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>
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@ -559,9 +559,11 @@ static void snd_usb_audio_disconnect(struct usb_device *dev,
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return;
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card = chip->card;
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mutex_lock(®ister_mutex);
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down_write(&chip->shutdown_rwsem);
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chip->shutdown = 1;
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up_write(&chip->shutdown_rwsem);
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mutex_lock(®ister_mutex);
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chip->num_interfaces--;
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if (chip->num_interfaces <= 0) {
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snd_card_disconnect(card);
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@ -582,11 +584,9 @@ static void snd_usb_audio_disconnect(struct usb_device *dev,
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snd_usb_mixer_disconnect(p);
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}
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usb_chip[chip->index] = NULL;
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up_write(&chip->shutdown_rwsem);
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mutex_unlock(®ister_mutex);
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snd_card_free_when_closed(card);
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} else {
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up_write(&chip->shutdown_rwsem);
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mutex_unlock(®ister_mutex);
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}
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}
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