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

[Jaroslav Kysela]

On Wed, 3 Jun 2009, Takashi Iwai wrote:

> 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...

Yes, thanks.

>> 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.

The patch has a unlock problem in 'if (snd_cards[i] && 
!strcmp(snd_cards[i]->id, id)) {' block.

Anyway, here is next cleanup which will remove locking outside the 
internal id function and fixes also next possible race in unique id 
allocation:

http://git.alsa-project.org/?p=alsa-kernel.git;a=commitdiff;h=13d55b5cb27a3c35d0fb2a48e1ee10ec89488569

 						Jaroslav

-----
Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project, Red Hat, Inc.