Re: pcm_meter.c issue at s16_update

[Jaroslav Kysela <perex@perex.cz>]

Dne 09. 08. 20 v 9:05 Pavel Hofman napsal(a):
> Dne 03. 08. 20 v 12:48 Pavel Hofman napsal(a):
>>
>>
>> Dne 03. 08. 20 v 9:22 Jaroslav Kysela napsal(a):
>>> Dne 03. 08. 20 v 8:17 Takashi Iwai napsal(a):
>>>> On Sun, 02 Aug 2020 19:50:44 +0200,
>>>>>>
>>>>>> Optionally the second case could be handled just like the first
>>>>>> case by
>>>>>> resetting s16->old, assuming the boundary wrap occurs very
>>>>>> infrequently.
>>>>>
>>>>> The following patch is tested to work OK, no CPU peaks and no meter
>>>>> output glitches when the size < 0 condition occurs:
>>>>>
>>>>> diff --git a/src/pcm/pcm_meter.c b/src/pcm/pcm_meter.c
>>>>> index 20b41876..48df5945 100644
>>>>> --- a/src/pcm/pcm_meter.c
>>>>> +++ b/src/pcm/pcm_meter.c
>>>>> @@ -1098,8 +1098,15 @@ static void s16_update(snd_pcm_scope_t *scope)
>>>>>          snd_pcm_sframes_t size;
>>>>>          snd_pcm_uframes_t offset;
>>>>>          size = meter->now - s16->old;
>>>>> -       if (size < 0)
>>>>> -               size += spcm->boundary;
>>>>> +       if (size < 0) {
>>>>> +               /**
>>>>> +                * Application pointer adjusted for delay (meter->now)
>>>>> has dropped compared
>>>>> +                * to the previous update cycle. Either spcm->boundary
>>>>> wraparound, pcm rewinding,
>>>>> +                * or pcm restart without s16->old properly reset.
>>>>> +                * In any case the safest solution is skipping this
>>>>> conversion cycle.
>>>>> +                */
>>>>> +               size = 0;
>>>>> +       }
>>>>>          offset = s16->old % meter->buf_size;
>>>>>          while (size > 0) {
>>>>>                  snd_pcm_uframes_t frames = size;
>>>>>
>>>>>
>>>>>
>>>>> Please will you accept this (workaround) bugfix? If so, I would send a
>>>>> proper patch.
>>>>
>>>> It looks OK, at least this must be safe.
>>>> So yes, I'll happily apply if you submit a proper patch.
>>>
>>> It would be probably better to check against the boundary / 2 value to
>>> check
>>> correctly the boundary wrap instead to drop all negative size values:
>>>
>>>    if (size < 0) {
>>>       if (size < -(spcm->boundary / 2))
>>>          size += spcm->boundary;
>>>       else
>>>          size = 0;
>>>    }
>>
>> Is there a reliable way to detect the boundary wraparound, at best using
>> some dedicated API? I could find any, IMO the wraparound does not create
>> any notification. The check is OK for a rewind, half of boundary is
>> usually a very large number too. I am not sure what would happen at
>> reset when application pointer was already past the boundary half - see
>> below.

Yes, it's a good argument. In this case, the s16->old value is not properly
synced during the reset operation, otherwise the boundary / 2 threshold
(change limit) is sufficient to detect the boundary wrap.

>>> The "hidden" pcm restart referred in the comment should not occur,
>>> otherwise
>>> it's another bug somewhere.
>>
>> I do not know the exact moments when plugin API methods are called. The
>> fact is Takashi's suggestion to call s16 reset explicitely in
>> snd_pcm_meter_reset created this order:
>>
>> snd_pcm_meter_reset -> s16->reset
>> s16_update: meter->now 22751, s16->old 22751, size 0
>> s16_update: meter->now 839, s16->old 22751, size -21912
>>
>> I.e. AFTER resetting meter/s16 the variable meter->now was still at the
>> original large 22751 (with s16->old equal to its value due to
>> s16->reset). The value of meter->now was reset to 839 (= app pointer -
>> delay) only in the next call of s16_update (when s16->old was still the
>> previous old value => size < 0 => huge size => high CPU load).  From
>> this I kind of conclude that the reset is buggy. Maybe the reset code
>> should re-calculate meter->now = appl.pointer - delay before aligning
>> s16->old = meter->now.
>>
>> Nevertheless all this (except for the boundary wraparound) would result
>> in the same size = 0, thus skipping samples from the last cycle, just
>> like what the proposed patch does.
>>
>>
> 
> Please can we reach a decision and close the problem so that affected
> use cases do not have to be patched with the next the alsa-lib version?

I think that this problem should be fixed for reset and rewind separately. The
meter->reset should be set in snd_pcm_meter_reset() inside the running_mutex
lock to serialize correctly the update operations in the
snd_pcm_meter_thread(). And perhaps, we can follow this logic for the rewind.

I mean, we should ensure to call the s16->reset at the proper time to avoid
broken old/now combinations inside the scope "clients".

Your proposed solution is just a workaround.

						Jaroslav

> 
> Thanks a lot in advance,
> 
> Pavel.
> 


-- 
Jaroslav Kysela <perex@perex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.