Re: [PATCH 1/2] tracing: Fix retry exhaustion in simple ring buffer reader swap

[Vincent Donnefort <vdonnefort@google.com>]

On Tue, Aug 25, 2026 at 01:20:37AM -0400, Ivan Immanuel Shaji via B4 Relay wrote:
> From: Ivan Immanuel Shaji <ivanimmanuel1234@gmail.com>
> 
> simple_ring_buffer_swap_reader_page() starts with retry set to 8 and
> post-decrements it only after a failed link replacement. On the final
> attempt, a successful replacement leaves retry at zero, while a failed
> replacement leaves it at -1.
> 
> The current !retry test reverses both outcomes. It returns an error after
> a successful final replacement, leaving the link update complete but the
> reader bookkeeping unfinished. After a failed final replacement, it
> falls through and updates the head and reader pointers as though the
> replacement succeeded, which can corrupt the ring.
> 
> Treat only a negative counter as exhaustion and return the documented
> -EBUSY error.
> 
> Fixes: 34e5b958bdad ("tracing: Introduce simple_ring_buffer")
> Cc: stable@vger.kernel.org
> Assisted-by: LLM sparse
> Signed-off-by: Ivan Immanuel Shaji <ivanimmanuel1234@gmail.com>

Reviewed-by: Vincent Donnefort <vdonnefort@google.com>

> ---
>  kernel/trace/simple_ring_buffer.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/trace/simple_ring_buffer.c b/kernel/trace/simple_ring_buffer.c
> index f4642f5adda3..49913bb0057a 100644
> --- a/kernel/trace/simple_ring_buffer.c
> +++ b/kernel/trace/simple_ring_buffer.c
> @@ -160,8 +160,8 @@ int simple_ring_buffer_swap_reader_page(struct simple_rb_per_cpu *cpu_buffer)
>  		overrun = cpu_buffer->meta->overrun;
>  	} while (!simple_bpage_unset_head_link(last, reader, SIMPLE_RB_LINK_NORMAL) && retry--);
>  
> -	if (!retry)
> -		return -EINVAL;
> +	if (retry < 0)
> +		return -EBUSY;
>  
>  	cpu_buffer->head_page = simple_bpage_from_link(reader->link.next);
>  	cpu_buffer->head_page->link.prev = &reader->link;
> 
> -- 
> 2.53.0
> 
>