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Date: Thu, 31 Aug 2017 09:08:05 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
cc: Borislav Petkov <bp@alien8.de>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: WARNING: possible circular locking dependency detected
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On Wed, 30 Aug 2017, Peter Zijlstra wrote:
> On offline it basically does perf_event_disable() for all CPU context
> events, and then adds HOTPLUG_OFFSET (-32) to arrive at: OFF +
> HOTPLUG_OFFSET = -33.
> 
> That's smaller than ERROR and thus perf_event_enable() no-ops on events
> for offline CPUs (maybe we should try and plumb an error return for
> IOC_ENABLE).
> 
> On online we subtract the HOTPLUG_OFFSET again and the event becomes a
> regular OFF, after which perf_event_enable() should work again.

I haven't come around to test that as I was busy cleaning up the unholy
mess in the watchdog code.

One other thing I stumbled over is:

    perf_event_create()
      ....
      x86_hw_reserve(event)

      if (__x86_pmu_event_init(event) < 0)
      	    event->destroy(event);
	    	x86_hw_release()
		    ....
		    cpus_read_lock();

If that happens from a hotplug function, we are doomed.

I mean, that particular watchdog event won't fail if the watchdog code
would verify that already at init time (which it does soon), but in general
event creation during hotplug is dangerous.

Thanks,

	tglx