sched: Fix deadlock between cpu hotplug and upmigrate change

There is a circular dependency between cpu_hotplug.lock and
HMP scheduler policy mutex. Prevent this by enforcing the
same lock order.

Here CPU0 and CPU4 are governed by different cpufreq policies.

----------------                        --------------------
    CPU 0                                          CPU 4
---------------                         --------------------

proc_sys_call_handler()                 cpu_up()

                                        --> acquired cpu_hotplug.lock

sched_hmp_proc_update_handler()         cpufreq_cpu_callback()

--> acquired policy_mutex

                                        cpufreq_governor_interactive()

get_online_cpus()                       sched_set_window()

--> waiting for cpu_hotplug.lock        --> waiting for policy_mutex

Change-Id: I39efc394f4f00815b72adc975021fdb16fe6e30a
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>

1 files changed, 19 insertions, 8 deletions

diff --git a/kernel/sched/hmp.c b/kernel/sched/hmp.c
index a8bf39c6d7d7..ab5587309760 100644
--- a/kernel/sched/hmp.c
+++ b/kernel/sched/hmp.c
@@ -1470,7 +1470,20 @@ int sched_hmp_proc_update_handler(struct ctl_table *table, int write,
 	int ret;
 	unsigned int old_val;
 	unsigned int *data = (unsigned int *)table->data;
-	int update_min_nice = 0;
+	int update_task_count = 0;
+
+	if (!sched_enable_hmp)
+		return 0;
+
+	/*
+	 * The policy mutex is acquired with cpu_hotplug.lock
+	 * held from cpu_up()->cpufreq_governor_interactive()->
+	 * sched_set_window(). So enforce the same order here.
+	 */
+	if (write && (data == &sysctl_sched_upmigrate_pct)) {
+		update_task_count = 1;
+		get_online_cpus();
+	}
 
 	mutex_lock(&policy_mutex);
 
@@ -1478,7 +1491,7 @@ int sched_hmp_proc_update_handler(struct ctl_table *table, int write,
 
 	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
 
-	if (ret || !write || !sched_enable_hmp)
+	if (ret || !write)
 		goto done;
 
 	if (write && (old_val == *data))
@@ -1500,20 +1513,18 @@ int sched_hmp_proc_update_handler(struct ctl_table *table, int write,
 	 * includes taking runqueue lock of all online cpus and re-initiatizing
 	 * their big counter values based on changed criteria.
 	 */
-	if ((data == &sysctl_sched_upmigrate_pct || update_min_nice)) {
-		get_online_cpus();
+	if (update_task_count)
 		pre_big_task_count_change(cpu_online_mask);
-	}
 
 	set_hmp_defaults();
 
-	if ((data == &sysctl_sched_upmigrate_pct || update_min_nice)) {
+	if (update_task_count)
 		post_big_task_count_change(cpu_online_mask);
-		put_online_cpus();
-	}
 
 done:
 	mutex_unlock(&policy_mutex);
+	if (update_task_count)
+		put_online_cpus();
 	return ret;
 }