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@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@msn.com Signed-off-by: Qing Cai caiqing@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;