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

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Message-ID: <20250813083244.GL4067720@noisy.programming.kicks-ass.net>
Date: Wed, 13 Aug 2025 10:32:44 +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 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;
> >
> >
> >