On Thu, 03 Mar 2016 13:40:42 +0100, IceBsi wrote:
From: Qing Cai caiqing@neusoft.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() at some point, there should 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%. Because the first user of the shared memory will get that dmix->shmptr->magic is 0, check dmix->shmptr->magic's value to determine if "we're the first user" is OK. Tests have been made 400+ times after this fix, and the issue no longer exists.
I think this is still racy. Multiple users can grab the shmem at the very same time. Maybe it looks as if working just because both users behavior as the first user and do clear and initialize.
The check of bus.shm_nattach=1 should be fine, per se. The problem is the magic key check of the secondary. In the current code, as you pointed out, this may happen before the first client finishes the initialization.
One option would be to use some lock like pthread mutex. Another option would be to spin for a while until it gets a non-zero value. I guess the latter is more suitable for dmix & co.
Thoughts?
thanks,
Takashi
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..93d6f3c 100644 --- a/src/pcm/pcm_direct.c +++ b/src/pcm/pcm_direct.c @@ -119,7 +119,7 @@ retryget: snd_pcm_direct_shm_discard(dmix); return err; }
- if (buf.shm_nattch == 1) { /* we're the first user, clear the segment */
- if (dmix->shmptr->magic != SND_PCM_DIRECT_MAGIC) { /* 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;
-- 2.7.2