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

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Message-ID: <20250812111522.GI4067720@noisy.programming.kicks-ass.net>
Date: Tue, 12 Aug 2025 13:15:22 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: tglx@...utronix.de
Cc: linux-kernel@...r.kernel.org, torvalds@...uxfoundation.org,
	mingo@...nel.org, namhyung@...nel.org, acme@...hat.com,
	kees@...nel.org
Subject: Re: [PATCH v3 14/15] perf: Identify the 0->1 transition for
 event::mmap_count


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@...utronix.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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;
>  }
> 
>