[PATCH V3 4/5] cpufreq: governor: Quit work-handlers early if governor is stopped

[Viresh Kumar]

cpufreq_governor_lock is abused by using it outside of cpufreq core,
i.e. in cpufreq-governors. But we didn't had a better solution to the
problem (described later) at that point of time, and following was the
only acceptable solution:

6f1e4efd882e ("cpufreq: Fix timer/workqueue corruption by protecting
reading governor_enabled")

The cpufreq governor core is fixed against possible races now and things
are in much better shape.

The original problem:

When a CPU is hot unplugged, we cancel delayed works for all
policy->cpus via gov_cancel_work(). If the work is already running on
any CPU, the workqueue code will wait for the work to finish, to prevent
the work items from re-queuing themselves.

This works most of the time, except for the case where the work handler
determines that it should adjust the delay for all other CPUs, that the
policy is managing. When this happens, the canceling CPU will cancel its
own work but can queue up the works on other policy->cpus.

For example, consider CPU 0-4 in a policy and we called
gov_cancel_work() for them. Workqueue core canceled the works for 0-3
and is waiting for the handler to finish on CPU4. At that time, handler
on CPU4 can restart works on CPU 0-3 again. Which makes 0-3 run works,
which the governor core thinks are canceled.

To fix that in a different (non-hacky) way, set set shared->policy to
false before trying to cancel the work. It should be updated within
timer_mutex, which will prevent the work-handlers to start. Once the
work-handlers finds that we are already trying to stop the governor, it
will exit early. And that will prevent queuing of works again as well.

Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
---
 drivers/cpufreq/cpufreq_governor.c | 33 +++++++++++++++++++++++----------
 1 file changed, 23 insertions(+), 10 deletions(-)

diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
index 750626d8fb03..931424ca96d9 100644
--- a/drivers/cpufreq/cpufreq_governor.c
+++ b/drivers/cpufreq/cpufreq_governor.c
@@ -171,10 +171,6 @@ void gov_queue_work(struct dbs_data *dbs_data, struct cpufreq_policy *policy,
 {
 	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.
@@ -188,9 +184,6 @@ void gov_queue_work(struct dbs_data *dbs_data, struct cpufreq_policy *policy,
 		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);
 
@@ -229,13 +222,24 @@ static void dbs_timer(struct work_struct *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;
 
@@ -252,6 +256,7 @@ static void dbs_timer(struct work_struct *work)
 	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);
 }
 
@@ -488,9 +493,17 @@ static int cpufreq_governor_stop(struct cpufreq_policy *policy,
 	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;
 }
-- 
2.4.0

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