Re: [PATCH V2 0/7] cpufreq: governors: Fix ABBA lockups

[Viresh Kumar]

On 04-02-16, 00:50, Rafael J. Wysocki wrote:
> This is exactly right.  We've avoided one deadlock only to trip into
> another one.
> 
> This happens because update_sampling_rate() acquires
> od_dbs_cdata.mutex which is held around cpufreq_governor_exit() by
> cpufreq_governor_dbs().
> 
> Worse yet, a deadlock can still happen without (the new)
> dbs_data->mutex, just between s_active and od_dbs_cdata.mutex if
> update_sampling_rate() runs in parallel with
> cpufreq_governor_dbs()->cpufreq_governor_exit() and the latter wins
> the race.
> 
> It looks like we need to drop the governor mutex before putting the
> kobject in cpufreq_governor_exit().

I have tried to explore all possible ways of fixing this, and every
other way looked to be racy in some way.

Does anyone else have a better idea (untested):

-------------------------8<-------------------------

Subject: [PATCH] cpufreq: ondemand: Shoot update_sampling_rate with a separate
 work

Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
---
 drivers/cpufreq/cpufreq_governor.h |  2 ++
 drivers/cpufreq/cpufreq_ondemand.c | 39 +++++++++++++++++++++++++++++---------
 2 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/drivers/cpufreq/cpufreq_governor.h b/drivers/cpufreq/cpufreq_governor.h
index 7bed63e14e7d..97e604356b20 100644
--- a/drivers/cpufreq/cpufreq_governor.h
+++ b/drivers/cpufreq/cpufreq_governor.h
@@ -141,6 +141,8 @@ struct od_dbs_tuners {
 	unsigned int powersave_bias;
 	unsigned int io_is_busy;
 	unsigned int min_sampling_rate;
+	struct work_struct work;
+	struct dbs_data *dbs_data;
 };
 
 struct cs_dbs_tuners {
diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
index 82ed490f7de0..93ad7a226aee 100644
--- a/drivers/cpufreq/cpufreq_ondemand.c
+++ b/drivers/cpufreq/cpufreq_ondemand.c
@@ -242,20 +242,27 @@ static struct common_dbs_data od_dbs_cdata;
  * reducing the sampling rate, we need to make the new value effective
  * immediately.
  */
-static void update_sampling_rate(struct dbs_data *dbs_data,
-		unsigned int new_rate)
+static void update_sampling_rate(struct work_struct *work)
 {
-	struct od_dbs_tuners *od_tuners = dbs_data->tuners;
+	struct od_dbs_tuners *od_tuners = container_of(work, struct
+						       od_dbs_tuners, work);
+	unsigned int new_rate = od_tuners->sampling_rate;
+	struct dbs_data *dbs_data = od_tuners->dbs_data;
 	struct cpumask cpumask;
 	int cpu;
 
-	od_tuners->sampling_rate = new_rate = max(new_rate,
-			od_tuners->min_sampling_rate);
-
 	/*
 	 * Lock governor so that governor start/stop can't execute in parallel.
+	 *
+	 * We can't do a regular mutex_lock() here, as that may deadlock against
+	 * another thread performing CPUFREQ_GOV_POLICY_EXIT event on the
+	 * governor, which might have already taken od_dbs_cdata.mutex and is
+	 * waiting for this work to finish.
 	 */
-	mutex_lock(&od_dbs_cdata.mutex);
+	if (!mutex_trylock(&od_dbs_cdata.mutex)) {
+		queue_work(system_wq, &od_tuners->work);
+		return;
+	}
 
 	cpumask_copy(&cpumask, cpu_online_mask);
 
@@ -311,13 +318,22 @@ static void update_sampling_rate(struct dbs_data *dbs_data,
 static ssize_t store_sampling_rate(struct dbs_data *dbs_data, const char *buf,
 		size_t count)
 {
+	struct od_dbs_tuners *od_tuners = dbs_data->tuners;
 	unsigned int input;
 	int ret;
 	ret = sscanf(buf, "%u", &input);
 	if (ret != 1)
 		return -EINVAL;
 
-	update_sampling_rate(dbs_data, input);
+	od_tuners->sampling_rate = max(input, od_tuners->min_sampling_rate);
+
+	/*
+	 * update_sampling_rate() requires to hold od_dbs_cdata.mutex, but we
+	 * can't take that from this thread, otherwise it results in ABBA
+	 * lockdep between s_active and od_dbs_cdata.mutex locks.
+	 */
+	queue_work(system_wq, &od_tuners->work);
+
 	return count;
 }
 
@@ -501,6 +517,8 @@ static int od_init(struct dbs_data *dbs_data, bool notify)
 	tuners->ignore_nice_load = 0;
 	tuners->powersave_bias = default_powersave_bias;
 	tuners->io_is_busy = should_io_be_busy();
+	INIT_WORK(&tuners->work, update_sampling_rate);
+	tuners->dbs_data = dbs_data;
 
 	dbs_data->tuners = tuners;
 	return 0;
@@ -508,7 +526,10 @@ static int od_init(struct dbs_data *dbs_data, bool notify)
 
 static void od_exit(struct dbs_data *dbs_data, bool notify)
 {
-	kfree(dbs_data->tuners);
+	struct od_dbs_tuners *tuners = dbs_data->tuners;
+
+	cancel_work_sync(&tuners->work);
+	kfree(tuners);
 }
 
 define_get_cpu_dbs_routines(od_cpu_dbs_info);