[alsa-devel] [PATCH - pcm 1/1] pcm: fix 'unable to create IPC shm instance' caused by fork from a thread

[Takashi Iwai]

On Thu, 10 Mar 2016 13:50:24 +0100,
bsiice wrote:
> 
> Hi Takashi Iwai:
> 
> Is this patch OK?
> 
> I have automatically tested 422 times after this fix, and
> 'unable to create IPC shm instance' doesn't happen, and so the multimedia
> application functions properly.

The patch looks good, and working as far as I've tested, so merged to
git tree now.  I added a few more changelog texts to explain what
exactly this patch does.

> I would be very happy if I could do a bit contribution to the open source
> world! :)

Glad to have a really nice improvement, too.  Thanks!


Takashi

> 
> 
> On 03/10/2016 08:40 PM, IceBsi wrote:
> > From: Qing Cai <bsiice at msn.com>
> > 
> > As stated in manpage SHMCTL(2), shm_nattch is "No. of current attaches"
> > (i.e., number of processes attached to the shared memeory). If an
> > application uses alsa-lib and invokes fork() from a thread of the
> > application, there may be the following execution sequence:
> >  1. execute the following statement:
> >       pcm_direct.c:110: dmix->shmptr = shmat(dmix->shmid, 0, 0)
> >     (shm_nattch becomes 1)
> >  2. invoke fork() in some thread.
> >     (shm_nattch becomes 2)
> >  3. execute the following statement:
> >       pcm_direct.c:122: if (buf.shm_nattch == 1)
> >  4. execute the following statement:
> >       pcm_direct.c:131: if (dmix->shmptr->magic != SND_PCM_DIRECT_MAGIC)
> >     (As stated in manpage SHMGET(2), "When a new shared memory segment
> >      is created, its contents are initialized to zero values", so
> >      dmix->shmptr->magic is 0)
> >  5. execute the following statements:
> >       pcm_direct.c:132: snd_pcm_direct_shm_discard(dmix)
> >       pcm_direct.c:133: return -EINVAL
> > The above execution sequence will cause the following error:
> >   unable to create IPC shm instance
> > This error causes multimedia application has no sound. This error rarely
> > occurs, probability is about 1%.
> > 
> > Signed-off-by: Qing Cai <bsiice at msn.com>
> > Signed-off-by: Qing Cai <caiqing at neusoft.com>
> > 
> > diff --git a/src/pcm/pcm_direct.c b/src/pcm/pcm_direct.c
> > index fd3877c..14de734 100644
> > --- a/src/pcm/pcm_direct.c
> > +++ b/src/pcm/pcm_direct.c
> > @@ -91,13 +91,19 @@ int snd_pcm_direct_semaphore_create_or_connect(snd_pcm_direct_t *dmix)
> >  int snd_pcm_direct_shm_create_or_connect(snd_pcm_direct_t *dmix)
> >  {
> >  	struct shmid_ds buf;
> > -	int tmpid, err;
> > +	int tmpid, err, first_instance = 0;
> >  	
> >  retryget:
> >  	dmix->shmid = shmget(dmix->ipc_key, sizeof(snd_pcm_direct_share_t),
> > -			     IPC_CREAT | dmix->ipc_perm);
> > +			     dmix->ipc_perm);
> > +	if (dmix->shmid < 0) {
> > +		if (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;
> > +	}
> >  	err = -errno;
> > -	if (dmix->shmid < 0){
> > +	if (dmix->shmid < 0) {
> >  		if (errno == EINVAL)
> >  		if ((tmpid = shmget(dmix->ipc_key, 0, dmix->ipc_perm)) != -1)
> >  		if (!shmctl(tmpid, IPC_STAT, &buf))
> > @@ -119,7 +125,7 @@ retryget:
> >  		snd_pcm_direct_shm_discard(dmix);
> >  		return err;
> >  	}
> > -	if (buf.shm_nattch == 1) {	/* we're the first user, clear the segment */
> > +	if (first_instance) {	/* we're the first user, clear the segment */
> >  		memset(dmix->shmptr, 0, sizeof(snd_pcm_direct_share_t));
> >  		if (dmix->ipc_gid >= 0) {
> >  			buf.shm_perm.gid = dmix->ipc_gid;
> > 
>