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Subject[PATCH 3.16 110/136] ALSA: timer: Fix mutex deadlock at releasing card
3.16.80-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Takashi Iwai <tiwai@suse.de>

commit a39331867335d4a94b6165e306265c9e24aca073 upstream.

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: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
sound/core/timer.c | 24 +++++++++++++++++-------
1 file changed, 17 insertions(+), 7 deletions(-)

--- a/sound/core/timer.c
+++ b/sound/core/timer.c
@@ -241,7 +241,8 @@ static int snd_timer_check_master(struct
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
@@ -253,6 +254,7 @@ int snd_timer_open(struct snd_timer_inst
{
struct snd_timer *timer;
struct snd_timer_instance *timeri = NULL;
+ struct device *card_dev_to_put = NULL;
int err;

mutex_lock(&register_mutex);
@@ -276,7 +278,7 @@ int snd_timer_open(struct snd_timer_inst
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;
@@ -328,7 +330,7 @@ int snd_timer_open(struct snd_timer_inst
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;
}
@@ -338,12 +340,15 @@ int snd_timer_open(struct snd_timer_inst
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;
}
@@ -352,7 +357,8 @@ int snd_timer_open(struct snd_timer_inst
* 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 = NULL;
struct snd_timer_instance *slave, *tmp;
@@ -404,7 +410,7 @@ static int snd_timer_close_locked(struct
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);
}

@@ -416,14 +422,18 @@ static int snd_timer_close_locked(struct
*/
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;
}

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