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@xxxxxxx>
>
> 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@xxxxxxx>
> Signed-off-by: Qing Cai <caiqing@xxxxxxxxxxx>
>
> 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;
>
_______________________________________________
Alsa-devel mailing list
Alsa-devel@xxxxxxxxxxxxxxxx
http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
