Re: [PATCH v2] ring-buffer: Handle a reader taking the new head in rb_handle_head_page()

[Breno Leitao <leitao@debian.org>]

Hello Steven,

On Thu, Nov 05, 2026 at 03:57:29PM -0500, Steven Rostedt wrote:
> Can you try this?
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index c64e001c3fc0..f0bb2d23d818 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -3721,6 +3721,14 @@ rb_handle_head_page(struct ring_buffer_per_cpu *cpu_buffer,
>  		local_sub(rb_page_commit(next_page), &cpu_buffer->entries_bytes);
>  		local_inc(&cpu_buffer->pages_lost);
>  
> +		/*
> +		 * To prevent a stale head_page from being between
> +		 * the UPDATE and new HEAD pages, set it here to have
> +		 * the reader check always find the update before
> +		 * finding the HEAD page.
> +		 */
> +		cpu_buffer->head_page = next_page;
> +
>  		if (cpu_buffer->ring_meta)
>  			rb_update_meta_head(cpu_buffer, next_page);
>  		/*
> 
> We may need to add memory barriers here, but for now, I think this could
> fix it. As it should prevent the search from ever starting in between the
> two heads.

This seems to be racy as well. Sashiko found the race, and it can be
reproduced by adding some delays in the code.

Thanks for looking at it,
--breno