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

[bsiice]

On 03/05/2016 03:36 PM, Takashi Iwai wrote:
> 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.

By 'severe problem', do you mean that alsa-lib may clear other program's
shmem? If the key is unique, will the 'severe problem' still happen?

> 
>> 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...

Oh, sorry, I thought it would be unique if all program/library use their
own full filename as arg to ftok(). Actually this doesn't guarantee
uniqueness. :)

What if alsa-lib changed to use POSIX shmem (i.e. shm_open()...)?


thanks,

Qing Cai

> 
> 
> thanks,
> 
> Takashi
> 
>