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

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Date:	Mon, 15 Feb 2016 15:17:55 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>
Cc:	Intel graphics driver community testing & development 
	<intel-gfx@...ts.freedesktop.org>,
	Linux kernel development <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	David Hildenbrand <dahi@...ux.vnet.ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
	Chris Wilson <chris@...is-wilson.co.uk>
Subject: Re: [PATCH] [RFC] kernel/cpu: Use lockref for online CPU reference
 counting

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.