Re: [PATCH v3 14/15] perf: Identify the 0->1 transition for event::mmap_count

[Peter Zijlstra <peterz@infradead.org>]

ljs noted that this Changelog was a tad terse, and he had a pending
question here.

On Tue, Aug 12, 2025 at 12:39:12PM +0200, Peter Zijlstra wrote:
> Needed because refcount_inc() doesn't allow the 0->1 transition.

Specifically, this is the case where we've created the RB, this means
there was no RB, and as such there could not have been an mmap.
Additionally we hold mmap_mutex to serialize everything.

This must be the first.

> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/events/core.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -7033,7 +7033,7 @@ static int perf_mmap_rb(struct vm_area_s
>  	perf_event_update_userpage(event);
>  
>  	perf_mmap_account(vma, user_extra, extra);
> -	atomic_inc(&event->mmap_count);
> +	atomic_set(&event->mmap_count, 1);
>  
>  	return 0;
>  }
> 
>