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