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Date: Fri, 1 Sep 2017 21:29:07 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Don Zickus <dzickus@redhat.com>
cc: LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
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        Nicholas Piggin <npiggin@gmail.com>,
        Chris Metcalf <cmetcalf@mellanox.com>,
        Ulrich Obergfell <uobergfe@redhat.com>
Subject: Re: [patch 10/29] lockup_detector/perf: Prevent cpu hotplug
 deadlock
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References: <20170831071558.995235362@linutronix.de> <20170831073053.770526691@linutronix.de> <20170901190208.pn4vq25udylxehph@redhat.com>
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On Fri, 1 Sep 2017, Don Zickus wrote:
> On Thu, Aug 31, 2017 at 09:16:08AM +0200, Thomas Gleixner wrote:
> > The following deadlock is possible in the watchdog hotplug code:
> > 
> >   cpus_write_lock()
> >     ...
> >       takedown_cpu()
> >         smpboot_park_threads()
> >           smpboot_park_thread()
> >             kthread_park()
> >               ->park() := watchdog_disable()
> >                 watchdog_nmi_disable()
> >                   perf_event_release_kernel();
> >                     put_event()
> >                       _free_event()
> >                         ->destroy() := hw_perf_event_destroy()
> >                           x86_release_hardware()
> >                             release_ds_buffers()
> >                               get_online_cpus()
> > 
> > when a per cpu watchdog perf event is destroyed which drops the last
> > reference to the PMU hardware. The cleanup code there invokes
> > get_online_cpus() which instantly deadlocks because the hotplug percpu
> > rwsem is write locked.
> 
> The main reason perf_event_release_kernel is in this path is because the
> oprofile folks complained they couldn't use the perf counters when the
> nmi_watchdog was disabled on the command line.

If the nmi watchdog is disabled on the command line then there are no
counters claimed at all.

Thanks,

	tglx