[PATCH 3.16 056/148] PM / devfreq: Lock devfreq in trans_stat_show

[Ben Hutchings]

3.16.82-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Leonard Crestez <leonard.crestez@xxxxxxx>

commit 2abb0d5268ae7b5ddf82099b1f8d5aa8414637d4 upstream.

There is no locking in this sysfs show function so stats printing can
race with a devfreq_update_status called as part of freq switching or
with initialization.

Also add an assert in devfreq_update_status to make it clear that lock
must be held by caller.

Fixes: 39688ce6facd ("PM / devfreq: account suspend/resume for stats")
Signed-off-by: Leonard Crestez <leonard.crestez@xxxxxxx>
Reviewed-by: Matthias Kaehlcke <mka@xxxxxxxxxxxx>
Reviewed-by: Chanwoo Choi <cw00.choi@xxxxxxxxxxx>
Signed-off-by: Chanwoo Choi <cw00.choi@xxxxxxxxxxx>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/drivers/devfreq/devfreq.c
+++ b/drivers/devfreq/devfreq.c
@@ -95,6 +95,7 @@ static int devfreq_update_status(struct
 	int lev, prev_lev, ret = 0;
 	unsigned long cur_time;
 
+	lockdep_assert_held(&devfreq->lock);
 	cur_time = jiffies;
 
 	prev_lev = devfreq_get_freq_level(devfreq, devfreq->previous_freq);
@@ -1054,9 +1055,13 @@ static ssize_t trans_stat_show(struct de
 	int i, j;
 	unsigned int max_state = devfreq->profile->max_state;
 
+	mutex_lock(&devfreq->lock);
 	if (!devfreq->stop_polling &&
-			devfreq_update_status(devfreq, devfreq->previous_freq))
+			devfreq_update_status(devfreq, devfreq->previous_freq)) {
+		mutex_unlock(&devfreq->lock);
 		return 0;
+	}
+	mutex_unlock(&devfreq->lock);
 
 	len = sprintf(buf, "   From  :   To\n");
 	len += sprintf(buf + len, "         :");