian@xxxxxxxxxxxxxxx>
> ---
>  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;