cpufreq: Fix locking issues with governors

The locking around governors handling isn't adequate currently.

The list of governors should never be traversed without the locking
in place. Also governor modules must not be removed while the code
in them is still in use.

Reported-by: Quentin Perret <qperret@google.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: All applicable <stable@vger.kernel.org>
[ rjw: Changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/cpufreq/cpufreq.c

@@ -621,6 +621,24 @@ static struct cpufreq_governor *find_governor(const char *str_governor)
 	return NULL;
 }
 
+static struct cpufreq_governor *get_governor(const char *str_governor)
+{
+	struct cpufreq_governor *t;
+
+	mutex_lock(&cpufreq_governor_mutex);
+	t = find_governor(str_governor);
+	if (!t)
+		goto unlock;
+
+	if (!try_module_get(t->owner))
+		t = NULL;
+
+unlock:
+	mutex_unlock(&cpufreq_governor_mutex);
+
+	return t;
+}
+
 static unsigned int cpufreq_parse_policy(char *str_governor)
 {
 	if (!strncasecmp(str_governor, "performance", CPUFREQ_NAME_LEN))
@@ -640,28 +658,14 @@ static struct cpufreq_governor *cpufreq_parse_governor(char *str_governor)
 {
 	struct cpufreq_governor *t;
 
-	mutex_lock(&cpufreq_governor_mutex);
+	t = get_governor(str_governor);
+	if (t)
+		return t;
 
-	t = find_governor(str_governor);
-	if (!t) {
-		int ret;
+	if (request_module("cpufreq_%s", str_governor))
+		return NULL;
 
-		mutex_unlock(&cpufreq_governor_mutex);
-
-		ret = request_module("cpufreq_%s", str_governor);
-		if (ret)
-			return NULL;
-
-		mutex_lock(&cpufreq_governor_mutex);
-
-		t = find_governor(str_governor);
-	}
-	if (t && !try_module_get(t->owner))
-		t = NULL;
-
-	mutex_unlock(&cpufreq_governor_mutex);
-
-	return t;
+	return get_governor(str_governor);
 }
 
 /**
@@ -815,12 +819,14 @@ static ssize_t show_scaling_available_governors(struct cpufreq_policy *policy,
 		goto out;
 	}
 
+	mutex_lock(&cpufreq_governor_mutex);
 	for_each_governor(t) {
 		if (i >= (ssize_t) ((PAGE_SIZE / sizeof(char))
 		    - (CPUFREQ_NAME_LEN + 2)))
-			goto out;
+			break;
 		i += scnprintf(&buf[i], CPUFREQ_NAME_PLEN, "%s ", t->name);
 	}
+	mutex_unlock(&cpufreq_governor_mutex);
 out:
 	i += sprintf(&buf[i], "\n");
 	return i;
@@ -1058,15 +1064,17 @@ static int cpufreq_init_policy(struct cpufreq_policy *policy)
 	struct cpufreq_governor *def_gov = cpufreq_default_governor();
 	struct cpufreq_governor *gov = NULL;
 	unsigned int pol = CPUFREQ_POLICY_UNKNOWN;
+	int ret;
 
 	if (has_target()) {
 		/* Update policy governor to the one used before hotplug. */
-		gov = find_governor(policy->last_governor);
+		gov = get_governor(policy->last_governor);
 		if (gov) {
 			pr_debug("Restoring governor %s for cpu %d\n",
 				 policy->governor->name, policy->cpu);
 		} else if (def_gov) {
 			gov = def_gov;
+			__module_get(gov->owner);
 		} else {
 			return -ENODATA;
 		}
@@ -1089,7 +1097,11 @@ static int cpufreq_init_policy(struct cpufreq_policy *policy)
 			return -ENODATA;
 	}
 
-	return cpufreq_set_policy(policy, gov, pol);
+	ret = cpufreq_set_policy(policy, gov, pol);
+	if (gov)
+		module_put(gov->owner);
+
+	return ret;
 }
 
 static int cpufreq_add_policy_cpu(struct cpufreq_policy *policy, unsigned int cpu)