From mboxrd@z Thu Jan  1 00:00:00 1970
From: Russell King - ARM Linux <linux@arm.linux.org.uk>
Subject: Re: 3.18: lockdep problems in cpufreq
Date: Mon, 15 Dec 2014 17:43:36 +0000
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To: Yadwinder Singh Brar <yadi.brar@samsung.com>
Cc: 'Viresh Kumar' <viresh.kumar@linaro.org>, "'Rafael J. Wysocki'" <rjw@rjwysocki.net>, linux-arm-kernel@lists.infradead.org, linux-pm@vger.kernel.org, 'Eduardo Valentin' <edubezval@gmail.com>

On Mon, Dec 15, 2014 at 08:24:05PM +0530, Yadwinder Singh Brar wrote:
> > The answer may well be to have finer grained locking here:
> > 
> > - one lock to protect cpufreq_dev_list, which is only ever taken when
> >   adding or removing entries from it
> > 
> > - a second lock to protect cpufreq_dev_count and the calls to
> >   cpufreq_register_notifier() and cpufreq_unregister_notifier()
> > 
> > and you would /never/ take either of those two locks inside each other.
> > In other words:
> > 
> > 	mutex_lock(&cooling_list_lock);
> > 	list_add(&cpufreq_dev->node, &cpufreq_dev_list);
> > 	mutex_unlock(&cooling_list_lock);
> > 
> > 	mutex_lock(&cooling_cpufreq_lock);
> > 	if (cpufreq_dev_count++ == 0)
> > 		cpufreq_register_notifier(&thermal_cpufreq_notifier_block,
> > 					  CPUFREQ_POLICY_NOTIFIER);
> > 	mutex_unlock(&cooling_cpufreq_lock);
> > 
> > and similar in the cleanup path.  The notifier itself would only ever
> > take the cooling_list_lock.
> > 
> 
> I agree with this approach, if its fine for others also, I can implement
> and post patch.

Yes, please do.

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