soc: qcom: msm_perf: Fix a race condition in hotplug callback

msm_performance relies on userspace to initialize cluster related
variables such as num_clusters, managed_cpus etc. Once num_clusters
is set the cluster related data structures are initialized and
userspace is allowed to set them.
However there could be a race where between num_clusters being set
and the cluster data structures being allocated, there is a hotplug
activity which would invoke the hotplug callback which in turn tries
to access managed_cpus. managed_cpus might not have been allocated by
that time resulting in a NULL pointer access in the callback causing
kernel panic.

Change-Id: Ia40af624322a89e0c0f9598bf7eea059e6969ebe
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>

drivers/soc/qcom/msm_performance.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014, The Linux Foundation. All rights reserved.
+ * Copyright (c) 2014-2015, The Linux Foundation. All rights reserved.
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 and
@@ -33,6 +33,7 @@ struct cpu_hp {
 	cpumask_var_t offlined_cpus;
 };
 static struct cpu_hp **managed_clusters;
+static bool clusters_inited;
 
 /* Work to evaluate the onlining/offlining CPUs */
 struct delayed_work evaluate_hotplug_work;
@@ -86,7 +87,7 @@ static int set_max_cpus(const char *buf, const struct kernel_param *kp)
 	const char *cp = buf;
 	int val;
 
-	if (!num_clusters)
+	if (!clusters_inited)
 		return -EINVAL;
 
 	while ((cp = strpbrk(cp + 1, ":")))
@@ -120,7 +121,7 @@ static int get_max_cpus(char *buf, const struct kernel_param *kp)
 {
 	int i, cnt = 0;
 
-	if (!num_clusters)
+	if (!clusters_inited)
 		return cnt;
 
 	for (i = 0; i < num_clusters; i++)
@@ -143,7 +144,7 @@ static int set_managed_cpus(const char *buf, const struct kernel_param *kp)
 	int i, ret;
 	struct cpumask tmp_mask;
 
-	if (!num_clusters)
+	if (!clusters_inited)
 		return -EINVAL;
 
 	ret = cpulist_parse(buf, &tmp_mask);
@@ -168,7 +169,7 @@ static int get_managed_cpus(char *buf, const struct kernel_param *kp)
 {
 	int i, cnt = 0;
 
-	if (!num_clusters)
+	if (!clusters_inited)
 		return cnt;
 
 	for (i = 0; i < num_clusters; i++) {
@@ -195,7 +196,7 @@ static int get_managed_online_cpus(char *buf, const struct kernel_param *kp)
 	struct cpumask tmp_mask;
 	struct cpu_hp *i_cpu_hp;
 
-	if (!num_clusters)
+	if (!clusters_inited)
 		return cnt;
 
 	for (i = 0; i < num_clusters; i++) {
@@ -498,6 +499,9 @@ static void __ref try_hotplug(struct cpu_hp *data)
 {
 	unsigned int i;
 
+	if (!clusters_inited)
+		return;
+
 	pr_debug("msm_perf: Trying hotplug...%d:%d\n",
 			num_online_managed(data->cpus),	num_online_cpus());
 
@@ -590,6 +594,9 @@ static int __ref msm_performance_cpu_callback(struct notifier_block *nfb,
 	unsigned int i;
 	struct cpu_hp *i_hp = NULL;
 
+	if (!clusters_inited)
+		return NOTIFY_OK;
+
 	for (i = 0; i < num_clusters; i++) {
 		if (cpumask_test_cpu(cpu, managed_clusters[i]->cpus)) {
 			i_hp = managed_clusters[i];
@@ -656,6 +663,7 @@ static int init_cluster_control(void)
 	INIT_DELAYED_WORK(&evaluate_hotplug_work, check_cluster_status);
 	mutex_init(&managed_cpus_lock);
 
+	clusters_inited = true;
 	return 0;
 }