linux-kernel - Re: Crashes in perf_event_ctx_lock

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Message-ID: <alpine.DEB.2.20.1711011922370.1942@nanos>
Date:   Wed, 1 Nov 2017 19:22:54 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Guenter Roeck <linux@...ck-us.net>
cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Don Zickus <dzickus@...hat.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: Crashes in perf_event_ctx_lock_nested

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@...ck-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