[alsa-devel] snd_pcsp locking mess

Stas Sergeev stsp at aknet.ru
Thu May 29 19:07:55 CEST 2008


Takashi Iwai wrote:
>> really be better if just snd_pcm_period_elapsed()
>> would raise a softirq. That's something
>> you proposed in the past IIRC.
>> Is this acceptable to have that change
>> not in the driver, or do you think it
>> is snd-pcsp-specific, and no other driver
>> will benefit?
> It would be good in general.
> However, better to be the next step after changing pcsp.
But why do you need both changes?
If you are going to change snd_pcm_period_elapsed()
itself, then how would the snd-pcsp
change be justified?

> One big disadvantage of the softirq is its (possible) timing
> inaccuracy.  (Note that this is about the normal non-preempt kernel.)
> The thing like pcsp driver does, flipping the bits at a high rate,
> isn't for softirq per se.
OK, I see.

> Well, it can be done in another away if you are so nervous about it.
> Simply add chip->timer_running atomic_t and reset it when the callback
> finished with HRTIMER_NORESTART.  In another side, wait until
> chip->timer_running becomes zero.
> But, it's just an uneeded addition of the code.
But for some reasons it looks safer
to me...

>>> Because the substream lock isn't necessary!  It just makes things more
>>> complicated -- if we take substream lock, AB/BA deadlocks occur again.
>> Even if the callback is called within
>> the softirq?
> The runtime callbacks, pointer and trigger, are called already inside they
> substream lock held by the PCM core.  Thus it doesn't matter whether
> it's in a hard or a softirq.
The difference is that in a softirq,
the hrtimer code doesn't take a lock.
The AB/BA problem was because of the
hrtimer' lock, which is not there
with the softirq callback. So what
exactly A and B do you mean here?

>> and yes, its nasty. But to me its nastiness
>> is only because it needs another lock to
>> protect the substream itself.
> No, you don't need to protect the substream at all.
But that was suggested by you in the
past. :) But adding a syncronization
point should be better I agree.
I'll see if some minimal change is
possible that will look sane to you
and wont make me ask 100 questions
about its safety at the same time. :)

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