Re: [patch 10/29] lockup_detector/perf: Prevent cpu hotplug deadlock

[Thomas Gleixner]

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