[alsa-devel] [PATCH] pcm: Fix shm initialization race-condition

[Takashi Iwai]

On Mon, 22 Aug 2016 18:04:33 +0200,
Ismael Luceno wrote:
> 
> Easily seen when two threads try at the same time, one of them will fail.
> 
> The bug was identified by using apulse with Skype.
> 
> Fixes: dec428c35221 ("pcm: fix 'unable to create IPC shm instance' caused by fork from a thread")
> Fixes: https://github.com/i-rinat/apulse/issues/38
> Signed-off-by: Ismael Luceno <ismael at iodev.co.uk>

OK, I'll apply this one, as this change alone is fine.

But in general, this implies that the application hitting this is
buggy.  You can't reliably call snd_pcm_open() for the same PCM stream
concurrently via multi threads.  You have to introduce the proper
protection in the caller side.


thanks,

Takashi


> ---
>  src/pcm/pcm_direct.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
> index c3925cc20fd3..643498375b34 100644
> --- a/src/pcm/pcm_direct.c
> +++ b/src/pcm/pcm_direct.c
> @@ -96,11 +96,12 @@ int snd_pcm_direct_shm_create_or_connect(snd_pcm_direct_t *dmix)
>  retryget:
>  	dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
>  			     dmix->ipc_perm);
> -	if (dmix->shmid < 0) {
> -		if (errno == ENOENT)
> +	if (dmix->shmid < 0 && errno == ENOENT) {
>  		if ((dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
>  					     IPC_CREAT | IPC_EXCL | dmix->ipc_perm)) != -1)
>  			first_instance = 1;
> +		else if (errno == EEXIST)
> +			goto retryget;
>  	}
>  	err = -errno;
>  	if (dmix->shmid < 0) {
> -- 
> 2.9.2
>