linux-kernel - Re: [RFC PATCH 15/19] cpufreq: remove useless usage of cpufreq_governor_mutex in __cpufreq

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Message-ID: <20160115163031.GU18603@e106622-lin>
Date:	Fri, 15 Jan 2016 16:30:31 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	peterz@...radead.org, rjw@...ysocki.net, mturquette@...libre.com,
	steve.muckle@...aro.org, vincent.guittot@...aro.org,
	morten.rasmussen@....com, dietmar.eggemann@....com
Subject: Re: [RFC PATCH 15/19] cpufreq: remove useless usage of
 cpufreq_governor_mutex in __cpufreq_governor

Hi,

On 12/01/16 16:36, Viresh Kumar wrote:
> On 11-01-16, 17:35, Juri Lelli wrote:
> > Commit 6f1e4efd882e ("cpufreq: Fix timer/workqueue corruption by
> > protecting reading governor_enabled") made policy->governor_enabled
> > guarded by cpufreq_governor_mutex in __cpufreq_governor. Now that
> > holding of policy->rwsem is asserted in __cpufreq_governor,
> > cpufreq_governor_mutex is overkilling.
> 
> I am sure that is going to break it. Try that x86, somehow I don't get
> it on my exynos boards.
> 

But governor_enabled seems to not be checked anymore outside cpufreq.c
(see also 01/19), as it was in the commit you are referring to. Now that
users of this should be holding policy->rwsem, so that should suffice
for protecting governor_enabled, as governor_enabled is only changed
inside here.

I run some test on a x86 box I setup and didn't see anything related to
this. I'll wait to get the first 0-day report anyway.

> > -	mutex_lock(&cpufreq_governor_mutex);
> >  	if ((policy->governor_enabled && event == CPUFREQ_GOV_START)
> >  	    || (!policy->governor_enabled
> >  	    && (event == CPUFREQ_GOV_LIMITS || event == CPUFREQ_GOV_STOP))) {
> > -		mutex_unlock(&cpufreq_governor_mutex);
> >  		return -EBUSY;
> >  	}
> 
> Actually the above checks should also be removed as the governors are
> responsible for maintaining their state machines. But
> userspace/powersave/performance don't have that support yet and so
> these checks save them from going into undefined states.
> 
> Over that, above and below checks are incomplete..
> 

You mean we need an additional patch that extends the checks performed?

Thanks,

- Juri

> > @@ -2006,8 +2004,6 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
> >  	else if (event == CPUFREQ_GOV_START)
> >  		policy->governor_enabled = true;
> >  
> > -	mutex_unlock(&cpufreq_governor_mutex);
> > -
> >  	ret = policy->governor->governor(policy, event);
> >  
> >  	if (!ret) {
> > @@ -2017,12 +2013,10 @@ static int __cpufreq_governor(struct cpufreq_policy *policy,
> >  			policy->governor->initialized--;
> >  	} else {
> >  		/* Restore original values */
> > -		mutex_lock(&cpufreq_governor_mutex);
> >  		if (event == CPUFREQ_GOV_STOP)
> >  			policy->governor_enabled = true;
> >  		else if (event == CPUFREQ_GOV_START)
> >  			policy->governor_enabled = false;
> > -		mutex_unlock(&cpufreq_governor_mutex);
> >  	}
> >  
> >  	if (((event == CPUFREQ_GOV_POLICY_INIT) && ret) ||
> > -- 
> > 2.2.2
> 
> -- 
> viresh
>