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From: Vince Weaver <vincent.weaver@maine.edu>
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Date: Mon, 29 Aug 2016 09:14:46 -0400 (EDT)
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To: Peter Zijlstra <peterz@infradead.org>
cc: Jiri Olsa <jolsa@redhat.com>, Vegard Nossum <vegard.nossum@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Stephane Eranian <eranian@google.com>,
        Vince Weaver <vincent.weaver@maine.edu>,
        Ingo Molnar <mingo@kernel.org>,
        David Carrillo-Cisneros <davidcc@google.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Kan Liang <kan.liang@intel.com>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        Paul Turner <pjt@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        linux-tip-commits@vger.kernel.org
Subject: Re: [tip:perf/core] perf/core: Check return value of the perf_event_read()
 IPI
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On Mon, 29 Aug 2016, Peter Zijlstra wrote:

> 
> The best I can come up with is something like:
> 
> 
> 	do {
> 		state = READ_ONCE(event->state);
> 		if (state != ACTIVE)
> 			break;
> 		smp_rmb();
> 		cpu = READ_ONCE(event->cpu);
> 		smp_rmb();
> 	} while (READ_ONCE(event->state) != state);
> 
> 
> And I suppose perf_event_restart() should do the same thing... Let me
> ponder this a wee bit more.

the perf_fuzzer trips over this warning fairly regularly too, so once you 
have a patch you're happy with I can give it a test.  

I ordered a whole crate of null modem cables so I've got a whole bunch of 
serial consoles ready and raring to go.  Maybe I can find out how the 
fuzzer crashes the new skylake machine, it doesn't leave anything useful 
in the syslog.

Vince