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Date: Mon, 28 Aug 2017 18:32:22 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Borislav Petkov <bp@alien8.de>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        lkml <linux-kernel@vger.kernel.org>
Subject: Re: WARNING: possible circular locking dependency detected
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On Mon, Aug 28, 2017 at 05:06:17PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 28, 2017 at 04:58:08PM +0200, Peter Zijlstra wrote:
> > On Fri, Aug 25, 2017 at 12:03:04PM +0200, Borislav Petkov wrote:
> > > Hey,
> > > 
> > > tglx says I have something for ya :-)
> > > 
> > > ======================================================
> > > WARNING: possible circular locking dependency detected
> > > 4.13.0-rc6+ #1 Not tainted
> > > ------------------------------------------------------
> > > watchdog/3/27 is trying to acquire lock:
> > >  (cpu_hotplug_lock.rw_sem){++++}, at: [<ffffffff8100c489>] release_ds_buffers+0x29/0xd0
> > > 
> > > but now in release context of a crosslock acquired at the following:
> > >  ((complete)&self->parked){+.+.}, at: [<ffffffff810895f6>] kthread_park+0x46/0x60
> > 
> > 
> > So I'm thinking this one is an actual deadlock.
> > 
> > So, as far as I can tell this ends up being:
> > 
> > CPU0                    CPU1
> > 
> > (smpboot_regiser_percpu_thread_cpumask)
> > 
> > get_online_cpus()
> > __smpboot_create_thread()
> >   kthread_park();
> >     wait_for_completion(&X)
> > 
> > 
> >                         (smpboot_thread_fn)
> > 
> >                         ->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()
> > 
> > 
> >                         kthread_parkme()
> >                           complete(&X)
> > 
> > 
> > 
> > So CPU0 holds cpus_hotplug_lock while wait_for_completion() and CPU1
> > needs to acquire before complete().
> > 
> > So if, in between, CPU2 does down_write(), things will get unstuck.
> > 
> > What's worse, there's also:
> > 
> >   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()
> > 
> > which as far as I can tell, spells instant deadlock..
> 
> Aah, but that latter will never happen.. because each CPU will have a
> &pmc_refcount and we can't unplug _all_ CPUs.
> 
> So the first one will only ever happen on boot, where we park() the very
> first watchdog thread and is a potential deadlock, but won't happen
> because nobody is around to do down_write() just yet.

I suspect however it is possible to interleave:

	sysctl.kernel.nmi_watchdog = 0
	  proc_watchdog_common()

with:

	hot un-plug
	 watchdog_disable()

to tickle that exact problem. Just needs a bit of luck.