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

[Takashi Iwai]

On Sat, 05 Mar 2016 07:37:34 +0100,
bsiice wrote:
> 
> On 03/05/2016 02:08 AM, Takashi Iwai wrote:
> > 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
> > 
> 
> Thank you for explanation! Now I understood.
> I have tried changing ipc_key many times when I have been investigating
> the problem, and changing ipc_key doesn't solve the problem I
> encounterd.
> 
> I think there is more benefits to do magic key check instead of
> shm_nattch check, because the situation that I encounterd happens more
> often than the situation that other program has a same ipc key with
> alsa-lib.

Yes, possibly.  However, which may cause a severe problem?  It's the
question, too.

> The problem left is to make ipc key as unique as possible.
> Every program/library should fulfil the responsibility of generating a
> unique key. As stated on http://www.tldp.org/LDP/lpg/node24.html, ftok()
> should be used to generate a unique key.

This doesn't guarantee the uniqueness completely, obviously :)
In addition, we'd need more than 256 variants...


thanks,

Takashi