[BUG] v4.11-rc1: CPUFREQ Circular locking dependency

[Russell King - ARM Linux]

On Fri, Mar 10, 2017 at 06:33:28PM +0000, Matthew Wilcox wrote:
> From cd5401d81633d5e48e39d67d4e65156e6759537e Mon Sep 17 00:00:00 2001
> From: Matthew Wilcox <mawilcox at microsoft.com>
> Date: Fri, 10 Mar 2017 13:22:53 -0500
> Subject: [PATCH] thermal: Fix potential deadlock in cpu_cooling
> 
> I expanded the scope of cooling_list_lock a little too far; it was
> not just covering cpufreq_dev_count, it was also covering the calls
> to cpufreq_register_notifier() and cpufreq_unregister_notifier().
> Since cooling_list_lock is also used within cpufreq_thermal_notifier(),
> lockdep reports a potential deadlock.  I don't think that's actually
> possible, but it's easy enough to make it impossible by testing the
> condition under cooling_list_lock and dropping the lock before calling
> cpufreq_register_notifier().
> 
> As a bonus, I noticed that cpufreq_dev_count is only used for the purpose
> of knowing whether this is the first or last cooling device registered,
> and we know that anyway because we know whether the list transitioned
> between empty and not-empty.  So we can delete that variable too.
> 
> Fixes: ae606089621ef0349402cfcbeca33a82abbd0fd0
> Reported-by: Russell King <linux at armlinux.org.uk>
> Signed-off-by: Matthew Wilcox <mawilcox at microsoft.com>

Thanks Matthew, appears to solve the problem.

Tested-by: Russell King <rmk+kernel at armlinux.org.uk>

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