PM / devfreq: Lock devfreq in trans_stat_show

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: 39688ce6 ("PM / devfreq: account suspend/resume for stats")
Cc: stable@vger.kernel.org
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Reviewed-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>

drivers/devfreq/devfreq.c

@@ -160,6 +160,7 @@ int devfreq_update_status(struct devfreq *devfreq, unsigned long freq)
 	int lev, prev_lev, ret = 0;
 	unsigned long cur_time;
 
+	lockdep_assert_held(&devfreq->lock);
 	cur_time = jiffies;
 
 	/* Immediately exit if previous_freq is not initialized yet. */
@@ -1397,12 +1398,17 @@ static ssize_t trans_stat_show(struct device *dev,
 	int i, j;
 	unsigned int max_state = devfreq->profile->max_state;
 
-	if (!devfreq->stop_polling &&
-			devfreq_update_status(devfreq, devfreq->previous_freq))
-		return 0;
 	if (max_state == 0)
 		return sprintf(buf, "Not Supported.\n");
 
+	mutex_lock(&devfreq->lock);
+	if (!devfreq->stop_polling &&
+			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, "           :");
 	for (i = 0; i < max_state; i++)