From: Srivatsa S. Bhat Date: Mon, 5 May 2014 07:22:39 +0000 (+0530) Subject: cpufreq: Catch double invocations of cpufreq_freq_transition_begin/end X-Git-Url: https://git.stricted.de/?a=commitdiff_plain;h=ca654dc3a93d3b47dddc0c24a98043060bbb256b;p=GitHub%2Fexynos8895%2Fandroid_kernel_samsung_universal8895.git cpufreq: Catch double invocations of cpufreq_freq_transition_begin/end Some cpufreq drivers were redundantly invoking the _begin() and _end() APIs around frequency transitions, and this double invocation (one from the cpufreq core and the other from the cpufreq driver) used to result in a self-deadlock, leading to system hangs during boot. (The _begin() API makes contending callers wait until the previous invocation is complete. Hence, the cpufreq driver would end up waiting on itself!). Now all such drivers have been fixed, but debugging this issue was not very straight-forward (even lockdep didn't catch this). So let us add a debug infrastructure to the cpufreq core to catch such issues more easily in the future. We add a new field called 'transition_task' to the policy structure, to keep track of the task which is performing the frequency transition. Using this field, we make note of this task during _begin() and print a warning if we find a case where the same task is calling _begin() again, before completing the previous frequency transition using the corresponding _end(). We have left out ASYNC_NOTIFICATION drivers from this debug infrastructure for 2 reasons: 1. At the moment, we have no way to avoid a particular scenario where this debug infrastructure can emit false-positive warnings for such drivers. The scenario is depicted below: Task A Task B /* 1st freq transition */ Invoke _begin() { ... ... } Change the frequency /* 2nd freq transition */ Invoke _begin() { ... //waiting for B to ... //finish _end() for ... //the 1st transition ... | Got interrupt for successful ... | change of frequency (1st one). ... | ... | /* 1st freq transition */ ... | Invoke _end() { ... | ... ... V } ... ... } This scenario is actually deadlock-free because, once Task A changes the frequency, it is Task B's responsibility to invoke the corresponding _end() for the 1st frequency transition. Hence it is perfectly legal for Task A to go ahead and attempt another frequency transition in the meantime. (Of course it won't be able to proceed until Task B finishes the 1st _end(), but this doesn't cause a deadlock or a hang). The debug infrastructure cannot handle this scenario and will treat it as a deadlock and print a warning. To avoid this, we exclude such drivers from the purview of this code. 2. Luckily, we don't _need_ this infrastructure for ASYNC_NOTIFICATION drivers at all! The cpufreq core does not automatically invoke the _begin() and _end() APIs during frequency transitions in such drivers. Thus, the driver alone is responsible for invoking _begin()/_end() and hence there shouldn't be any conflicts which lead to double invocations. So, we can skip these drivers, since the probability that such drivers will hit this problem is extremely low, as outlined above. Signed-off-by: Srivatsa S. Bhat Acked-by: Viresh Kumar Signed-off-by: Rafael J. Wysocki --- diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c index a517da996aaf..bfe82b63875f 100644 --- a/drivers/cpufreq/cpufreq.c +++ b/drivers/cpufreq/cpufreq.c @@ -365,6 +365,18 @@ static void cpufreq_notify_post_transition(struct cpufreq_policy *policy, void cpufreq_freq_transition_begin(struct cpufreq_policy *policy, struct cpufreq_freqs *freqs) { + + /* + * Catch double invocations of _begin() which lead to self-deadlock. + * ASYNC_NOTIFICATION drivers are left out because the cpufreq core + * doesn't invoke _begin() on their behalf, and hence the chances of + * double invocations are very low. Moreover, there are scenarios + * where these checks can emit false-positive warnings in these + * drivers; so we avoid that by skipping them altogether. + */ + WARN_ON(!(cpufreq_driver->flags & CPUFREQ_ASYNC_NOTIFICATION) + && current == policy->transition_task); + wait: wait_event(policy->transition_wait, !policy->transition_ongoing); @@ -376,6 +388,7 @@ wait: } policy->transition_ongoing = true; + policy->transition_task = current; spin_unlock(&policy->transition_lock); @@ -392,6 +405,7 @@ void cpufreq_freq_transition_end(struct cpufreq_policy *policy, cpufreq_notify_post_transition(policy, freqs, transition_failed); policy->transition_ongoing = false; + policy->transition_task = NULL; wake_up(&policy->transition_wait); } diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h index 77a5fa191502..f3822f836e14 100644 --- a/include/linux/cpufreq.h +++ b/include/linux/cpufreq.h @@ -110,6 +110,7 @@ struct cpufreq_policy { bool transition_ongoing; /* Tracks transition status */ spinlock_t transition_lock; wait_queue_head_t transition_wait; + struct task_struct *transition_task; /* Task which is doing the transition */ }; /* Only for ACPI */