cpufreq: governor: Quit work-handlers early if governor is stopped

gov_queue_work() acquires cpufreq_governor_lock to allow
cpufreq_governor_stop() to drain delayed work items possibly scheduled
on CPUs that share the policy with a CPU being taken offline.

However, the same goal may be achieved in a more straightforward way if
the policy pointer in the struct cpu_dbs_info matching the policy CPU is
reset upfront by cpufreq_governor_stop() under the timer_mutex belonging
to it and checked against NULL, under the same lock, at the beginning of
dbs_timer().

In that case every instance of dbs_timer() run for a struct cpu_dbs_info
sharing the policy pointer in question after cpufreq_governor_stop() has
started will notice that that pointer is NULL and bail out immediately
without queuing up any new work items. In turn, gov_cancel_work()
called by cpufreq_governor_stop() before destroying timer_mutex will
wait for all of the delayed work items currently running on the CPUs
sharing the policy to drop the mutex, so it may be destroyed safely.

Make cpufreq_governor_stop() and dbs_timer() work as described and
modify gov_queue_work() so it does not acquire cpufreq_governor_lock any
more.

Signed-off-by: Viresh Kumar <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/cpufreq/cpufreq_governor.c

···
 {
 	int i;
 
-	mutex_lock(&cpufreq_governor_lock);
-	if (!policy->governor_enabled)
-		goto out_unlock;
-
 	if (!all_cpus) {
 		/*
 		 * Use raw_smp_processor_id() to avoid preemptible warnings.
···
 		for_each_cpu(i, policy->cpus)
 			__gov_queue_work(i, dbs_data, delay);
 	}
-
-out_unlock:
-	mutex_unlock(&cpufreq_governor_lock);
 }
 EXPORT_SYMBOL_GPL(gov_queue_work);
 
···
 	struct cpu_dbs_info *cdbs = container_of(work, struct cpu_dbs_info,
 						 dwork.work);
 	struct cpu_common_dbs_info *shared = cdbs->shared;
-	struct cpufreq_policy *policy = shared->policy;
-	struct dbs_data *dbs_data = policy->governor_data;
+	struct cpufreq_policy *policy;
+	struct dbs_data *dbs_data;
 	unsigned int sampling_rate, delay;
 	bool modify_all = true;
 
 	mutex_lock(&shared->timer_mutex);
+
+	policy = shared->policy;
+
+	/*
+	 * Governor might already be disabled and there is no point continuing
+	 * with the work-handler.
+	 */
+	if (!policy)
+		goto unlock;
+
+	dbs_data = policy->governor_data;
 
 	if (dbs_data->cdata->governor == GOV_CONSERVATIVE) {
 		struct cs_dbs_tuners *cs_tuners = dbs_data->tuners;
···
 	delay = dbs_data->cdata->gov_dbs_timer(cdbs, dbs_data, modify_all);
 	gov_queue_work(dbs_data, policy, delay, modify_all);
 
+unlock:
 	mutex_unlock(&shared->timer_mutex);
 }
 
···
 	if (!shared || !shared->policy)
 		return -EBUSY;
 
+	/*
+	 * Work-handler must see this updated, as it should not proceed any
+	 * further after governor is disabled. And so timer_mutex is taken while
+	 * updating this value.
+	 */
+	mutex_lock(&shared->timer_mutex);
+	shared->policy = NULL;
+	mutex_unlock(&shared->timer_mutex);
+
 	gov_cancel_work(dbs_data, policy);
 
-	shared->policy = NULL;
 	mutex_destroy(&shared->timer_mutex);
 	return 0;
 }