linux-kernel - Re: [PATCH v3 13/15] perf: Use scoped_guard() for mmap_mutex in perf

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Message-ID: <a9085638-b560-43a6-861e-74e52b431783@lucifer.local>
Date: Wed, 13 Aug 2025 07:42:41 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: tglx@...utronix.de, linux-kernel@...r.kernel.org,
        torvalds@...uxfoundation.org, mingo@...nel.org, namhyung@...nel.org,
        acme@...hat.com, kees@...nel.org
Subject: Re: [PATCH v3 13/15] perf: Use scoped_guard() for mmap_mutex in
 perf_mmap()

On Tue, Aug 12, 2025 at 12:39:11PM +0200, Peter Zijlstra wrote:
> Mostly just re-indent noise.
>
> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

> ---
>  kernel/events/core.c |   32 ++++++++++++++------------------
>  1 file changed, 14 insertions(+), 18 deletions(-)
>
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -7143,27 +7143,23 @@ static int perf_mmap(struct file *file,
>  	if (vma_size != PAGE_SIZE * nr_pages)
>  		return -EINVAL;
>
> -	mutex_lock(&event->mmap_mutex);

Yeah getting rid of this lock goto stuff is great... mm is at risk of some
more Lorenzo churn to use this stuff over there... :)

> -	ret = -EINVAL;

You see, I'm getting in a pattern here with comment on prior patch and you
reading my mind and fixing it on next ;)

> +	scoped_guard (mutex, &event->mmap_mutex) {
> +		/*
> +		 * This relies on __pmu_detach_event() taking mmap_mutex after marking
> +		 * the event REVOKED. Either we observe the state, or __pmu_detach_event()
> +		 * will detach the rb created here.
> +		 */
> +		if (event->state <= PERF_EVENT_STATE_REVOKED) {
> +			ret = -ENODEV;
> +			break;

I don't absolutely love this break-for-what-is-not-obviously-a-for-loop
formulation (I know scoped_guard in practice _is_ a for loop, but obviously
that's hidden by macro), but I guess hey it's C, and we have to do what we
have to do :)

> +		}
>
> -	/*
> -	 * This relies on __pmu_detach_event() taking mmap_mutex after marking
> -	 * the event REVOKED. Either we observe the state, or __pmu_detach_event()
> -	 * will detach the rb created here.
> -	 */
> -	if (event->state <= PERF_EVENT_STATE_REVOKED) {
> -		ret = -ENODEV;
> -		goto unlock;
> +		if (vma->vm_pgoff == 0)
> +			ret = perf_mmap_rb(vma, event, nr_pages);
> +		else
> +			ret = perf_mmap_aux(vma, event, nr_pages);
>  	}
>
> -	if (vma->vm_pgoff == 0)
> -		ret = perf_mmap_rb(vma, event, nr_pages);
> -	else
> -		ret = perf_mmap_aux(vma, event, nr_pages);
> -
> -unlock:
> -	mutex_unlock(&event->mmap_mutex);
> -
>  	if (ret)
>  		return ret;
>
>
>