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From: Peter Zijlstra <peterz@infradead.org>
To: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Yabin Cui <yabinc@google.com>, Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] perf/ring_buffer: Fix exposing a temporarily decreased data_head.
Date: Wed, 15 May 2019 11:43:52 +0200	[thread overview]
Message-ID: <20190515094352.GC2623@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <87ef50xlb8.fsf@ashishki-desk.ger.corp.intel.com>

On Wed, May 15, 2019 at 09:51:07AM +0300, Alexander Shishkin wrote:
> Yabin Cui <yabinc@google.com> writes:
> 
> > diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
> > index 674b35383491..0b9aefe13b04 100644
> > --- a/kernel/events/ring_buffer.c
> > +++ b/kernel/events/ring_buffer.c
> > @@ -54,8 +54,10 @@ static void perf_output_put_handle(struct perf_output_handle *handle)
> >  	 * IRQ/NMI can happen here, which means we can miss a head update.
> >  	 */
> >  
> > -	if (!local_dec_and_test(&rb->nest))
> > +	if (local_read(&rb->nest) > 1) {
> > +		local_dec(&rb->nest);
> 
> What stops rb->nest changing between local_read() and local_dec()?

Nothing, however it must remain the same :-)

That is the cryptic way of saying that since these buffers are strictly
per-cpu, the only change can come from interrupts, and they must have a
net 0 change. Or rather, an equal amount of decrements to increments.

So if it changes, it must also change back to where it was.

  reply	other threads:[~2019-05-15  9:44 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-15  0:30 Yabin Cui
2019-05-15  6:51 ` Alexander Shishkin
2019-05-15  9:43   ` Peter Zijlstra [this message]
2019-05-15 10:04     ` Alexander Shishkin
2019-05-15 10:51       ` Peter Zijlstra
2019-05-15 10:48 ` Peter Zijlstra
2019-05-16 18:37   ` Yabin Cui
2019-05-16 18:40 ` [PATCH v2] " Yabin Cui
2019-05-17  8:47   ` Peter Zijlstra
2019-05-17 17:59     ` [PATCH] " Yabin Cui

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