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Date: Wed, 1 Nov 2017 19:22:54 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Guenter Roeck <linux@roeck-us.net>
cc: Peter Zijlstra <peterz@infradead.org>, linux-kernel@vger.kernel.org,
        Don Zickus <dzickus@redhat.com>, Ingo Molnar <mingo@kernel.org>
Subject: Re: Crashes in perf_event_ctx_lock_nested
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On Tue, 31 Oct 2017, Guenter Roeck wrote:
> On Tue, Oct 31, 2017 at 10:32:00PM +0100, Thomas Gleixner wrote:
> 
> [ ...] 
> 
> > So we have to revert
> > 
> > a33d44843d45 ("watchdog/hardlockup/perf: Simplify deferred event destroy")
> > 
> > Patch attached.
> > 
> 
> Tested-by: Guenter Roeck <linux@roeck-us.net>
> 
> There is still a problem. When running
> 
> echo 6 > /proc/sys/kernel/watchdog_thresh
> echo 5 > /proc/sys/kernel/watchdog_thresh
> 
> repeatedly, the message
> 
> NMI watchdog: Enabled. Permanently consumes one hw-PMU counter.
> 
> stops after a while (after ~10-30 iterations, with fluctuations).
> After adding trace messages into hardlockup_detector_perf_disable()
> and hardlockup_detector_perf_enable(), I see:
> 
> hardlockup_detector_perf_disable: disable(0): Number of CPUs: 3
> hardlockup_detector_perf_disable: disable(1): Number of CPUs: 2
> hardlockup_detector_perf_disable: disable(2): Number of CPUs: 1
> hardlockup_detector_perf_disable: disable(3): Number of CPUs: 0
> ...
> hardlockup_detector_perf_disable: disable(0): Number of CPUs: 2
> hardlockup_detector_perf_disable: disable(1): Number of CPUs: 1
> hardlockup_detector_perf_disable: disable(2): Number of CPUs: 0
> hardlockup_detector_perf_disable: disable(3): Number of CPUs: -1
> ...
> hardlockup_detector_perf_enable: enable(1): Number of CPUs: -6
> hardlockup_detector_perf_enable: enable(3): Number of CPUs: -5
> hardlockup_detector_perf_enable: enable(2): Number of CPUs: -4
> hardlockup_detector_perf_enable: enable(0): Number of CPUs: -3
> 
> Maybe watchdog_cpus needs to be atomic ?

Indeed.

Thanks,

	tglx