[media] dvb_demux: fix deadlock in dmx_section_feed_release_filter()

dmx_section_feed_release_filter() locks dvbdmx->mutex and
if the feed is still filtering, it calls feed->stop_filtering(feed).
stop_filtering() is implemented by dmx_section_feed_stop_filtering()
that first of all try to lock the same mutex: dvbdmx->mutex.
That leads to a deadlock.
It does not happen often in practice because all callers of
release_filter() stop filtering by themselves.
So the problem can happen in case of race condition only.
The patch releases dvbdmx->mutex before call to feed->stop_filtering(feed)
and reacquires the mutex after that.
Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Michael Krufky <mkrufky@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>

drivers/media/dvb-core/dvb_demux.c

@@ -1032,8 +1032,13 @@ static int dmx_section_feed_release_filter(struct dmx_section_feed *feed,
 		return -EINVAL;
 	}
 
-	if (feed->is_filtering)
+	if (feed->is_filtering) {
+		/* release dvbdmx->mutex as far as 
+		   it is acquired by stop_filtering() itself */
+		mutex_unlock(&dvbdmx->mutex);
 		feed->stop_filtering(feed);
+		mutex_lock(&dvbdmx->mutex);
+	}
 
 	spin_lock_irq(&dvbdmx->lock);
 	f = dvbdmxfeed->filter;