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Date: Mon, 6 Feb 2012 15:38:45 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Stephane Eranian <eranian@google.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
        Markus Trippelsdorf <markus@trippelsdorf.de>,
        linux-kernel@vger.kernel.org, Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Paul Mackerras <paulus@samba.org>
Subject: Re: WARNING: at arch/x86/kernel/cpu/perf_event.c:989
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* Stephane Eranian <eranian@google.com> wrote:

> Ok, I found the problem!
> 
> it comes from perf_adjust_freq_unthr_context() vs perf_adjust_period().
> The latter can under certain condition stop and restart the event. So we
> had:
> 
>       stop()
>       if (delta > 0) {
>            perf_adjust_period() {
>                if (period > 8*...) {
>                    stop()
>                    ...
>                    start()
>                }
>            }
>        }
>        start()
> 
> Could have a double stop() and double start(), thus triggering the warning in
> x86_pmu_start().
> 
> Will post a patch shortly to fix this.

Please send a patch against tip:master (or perf/urgent), on top 
of:

  84f2b9b2edc0: perf: Remove deprecated WARN_ON_ONCE()

Note that I updated that patch to removed one more WARN_ON() 
than you originally sent - please add it back as appropriate, in 
the delta fix patch.

Thanks,

	Ingo