Re: [alsa-devel] [PATCH] ALSA: Fix double locking of card list in snd_card_register()

At Wed, 3 Jun 2009 20:43:29 +0100, Mark Brown wrote:

The introduction of snd_card_set_id() added a lock on the card list to the old choose_default_id() function when using it to implement the new API call. This lock is needed to allow us to walk the list and check to see if our new name is a duplicate. Unfortunately this causes a lockup when called from snd_card_register() (in cases where no ID is supplied for the card) since the card list is already locked there.

Oh, thanks for catching this. Obviously I haven't tested the latest code enough...

Fix this fairly hideously by factoring out the implementation and using a flag to indicate if the lock should be held. A better fix would probably be to refactor snd_card_register() to move the _set_id() outside the locking region but I can't immediately see anything I can convince myself is safe.

Agreed, there can be a cleaner solution but your fix is definitely safe. So I applied it as a quick fix.

Takashi

Signed-off-by: Mark Brown broonie@opensource.wolfsonmicro.com

This patch isn't terribly nice but I figured it was a more useful report than just the deadlock.

sound/core/init.c | 31 +++++++++++++++++++------------ 1 files changed, 19 insertions(+), 12 deletions(-)

diff --git a/sound/core/init.c b/sound/core/init.c index 6557dd8..a578d05 100644 --- a/sound/core/init.c +++ b/sound/core/init.c @@ -476,15 +476,8 @@ int snd_card_free(struct snd_card *card)

EXPORT_SYMBOL(snd_card_free);

-/**

• • snd_card_set_id - set card identification name
• • @card: soundcard structure
• • @nid: new identification string
• • This function sets the card identification and checks for name
• • collisions.
• */

-void snd_card_set_id(struct snd_card *card, const char *nid) +static void snd_card_set_id_internal(struct snd_card *card, const char *nid,

• 		     int do_locking)
  

{ int i, len, idx_flag = 0, loops = SNDRV_CARDS; const char *spos, *src; @@ -529,14 +522,16 @@ void snd_card_set_id(struct snd_card *card, const char *nid) } if (!snd_info_check_reserved_words(id)) goto __change;

• mutex_lock(&snd_card_mutex);
  

• if (do_locking)
  

• 	mutex_lock(&snd_card_mutex);
  

  for (i = 0; i < snd_ecards_limit; i++) { if (snd_cards[i] && !strcmp(snd_cards[i]->id, id)) { mutex_unlock(&snd_card_mutex); goto __change; } }

• mutex_unlock(&snd_card_mutex);
  

• if (do_locking)
  

• 	mutex_unlock(&snd_card_mutex);
  

  break;

  __change:
  

@@ -561,6 +556,18 @@ void snd_card_set_id(struct snd_card *card, const char *nid) } }

+/**

• • snd_card_set_id - set card identification name
• • @card: soundcard structure
• • @nid: new identification string
• • This function sets the card identification and checks for name
• • collisions.
• */

+void snd_card_set_id(struct snd_card *card, const char *nid) +{

• snd_card_set_id_internal(card, nid, 1);

+} EXPORT_SYMBOL(snd_card_set_id);

#ifndef CONFIG_SYSFS_DEPRECATED @@ -657,7 +664,7 @@ int snd_card_register(struct snd_card *card) return 0; } if (card->id[0] == '\0')

• snd_card_set_id(card, NULL);
  

• snd_card_set_id_internal(card, NULL, 0);
  

  snd_cards[card->number] = card; mutex_unlock(&snd_card_mutex); init_info_for_card(card);

-- 1.6.3.1