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Date: Mon, 28 Aug 2017 16:58:08 +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 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..