sched: fix race conditions where HMP tunables change

When multiple threads race to update HMP scheduler tunables, at present,
the tunables which require big/small task count fix-up can be updated
without fix-up and it can trigger BUG_ON().
This happens because sched_hmp_proc_update_handler() acquires rq locks and
does fix-up only when number of big/small tasks affecting tunables are
updated even though the function sched_hmp_proc_update_handler() calls
set_hmp_defaults() which re-calculates all sysctl input data at that
point.  Consequently a thread that is trying to update a tunable which does
not affect big/small task count can call set_hmp_defaults() and update
big/small task count affecting tunable without fix-up if there is another
thread and it just set fix-up needed sysctl value.

Example of problem scenario :
thread 0                               thread 1
Set sched_small_task – needs fix up.
                                       Set sched_init_task_load – no fix
                                       up needed.
proc_dointvec_minmax() completed
which means sysctl_sched_small_task has
new value.
                                       Call set_hmp_defaults() without
                                       lock/fixup. set_hmp_defaults() still
                                       updates sched_small_tasks with new
                                       sysctl_sched_small_task value by
                                       thread 0.

Fix such issue by embracing proc update handler with already existing
policy mutex.

CRs-fixed: 812443
Change-Id: I7aa4c0efc1ca56e28dc0513480aca3264786d4f7
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>

1 files changed, 17 insertions, 11 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c1b7bf841fdf..c7d456ba3960 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3965,37 +3965,41 @@ int sched_hmp_proc_update_handler(struct ctl_table *table, int write,
 		loff_t *ppos)
 {
 	int ret;
+	unsigned int old_val;
 	unsigned int *data = (unsigned int *)table->data;
-	unsigned int old_val = *data;
 	int update_min_nice = 0;
 
+	mutex_lock(&policy_mutex);
+
+	old_val = *data;
+
 	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
 
 	if (ret || !write || !sched_enable_hmp)
-		return ret;
+		goto done;
 
 	if (write && (old_val == *data))
-		return 0;
+		goto done;
 
 	if (data == &sysctl_sched_min_runtime) {
 		sched_min_runtime = ((u64) sysctl_sched_min_runtime) * 1000;
-		return 0;
+		goto done;
 	}
 
-	if (data == (unsigned int *)&sysctl_sched_upmigrate_min_nice)
-		update_min_nice = 1;
-
-	if (update_min_nice) {
+	if (data == (unsigned int *)&sysctl_sched_upmigrate_min_nice) {
 		if ((*(int *)data) < -20 || (*(int *)data) > 19) {
 			*data = old_val;
-			return -EINVAL;
+			ret = -EINVAL;
+			goto done;
 		}
+		update_min_nice = 1;
 	} else {
 		/* all tunables other than min_nice are in percentage */
 		if (sysctl_sched_downmigrate_pct >
 		    sysctl_sched_upmigrate_pct || *data > 100) {
 			*data = old_val;
-			return -EINVAL;
+			ret = -EINVAL;
+			goto done;
 		}
 	}
 
@@ -4021,7 +4025,9 @@ int sched_hmp_proc_update_handler(struct ctl_table *table, int write,
 		put_online_cpus();
 	}
 
-	return 0;
+done:
+	mutex_unlock(&policy_mutex);
+	return ret;
 }
 
 /*