1 files changed, 12 insertions, 0 deletions

diff --git a/drivers/cpufreq/cpufreq_conservative.c b/drivers/cpufreq/cpufreq_conservative.c
index e07a35487bde..8878a154ed43 100644
--- a/drivers/cpufreq/cpufreq_conservative.c
+++ b/drivers/cpufreq/cpufreq_conservative.c
@@ -72,6 +72,14 @@ 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
+ */
 static DEFINE_MUTEX     (dbs_mutex);
 static DECLARE_WORK     (dbs_work, do_dbs_timer, NULL);
 
@@ -414,12 +422,14 @@ static void dbs_check_cpu(int cpu)
 static void do_dbs_timer(void *data)
 { 
         int i;
+        lock_cpu_hotplug();
         mutex_lock(&dbs_mutex);
         for_each_online_cpu(i)
                 dbs_check_cpu(i);
         schedule_delayed_work(&dbs_work, 
                         usecs_to_jiffies(dbs_tuners_ins.sampling_rate));
         mutex_unlock(&dbs_mutex);
+        unlock_cpu_hotplug();
 } 
 
 static inline void dbs_timer_init(void)
@@ -514,6 +524,7 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
                 break;
 
         case CPUFREQ_GOV_LIMITS:
+                lock_cpu_hotplug();
                 mutex_lock(&dbs_mutex);
                 if (policy->max < this_dbs_info->cur_policy->cur)
                         __cpufreq_driver_target(
@@ -524,6 +535,7 @@ static int cpufreq_governor_dbs(struct cpufreq_policy *policy,
                                         this_dbs_info->cur_policy,
                                         policy->min, CPUFREQ_RELATION_L);
                 mutex_unlock(&dbs_mutex);
+                unlock_cpu_hotplug();
                 break;
         }
         return 0;