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
