* Re: [PATCH v2] tracing: Fix nr_subbufs initialization in simple_ring_buffer_init_mm()
[not found] ` <20260319094126.109691-1-devnexen@gmail.com>
@ 2026-05-12 13:26 ` Vincent Donnefort
0 siblings, 0 replies; only message in thread
From: Vincent Donnefort @ 2026-05-12 13:26 UTC (permalink / raw)
To: David Carlier
Cc: Steven Rostedt, Mathieu Desnoyers, Masami Hiramatsu,
Catalin Marinas, Ryan Roberts, linux-kernel
On Thu, Mar 19, 2026 at 09:41:26AM +0000, David Carlier wrote:
> nr_subbufs in the ring buffer metadata is always initialized to zero
> because it is assigned from cpu_buffer->nr_pages before the page
> initialization loop has run. While nr_subbufs is not currently read
> by the kernel, it should reflect the actual buffer geometry in the
> meta page for correctness.
>
> Move the assignment after the page loop so that cpu_buffer->nr_pages
> holds the final count. Bug discover by Opus AI agent.
nit: I believe we can put that new Assisted-by: tag now.
>
> Fixes: 34e5b958bdad ("tracing: Introduce simple_ring_buffer")
> Signed-off-by: David Carlier <devnexen@gmail.com>
> ---
> kernel/trace/simple_ring_buffer.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/trace/simple_ring_buffer.c b/kernel/trace/simple_ring_buffer.c
> index 02af2297ae5a..f731f14d0ff7 100644
> --- a/kernel/trace/simple_ring_buffer.c
> +++ b/kernel/trace/simple_ring_buffer.c
> @@ -395,7 +395,6 @@ int simple_ring_buffer_init_mm(struct simple_rb_per_cpu *cpu_buffer,
>
> memset(cpu_buffer->meta, 0, sizeof(*cpu_buffer->meta));
> cpu_buffer->meta->meta_page_size = PAGE_SIZE;
> - cpu_buffer->meta->nr_subbufs = cpu_buffer->nr_pages;
>
> /* The reader page is not part of the ring initially */
> page = load_page(desc->page_va[0]);
> @@ -437,6 +436,7 @@ int simple_ring_buffer_init_mm(struct simple_rb_per_cpu *cpu_buffer,
> return ret;
> }
>
> + cpu_buffer->meta->nr_subbufs = cpu_buffer->nr_pages;
> /* Close the ring */
> bpage->link.next = &cpu_buffer->tail_page->link;
> cpu_buffer->tail_page->link.prev = &bpage->link;
> --
> 2.53.0
>
Reviewed-by: Vincent Donnefort <vdonnefort@google.com>
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