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

[Takashi Iwai]

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 at 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
>