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