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@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;
> >
>
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