Re: [PATCH 2/2] ring-buffer: Stop remote reader update when page swap fails

[Vincent Donnefort <vdonnefort@google.com>]

On Tue, Aug 25, 2026 at 01:20:38AM -0400, Ivan Immanuel Shaji via B4 Relay wrote:
> From: Ivan Immanuel Shaji <ivanimmanuel1234@gmail.com>
> 
> The remote swap_reader_page callback can fail when the writer moves the
> head before the remote catches it. __rb_get_reader_page_from_remote()
> warns about that failure but continues with the unchanged reader ID and
> rearranges the local page list as though the swap succeeded.
> 
> Stop immediately after warning. Callers already handle a NULL reader
> page as a failed attempt, and this avoids splicing the same page as both
> the previous and new reader.
> 
> Fixes: 2e67fabd8b77 ("ring-buffer: Introduce ring-buffer remotes")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM sparse
> Signed-off-by: Ivan Immanuel Shaji <ivanimmanuel1234@gmail.com>
> ---
>  kernel/trace/ring_buffer.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index 3c3ed639923d..d11b2759caac 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -5805,8 +5805,9 @@ __rb_get_reader_page_from_remote(struct ring_buffer_per_cpu *cpu_buffer)
>  
>  	prev_reader = cpu_buffer->subbuf_ids[cpu_buffer->meta_page->reader.id];
>  
> -	WARN_ON_ONCE(cpu_buffer->remote->swap_reader_page(cpu_buffer->cpu,
> -							  cpu_buffer->remote->priv));
> +	if (WARN_ON_ONCE(cpu_buffer->remote->swap_reader_page(cpu_buffer->cpu,
> +							      cpu_buffer->remote->priv)))
> +		return NULL;
>  	/* nr_pages doesn't include the reader page */
>  	if (WARN_ON_ONCE(cpu_buffer->meta_page->reader.id > cpu_buffer->nr_pages))
>  		return NULL;
> 
> -- 
> 2.53.0
> 
> 

If handled gracefully and as this can be triggered by a storm of events on small
buffer (when -EBUSY) I wonder if we shouldn't use a pr_warn_ratelimited()
instead of a WARN()?

-- 
Vincent