Re: [RFC PATCH] tracing/ring-buffer: Drop inappropriate WARN in rb_set_head_page()

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 1 Mar 2023 11:47:02 +0800
Zheng Yejian <zhengyejian1@huawei.com> wrote:

> Following WARNING appears several times during test on v5.10 but
> mainline kernel should have the same problem. However I currently
> can't find the reproduction method.
> 
> WARNING: CPU: 29 PID: 686834 at kernel/trace/ring_buffer.c:1357
>            rb_set_head_page+0x168/0x264
> Link: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/kernel/trace/ring_buffer.c?h=linux-5.10.y#n1357
> Call trace:
>  rb_set_head_page+0x168/0x264
>  rb_per_cpu_empty+0x34/0x15c
>  ring_buffer_empty_cpu.part.0.isra.0+0x1a4/0x3f0
>  ring_buffer_empty_cpu+0x74/0xb4
>  __find_next_entry+0x14c/0x2f4
>  trace_find_next_entry_inc+0x48/0x13c
>  tracing_read_pipe+0x2c8/0x6b4
>  vfs_read+0x144/0x324
>  ksys_read+0x104/0x220
>  __arm64_sys_read+0x54/0x70
>  el0_svc_common.constprop.0+0xd8/0x37c
>  do_el0_svc+0x50/0x120
>  el0_svc+0x24/0x3c
>  el0_sync_handler+0x17c/0x180
>  el0_sync+0x160/0x180
> 
> The WARNING appears because rb_set_head_page() didn't grab the header
> after three loops traversing buffer pages. This was not considered
> to be expected, as comment said, writer possibly moves the header in
> one loop.
> 
> However, supposing writer keeps moving the header, we may miss more
> loops and it seems normal not to grab the header within three loops
> in rb_set_head_page(). Therefore drop that RB_WARN_ON().

It's normal to grab the header in two loops. I only made it three in case I
was wrong. If it took 4 tries, something is wrong. Just returning NULL
without setting the header will cause bugs elsewhere.

-- Steve


> 
> Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
> ---
>  kernel/trace/ring_buffer.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index af50d931b020..cbfa306570d3 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -1471,9 +1471,7 @@ rb_set_head_page(struct ring_buffer_per_cpu *cpu_buffer)
>  	page = head = cpu_buffer->head_page;
>  	/*
>  	 * It is possible that the writer moves the header behind
> -	 * where we started, and we miss in one loop.
> -	 * A second loop should grab the header, but we'll do
> -	 * three loops just because I'm paranoid.
> +	 * where we started, so we try three loops to grab the header.
>  	 */
>  	for (i = 0; i < 3; i++) {
>  		do {
> @@ -1485,8 +1483,6 @@ rb_set_head_page(struct ring_buffer_per_cpu *cpu_buffer)
>  		} while (page != head);
>  	}
>  
> -	RB_WARN_ON(cpu_buffer, 1);
> -
>  	return NULL;
>  }
>