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

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Message-ID: <20250813084448.GB4068168@noisy.programming.kicks-ass.net>
Date: Wed, 13 Aug 2025 10:44:48 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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 Wed, Aug 13, 2025 at 10:32:44AM +0200, Peter Zijlstra wrote:
> On Wed, Aug 13, 2025 at 07:42:41AM +0100, Lorenzo Stoakes wrote:
> 
> > > +	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 :)
> 
> Right, don't love it either, but the alternative was a goto and that's
> arguably worse, so meh.
> 
> > > +		}
> > >
> > > +		if (vma->vm_pgoff == 0)
> > > +			ret = perf_mmap_rb(vma, event, nr_pages);
> > > +		else
> > > +			ret = perf_mmap_aux(vma, event, nr_pages);
> > >  	}
> > >
> > >  	if (ret)
> > >  		return ret;
> > >

Nah, I'm an idiot.. How's this?

---

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 20189a3354f2..4b82f8ed6b4e 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7151,20 +7151,17 @@ static int perf_mmap(struct file *file, struct vm_area_struct *vma)
 		 * 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;
-		}
+		if (event->state <= PERF_EVENT_STATE_REVOKED)
+			return -ENODEV;
 
 		if (vma->vm_pgoff == 0)
 			ret = perf_mmap_rb(vma, event, nr_pages);
 		else
 			ret = perf_mmap_aux(vma, event, nr_pages);
+		if (ret)
+			return ret;
 	}
 
-	if (ret)
-		return ret;
-
 	/*
 	 * Since pinned accounting is per vm we cannot allow fork() to copy our
 	 * vma.