Re: [RFC PATCH 15/19] cpufreq: remove useless usage of cpufreq_governor_mutex in __cpufreq_governor

From: Juri Lelli
Date: Fri Jan 15 2016 - 11:30:23 EST


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
>