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

[Zhang Rui]

On Fri, 2017-03-10 at 23:43 +0000, Russell King - ARM Linux wrote:
> 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>
> 

patch applied.

thanks,
rui