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.
I would be very happy if I could do a bit contribution to the open source world! :)
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:
- execute the following statement: pcm_direct.c:110: dmix->shmptr = shmat(dmix->shmid, 0, 0) (shm_nattch becomes 1)
- invoke fork() in some thread. (shm_nattch becomes 2)
- execute the following statement: pcm_direct.c:122: if (buf.shm_nattch == 1)
- 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)
- 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;