linux-kernel - Re: [RFC PATCH 1/1] perf/core: Wake up parent event if inherited event has no ring buffer

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Date:   Tue, 25 Jan 2022 20:12:58 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     James Clark <james.clark@....com>
Cc:     mingo@...hat.com, acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, jolsa@...hat.com,
        namhyung@...nel.org, linux-perf-users@...r.kernel.org,
        leo.yan@...aro.com, Suzuki.Poulose@....com,
        Ruben Ayrapetyan <Ruben.Ayrapetyan@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] perf/core: Wake up parent event if inherited
 event has no ring buffer

On Mon, Jan 24, 2022 at 02:58:18PM +0000, James Clark wrote:

> > Would this be the better patch?
> 
> Yes I tested this and it also works.

Excellent!

> There is one other suspicious access
> of ->rb followed by if(rb) here in perf_poll(), but maybe it works out ok?
> 
> 	mutex_lock(&event->mmap_mutex);
> 	rb = event->rb;
> 	if (rb)
> 		events = atomic_xchg(&rb->poll, 0);
> 

That case must be good since it is the event from a file. Those cannot
be clones.

> We also have a Perf self test that covers this failure for Arm SPE now, I'm not
> sure if I should post that separately or with your new version of this fix?

They'd be routed through separate trees anyway, Arnaldo takes the
userspace stuff.