Re: [RFC v3] cpufreq: Make sure frequency transitions are serialized

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 03/20/2014 02:07 PM, Viresh Kumar wrote:
> On 20 March 2014 14:02, Srivatsa S. Bhat
> <srivatsa.bhat@linux.vnet.ibm.com> wrote:
>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>> index 199b52b..5283f10 100644
>> --- a/drivers/cpufreq/cpufreq.c
>> +++ b/drivers/cpufreq/cpufreq.c
>> @@ -349,6 +349,39 @@ void cpufreq_notify_post_transition(struct cpufreq_policy *policy,
>>  EXPORT_SYMBOL_GPL(cpufreq_notify_post_transition);
>>
>>
>> +void cpufreq_freq_transition_begin(struct cpufreq_policy *policy,
>> +               struct cpufreq_freqs *freqs, unsigned int state)
>> +{
>> +wait:
>> +       wait_event(&policy->transition_wait, !policy->transition_ongoing);
>> +
>> +       mutex_lock(&policy->transition_lock);
>> +
>> +       if (policy->transition_ongoing) {
>> +               mutex_unlock(&policy->transition_lock);
>> +               goto wait;
>> +       }
>> +
>> +       policy->transition_ongoing = true;
>> +
>> +       mutex_unlock(&policy->transition_lock);
>> +
>> +       cpufreq_notify_transition(policy, freqs, CPUFREQ_PRECHANGE);
>> +}
>> +
>> +void cpufreq_freq_transition_end(struct cpufreq_policy *policy,
>> +               struct cpufreq_freqs *freqs, unsigned int state)
>> +{
>> +       cpufreq_notify_transition(policy, freqs, CPUFREQ_POSTCHANGE);
>> +
>> +       mutex_lock(&policy->transition_lock);
> 
> Why do we need locking here? You explained that earlier :)
> 

Hmm.. I had thought of some complex race condition which would make
tasks miss the wake-up event and sleep forever, and hence added
the locking there to prevent that. But now that I think more closely,
I'm not able to recall that race... I will give some more thought to
it and if I can't find any loopholes in doing the second update to
the ongoing flag without locks, then I'll post the patchset with
that lockless version itself.

> Also, I would like to add this here:
> 
>     WARN_ON(policy->transition_ongoing);
>

Hmm? Won't it always be true? We are the ones who set that flag to
true earlier, right? I guess you meant WARN_ON(!policy->transition_ongoing)
perhaps? I'm not sure whether its really worth it, because it kinda looks
obvious. Not sure what kind of bugs it would catch. I can't think of any
such scenario :-(
 
>> +       policy->transition_ongoing = false;
>> +       mutex_unlock(&policy->transition_lock);
>> +
>> +       wake_up(&policy->transition_wait);
>> +}
> 

Regards,
Srivatsa S. Bhat