[alsa-devel] [PATCH - pcm 1/1] pcm: fix "unable to create IPC shm instance" in some case

[Takashi Iwai]

On Fri, 04 Mar 2016 18:36:46 +0100,
bsiice wrote:
> 
> On 03/05/2016 12:37 AM, Takashi Iwai wrote:
> > On Fri, 04 Mar 2016 17:22:58 +0100,
> > bsiice wrote:
> >>
> >> On 03/03/2016 09:53 PM, Takashi Iwai wrote:
> >>> On Thu, 03 Mar 2016 13:40:42 +0100,
> >>> IceBsi wrote:
> >>>>
> >>>> From: Qing Cai <caiqing at 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:
> >>>>  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%.
> >>>> 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.
> >> I think this won't be a race condition. Since
> >> snd_pcm_direct_shm_create_or_connect() is protected by
> >> snd_pcm_direct_semaphore_down() and snd_pcm_direct_semaphore_up(),
> >> multiple processes won't do clear and initialization concurrently.
> > 
> > Hrm, OK, now understood the situation.
> > 
> >>> 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.
> >> I think the check of dmix->shmptr->magic!=SND_PCM_DIRECT_MAGIC is more
> >> robust. Assume there are two clients of shmem and shmem is initialized,
> >> if one of the clients exits, shm_nattach will become 1 and shmem may be
> >> initialized again.
> > 
> > Yeah, in the fork from a thread, the shm_nattch check doesn't work
> > reliably, indeed.
> > 
> >> In the current code, because there is semaphore, magic key check won't
> >> happen before initialization of shmem.
> >> In the scenario that I want to tell, there is only one process, and only
> >> one thread doning the clear and initialization of shmem, and the fork()
> >> is invoked in another thread of non alsa-lib context (i.e., the fork()
> >> code is not in alsa-lib). If the fork() is invoked before shmat(), the
> >> problem won't happen; If the fork() is invoked after the check of
> >> bus.shm_nattach=1, the problem won't happen too. Only if the fork() is
> >> invoked just after shmat() and just before the check of
> >> bus.shm_nattach=1, the problem happens.
> > 
> > Well, the reason why the magic key check was introduced was about the
> > safety.  Since the shmid is given explicitly, it might override some
> > shmem usages other than dmix accidentally.  With your patch, it
> > forcibly clears the region -- this is quite opposite to the intention 
> > of the magic key check.
> 
> I just think it clears the region only once at the first time. Could you
> please show me the scenario in detail?

The shmid ipc key is given explicitly by an alsa-lib configuration,
but there is no guarantee that this key has never been used by some
other programs for a completely different purpose.  So, for non-first
user, it verifies whether the attached region really belongs to
alsa-lib dmix.


Takashi