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. 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. thanks, Qing Cai _______________________________________________ Alsa-devel mailing list Alsa-devel@xxxxxxxxxxxxxxxx http://mailman.alsa-project.org/mailman/listinfo/alsa-devel
