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Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference counting

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On Mon, Feb 15, 2016 at 02:36:43PM +0200, Joonas Lahtinen wrote:
> Instead of implementing a custom locked reference counting, use lockref.
> 
> Current implementation leads to a deadlock splat on Intel SKL platforms
> when lockdep debugging is enabled.
> 
> This is due to few of CPUfreq drivers (including Intel P-state) having this;
> policy->rwsem is locked during driver initialization and the functions called
> during init that actually apply CPU limits use get_online_cpus (because they
> have other calling paths too), which will briefly lock cpu_hotplug.lock to
> increase cpu_hotplug.refcount.
> 
> On later calling path, when doing a suspend, when cpu_hotplug_begin() is called
> in disable_nonboot_cpus(), callbacks to CPUfreq functions get called after,
> which will lock policy->rwsem and cpu_hotplug.lock is already held by
> cpu_hotplug_begin() and we do have a potential deadlock scenario reported by
> our CI system (though it is a very unlikely one). See the Bugzilla link for more
> details.

I've been meaning to change the thing into a percpu-rwsem, I just
haven't had time to look into the lockdep splat that generated.

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[PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference counting Joonas Lahtinen <joonas.lahtinen@linux.intel.com> - 2016-02-15 13:40 +0100
  Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference  counting Peter Zijlstra <peterz@infradead.org> - 2016-02-15 15:20 +0100
    Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference  counting Peter Zijlstra <peterz@infradead.org> - 2016-02-15 18:10 +0100
      Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference  counting Joonas Lahtinen <joonas.lahtinen@linux.intel.com> - 2016-02-16 09:50 +0100
        Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference  counting Peter Zijlstra <peterz@infradead.org> - 2016-02-16 10:20 +0100
          Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference  counting Joonas Lahtinen <joonas.lahtinen@linux.intel.com> - 2016-02-16 12:00 +0100
            Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference  counting Peter Zijlstra <peterz@infradead.org> - 2016-02-16 12:10 +0100
              Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference  counting Joonas Lahtinen <joonas.lahtinen@linux.intel.com> - 2016-02-17 13:50 +0100
                Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference  counting Peter Zijlstra <peterz@infradead.org> - 2016-02-17 15:30 +0100
                Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference  counting Peter Zijlstra <peterz@infradead.org> - 2016-02-17 17:20 +0100
                Re: [Intel-gfx] [PATCH] [RFC] kernel/cpu: Use lockref for online CPU  reference counting Daniel Vetter <daniel@ffwll.ch> - 2016-02-17 17:40 +0100
                Re: [Intel-gfx] [PATCH] [RFC] kernel/cpu: Use lockref for online CPU  reference counting Peter Zijlstra <peterz@infradead.org> - 2016-02-17 17:40 +0100
                Re: [Intel-gfx] [PATCH] [RFC] kernel/cpu: Use lockref for online  CPU reference counting Joonas Lahtinen <joonas.lahtinen@linux.intel.com> - 2016-02-18 11:40 +0100
                Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference  counting Daniel Vetter <daniel@ffwll.ch> - 2016-02-17 17:20 +0100
      Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference  counting Joonas Lahtinen <joonas.lahtinen@linux.intel.com> - 2016-02-18 12:00 +0100
    Re: [Intel-gfx] [PATCH] [RFC] kernel/cpu: Use lockref for online CPU  reference counting Daniel Vetter <daniel@ffwll.ch> - 2016-02-15 18:20 +0100

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