[patch 2/3] cpufreq: Define dbs_mutex purpose and cleanup its usage

[venkatesh.pallipadi-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org]

[-- Attachment #1: 0002-cpufreq-Define-dbs_mutex-purpose-and-cleanup-its-us.patch --]
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Commit b14893a62c73af0eca414cfed505b8c09efc613c although it was very
much needed to cleanup ondemand timer cleanly, openup a can of worms
related to locking dependencies in cpufreq.

Patch here defines the need for dbs_mutex and cleans up its usage in
ondemand governor. This also resolves the lockdep warnings reported here

http://lkml.indiana.edu/hypermail/linux/kernel/0906.1/01925.html

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
---
 drivers/cpufreq/cpufreq_ondemand.c |   37 +++++++++++++++--------------------
 1 files changed, 16 insertions(+), 21 deletions(-)

diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
index 1911d17..b2d2106 100644
--- a/drivers/cpufreq/cpufreq_ondemand.c
+++ b/drivers/cpufreq/cpufreq_ondemand.c
@@ -78,15 +78,14 @@ static DEFINE_PER_CPU(struct cpu_dbs_info_s, cpu_dbs_info);
 static unsigned int dbs_enable;	/* number of CPUs using this policy */
 
 /*
- * DEADLOCK ALERT! There is a ordering requirement between cpu_hotplug
- * lock and dbs_mutex. cpu_hotplug lock should always be held before
- * dbs_mutex. If any function that can potentially take cpu_hotplug lock
- * (like __cpufreq_driver_target()) is being called with dbs_mutex taken, then
- * cpu_hotplug lock should be taken before that. Note that cpu_hotplug lock
- * is recursive for the same process. -Venki
- * DEADLOCK ALERT! (2) : do_dbs_timer() must not take the dbs_mutex, because it
- * would deadlock with cancel_delayed_work_sync(), which is needed for proper
- * raceless workqueue teardown.
+ * dbs_mutex protects data in dbs_tuners_ins from concurrent changes on
+ * different CPUs. It also serializes dbs_enable usage in CPUFREQ_GOV_START
+ * and CPUFREQ_GOV_STOP.
+ *
+ * dbs_mutex should be always held after lock_policy_rwsem whenever needed.
+ * do_dbs_timer() must not take the dbs_mutex, because it would deadlock
+ * with cancel_delayed_work_sync(), which is needed for proper raceless
+ * workqueue teardown.
  */
 static DEFINE_MUTEX(dbs_mutex);
 
@@ -240,12 +239,10 @@ static ssize_t store_sampling_rate(struct cpufreq_policy *unused,
 	unsigned int input;
 	int ret;
 	ret = sscanf(buf, "%u", &input);
+	if (ret != 1)
+		return -EINVAL;
 
 	mutex_lock(&dbs_mutex);
-	if (ret != 1) {
-		mutex_unlock(&dbs_mutex);
-		return -EINVAL;
-	}
 	dbs_tuners_ins.sampling_rate = max(input, min_sampling_rate);
 	mutex_unlock(&dbs_mutex);
 
@@ -258,14 +255,12 @@ static ssize_t store_up_threshold(struct cpufreq_policy *unused,
 	unsigned int input;
 	int ret;
 	ret = sscanf(buf, "%u", &input);
-
-	mutex_lock(&dbs_mutex);
 	if (ret != 1 || input > MAX_FREQUENCY_UP_THRESHOLD ||
 			input < MIN_FREQUENCY_UP_THRESHOLD) {
-		mutex_unlock(&dbs_mutex);
 		return -EINVAL;
 	}
 
+	mutex_lock(&dbs_mutex);
 	dbs_tuners_ins.up_threshold = input;
 	mutex_unlock(&dbs_mutex);
 
@@ -324,8 +319,8 @@ static ssize_t store_powersave_bias(struct cpufreq_policy *unused,
 
 	mutex_lock(&dbs_mutex);
 	dbs_tuners_ins.powersave_bias = input;
-	ondemand_powersave_bias_init();
 	mutex_unlock(&dbs_mutex);
+	ondemand_powersave_bias_init();
 
 	return count;
 }
@@ -598,14 +593,16 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
 				max(min_sampling_rate,
 				    latency * LATENCY_MULTIPLIER);
 		}
+		mutex_unlock(&dbs_mutex);
+
 		dbs_timer_init(this_dbs_info);
 
-		mutex_unlock(&dbs_mutex);
 		break;
 
 	case CPUFREQ_GOV_STOP:
-		mutex_lock(&dbs_mutex);
 		dbs_timer_exit(this_dbs_info);
+
+		mutex_lock(&dbs_mutex);
 		sysfs_remove_group(&policy->kobj, &dbs_attr_group);
 		dbs_enable--;
 		mutex_unlock(&dbs_mutex);
@@ -613,14 +610,12 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
 		break;
 
 	case CPUFREQ_GOV_LIMITS:
-		mutex_lock(&dbs_mutex);
 		if (policy->max < this_dbs_info->cur_policy->cur)
 			__cpufreq_driver_target(this_dbs_info->cur_policy,
 				policy->max, CPUFREQ_RELATION_H);
 		else if (policy->min > this_dbs_info->cur_policy->cur)
 			__cpufreq_driver_target(this_dbs_info->cur_policy,
 				policy->min, CPUFREQ_RELATION_L);
-		mutex_unlock(&dbs_mutex);
 		break;
 	}
 	return 0;
-- 
1.6.0.6

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