Commit a3933186 authored by Takashi Iwai's avatar Takashi Iwai
Browse files

ALSA: timer: Fix mutex deadlock at releasing card 



When a card is disconnected while in use, the system waits until all
opened files are closed then releases the card.  This is done via
put_device() of the card device in each device release code.

The recently reported mutex deadlock bug happens in this code path;
snd_timer_close() for the timer device deals with the global
register_mutex and it calls put_device() there.  When this timer
device is the last one, the card gets freed and it eventually calls
snd_timer_free(), which has again the protection with the global
register_mutex -- boom.

Basically put_device() call itself is race-free, so a relative simple
workaround is to move this put_device() call out of the mutex.  For
achieving that, in this patch, snd_timer_close_locked() got a new
argument to store the card device pointer in return, and each caller
invokes put_device() with the returned object after the mutex unlock.

Reported-and-tested-by: default avatarKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
parent 302d5a80

sound/core/timer.c

+17 −7
Original line number Diff line number Diff line
@@ -226,7 +226,8 @@ static int snd_timer_check_master(struct snd_timer_instance *master) 
	return 0;

}



static int snd_timer_close_locked(struct snd_timer_instance *timeri);

static int snd_timer_close_locked(struct snd_timer_instance *timeri,

				  struct device **card_devp_to_put);



/*

 * open a timer instance
@@ -238,6 +239,7 @@ int snd_timer_open(struct snd_timer_instance **ti, 
{

	struct snd_timer *timer;

	struct snd_timer_instance *timeri = NULL;

	struct device *card_dev_to_put = NULL;

	int err;



	mutex_lock(&register_mutex);
@@ -261,7 +263,7 @@ int snd_timer_open(struct snd_timer_instance **ti, 
		list_add_tail(&timeri->open_list, &snd_timer_slave_list);

		err = snd_timer_check_slave(timeri);

		if (err < 0) {

			snd_timer_close_locked(timeri);

			snd_timer_close_locked(timeri, &card_dev_to_put);

			timeri = NULL;

		}

		goto unlock;
@@ -313,7 +315,7 @@ int snd_timer_open(struct snd_timer_instance **ti, 
			timeri = NULL;



			if (timer->card)

				put_device(&timer->card->card_dev);

				card_dev_to_put = &timer->card->card_dev;

			module_put(timer->module);

			goto unlock;

		}
@@ -323,12 +325,15 @@ int snd_timer_open(struct snd_timer_instance **ti, 
	timer->num_instances++;

	err = snd_timer_check_master(timeri);

	if (err < 0) {

		snd_timer_close_locked(timeri);

		snd_timer_close_locked(timeri, &card_dev_to_put);

		timeri = NULL;

	}



 unlock:

	mutex_unlock(&register_mutex);

	/* put_device() is called after unlock for avoiding deadlock */

	if (card_dev_to_put)

		put_device(card_dev_to_put);

	*ti = timeri;

	return err;

}
@@ -338,7 +343,8 @@ EXPORT_SYMBOL(snd_timer_open); 
 * close a timer instance

 * call this with register_mutex down.

 */

static int snd_timer_close_locked(struct snd_timer_instance *timeri)

static int snd_timer_close_locked(struct snd_timer_instance *timeri,

				  struct device **card_devp_to_put)

{

	struct snd_timer *timer = timeri->timer;

	struct snd_timer_instance *slave, *tmp;
@@ -395,7 +401,7 @@ static int snd_timer_close_locked(struct snd_timer_instance *timeri) 
			timer->hw.close(timer);

		/* release a card refcount for safe disconnection */

		if (timer->card)

			put_device(&timer->card->card_dev);

			*card_devp_to_put = &timer->card->card_dev;

		module_put(timer->module);

	}
@@ -407,14 +413,18 @@ static int snd_timer_close_locked(struct snd_timer_instance *timeri) 
 */

int snd_timer_close(struct snd_timer_instance *timeri)

{

	struct device *card_dev_to_put = NULL;

	int err;



	if (snd_BUG_ON(!timeri))

		return -ENXIO;



	mutex_lock(&register_mutex);

	err = snd_timer_close_locked(timeri);

	err = snd_timer_close_locked(timeri, &card_dev_to_put);

	mutex_unlock(&register_mutex);

	/* put_device() is called after unlock for avoiding deadlock */

	if (card_dev_to_put)

		put_device(card_dev_to_put);

	return err;

}

EXPORT_SYMBOL(snd_timer_close);

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