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Date:   Wed, 15 May 2019 11:43:52 +0200
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.
Message-ID: <20190515094352.GC2623@hirez.programming.kicks-ass.net>
References: <20190515003059.23920-1-yabinc@google.com>
 <87ef50xlb8.fsf@ashishki-desk.ger.corp.intel.com>
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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.