pcm: fix 'unable to create IPC shm instance' caused by fork from a thread

pcm: fix 'unable to create IPC shm instance' caused by fork from a thread

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%.

More notes about this patch:
this patch tries to address the race above by changing the condition
to identify "the first user".  Until now, the first user was
identified by checking shm_nattch.  But this is racy, as stated in the
above.

In this version, we try to assign a shm at first without IPC_CREAT.
If this succeeds, we are not alone, so we must not be the first user.
Only when this fails, try to get a shmem with IPC_CREAT and IPC_EXCL.
If this succeeds, we are the first user.  And, one more notable point
is that the race of this function call itself is protected by
semaphore in the caller side.  The only point to avoid is the race
after shmget() and the first initialization, and this method should
work around that.

Signed-off-by: Qing Cai <bsiice@msn.com>
Signed-off-by: Qing Cai <caiqing@neusoft.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>