[PATCH 095/241] ALSA: usb-audio: Fix mutex deadlock at disconnection

[Herton Ronaldo Krzesinski]

3.5.7.2 -stable review patch.  If anyone has any objections, please let me know.

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

From: Takashi Iwai <ti...@suse.de>

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 <jsl...@suse.cz>
Signed-off-by: Takashi Iwai <ti...@suse.de>
Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesin...@canonical.com>
---
 sound/usb/card.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/sound/usb/card.c b/sound/usb/card.c
index 486b4a6..b3f5ad4 100644
--- a/sound/usb/card.c
+++ b/sound/usb/card.c
@@ -559,9 +559,11 @@ 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);
@@ -582,11 +584,9 @@ 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);
        }
 }
-- 
1.7.9.5

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