[PATCH 5.15.y] ALSA: seq: Fix port lock leak in deliver_to_subscribers()

[Tzung-Bi Shih <tzungbi@kernel.org>]

Commit 46397622a3fa ("ALSA: seq: Add UMP support") mistakenly removed
the `snd_seq_port_unlock(src_port)` call in deliver_to_subscribers().
As a result, every time an event is delivered to a subscriber, the
source port's `use_lock` atomic counter is increased but never
decreased.

The leak prevents ALSA sequencer ports from closing cleanly.  When the
client process exits or destroys the port, port_delete() hangs forever
at snd_use_lock_sync().  The process becomes stuck in an unkillable D
state, triggering the following warning:

    ALSA: seq_lock: waiting [1 left] in sound/core/seq/seq_ports.c:250

Restore the missing `snd_seq_port_unlock(src_port)`.

Note that commit 177ccf811df4 ("ALSA: seq: Support MIDI 2.0 UMP Endpoint
port") incidentally resolved this issue upstream when it refactored the
locking logic into a wrapper function.  However, since the commit
introduces a new feature and might not be suitable for stable release
branches, this targeted fix is necessary.

Fixes: 6389f2c13531 ("ALSA: seq: Add UMP support")
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
---
 sound/core/seq/seq_clientmgr.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/sound/core/seq/seq_clientmgr.c b/sound/core/seq/seq_clientmgr.c
index f2feeb97765c..bd2d7a9fab5b 100644
--- a/sound/core/seq/seq_clientmgr.c
+++ b/sound/core/seq/seq_clientmgr.c
@@ -733,6 +733,7 @@ static int deliver_to_subscribers(struct snd_seq_client *client,
 		read_unlock(&grp->list_lock);
 	else
 		up_read(&grp->list_mutex);
+	snd_seq_port_unlock(src_port);
 	memcpy(event, &event_saved, saved_size);
 	return (result < 0) ? result : num_ev;
 }
-- 
2.55.0.979.g7e5102b832-goog