Re: [PATCH 4/6] ring-buffer: Fix subbuf resize concurrency

[Steven Rostedt <rostedt@goodmis.org>]

On Mon, 10 Aug 2026 08:54:22 +0100
Vincent Donnefort <vdonnefort@google.com> wrote:

> This is probably the issue.
> 
> diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> index c2cbdb3ef727..a64cdc925078 100644
> --- a/kernel/trace/ring_buffer.c
> +++ b/kernel/trace/ring_buffer.c
> @@ -3534,7 +3534,7 @@ rb_event_index(struct ring_buffer_per_cpu *cpu_buffer, struct ring_buffer_event
>  {
>         unsigned long addr = (unsigned long)event;
>  
> -       addr &= rb_subbuf_size(cpu_buffer->buffer) - 1;
> +       addr &= (unsigned long)rb_subbuf_size(cpu_buffer->buffer) - 1;
>  
>         return addr - BUF_PAGE_HDR_SIZE;
>  }
> @@ -4123,7 +4123,7 @@ rb_try_to_discard(struct ring_buffer_per_cpu *cpu_buffer,
>         new_index = rb_event_index(cpu_buffer, event);
>         old_index = new_index + rb_event_ts_length(event);
>         addr = (unsigned long)event;
> -       addr &= ~(rb_subbuf_size(cpu_buffer->buffer) - 1);
> +       addr &= ~((unsigned long)rb_subbuf_size(cpu_buffer->buffer) - 1);
>  
>         bpage = READ_ONCE(cpu_buffer->tail_page);
>  
> @@ -5033,7 +5033,7 @@ rb_decrement_entry(struct ring_buffer_per_cpu *cpu_buffer,
>         struct buffer_page *bpage = cpu_buffer->commit_page;
>         struct buffer_page *start;
>  
> -       addr &= ~(rb_subbuf_size(cpu_buffer->buffer) - 1);
> +       addr &= ~((unsigned long)rb_subbuf_size(cpu_buffer->buffer) - 1);

What about just making those functions return unsigned long instead of
having to add typecasts all over. It shouldn't hurt the places it's made an
int? Although I'm not sure if bots will complain. :-/

>  
>         /* Do the likely case first */
>         if (likely(bpage->page == (void *)addr)) {
> 
> 
> I will resend a V3 dropping the 3 first patches you have applied already.

Yeah, they are already in Linus's tree.

-- Steve