ALSA: usb-audio: Fix mutex deadlock at disconnection

commit 10e44239f67d0b6fb74006e61a7e883b8075247a upstream.

The recent change for USB-audio disconnection race fixes introduced a
mutex deadlock again.  There is a circular dependency between
chip->shutdown_rwsem and pcm->open_mutex, depicted like below, when a
device is opened during the disconnection operation:

A. snd_usb_audio_disconnect() ->
     card.c::register_mutex ->
       chip->shutdown_rwsem (write) ->
         snd_card_disconnect() ->
           pcm.c::register_mutex ->
             pcm->open_mutex

B. snd_pcm_open() ->
     pcm->open_mutex ->
       snd_usb_pcm_open() ->
         chip->shutdown_rwsem (read)

Since the chip->shutdown_rwsem protection in the case A is required
only for turning on the chip->shutdown flag and it doesn't have to be
taken for the whole operation, we can reduce its window in
snd_usb_audio_disconnect().

Reported-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

1 files changed, 3 insertions, 3 deletions

diff --git a/sound/usb/card.c b/sound/usb/card.c
index 7003c131772..566acb31958 100644
--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -555,9 +555,11 @@ static void snd_usb_audio_disconnect(struct usb_device *dev,
 		return;
 
 	card = chip->card;
-	mutex_lock(&register_mutex);
 	down_write(&chip->shutdown_rwsem);
 	chip->shutdown = 1;
+	up_write(&chip->shutdown_rwsem);
+
+	mutex_lock(&register_mutex);
 	chip->num_interfaces--;
 	if (chip->num_interfaces <= 0) {
 		snd_card_disconnect(card);
@@ -574,11 +576,9 @@ static void snd_usb_audio_disconnect(struct usb_device *dev,
 			snd_usb_mixer_disconnect(p);
 		}
 		usb_chip[chip->index] = NULL;
-		up_write(&chip->shutdown_rwsem);
 		mutex_unlock(&register_mutex);
 		snd_card_free_when_closed(card);
 	} else {
-		up_write(&chip->shutdown_rwsem);
 		mutex_unlock(&register_mutex);
 	}
 }