Commit 10e44239 authored Nov 13, 2012 by Takashi Iwai

ALSA: usb-audio: Fix mutex deadlock at disconnection



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>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

parent effded75

sound/usb/card.c

+3 −3

Original line number	Diff line number	Diff line
		@@ -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);
		}
		}

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