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@xxxxxxxxxxx> > >>>>>> > >>>>>> 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 _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
