Re: [Bugme-new] [Bug 10701] New: snd_pcsp lockdep warning

[Takashi Iwai]

At Sat, 17 May 2008 17:24:22 +0200 (CEST),
Thomas Gleixner wrote:
> 
> On Sat, 17 May 2008, Takashi Iwai wrote:
> > At Fri, 16 May 2008 21:32:49 +0200 (CEST),
> > Thomas Gleixner wrote:
> > > 
> > > On Fri, 16 May 2008, Andrew Morton wrote:
> > > > > So the author was well aware of locking problem and the whole code is
> > > > > just a stupid hack around the problem without solving it. This code is
> > > > > not at all suited for HRTIMER_CB_IRQSAFE.
> > > > 
> > > > You sound impressed!  So what's the fix?  HRTIMER_CB_SOFTIRQ?
> > > 
> > > Yeah, impressed by creativity. HRTIMER_CB_SOFTIRQ should be the right
> > > thing.
> > 
> > Well, it'd be basically a similar way like snd-pcsp currently does ("a
> > stupid hack" :)  But, it's good to have a fix, anyway, since this
> > sounds like a generic problem with a callback in a spinlock.  If the
> > callback requires another own lock, this can easily lead to a AB/BA
> > deadlock.  Actually, ALSA PCM core had sometimes similar problems,
> > too.
> 
> HRTIMER_CB_SOFTIRQ is not a stupid hack :) It's the default for
> hrtimers and it does not hold any locks when calling the callback. We
> really want to avoid tons of callbacks in the timer interrupt itself.

Yes, this would be an easy workaround for snd-pcsp problem, I guess.

A better fix would be to make snd-pcsp hrtimer calback just change the
pc-speaker port (0x61 and 0x42) and update the position.  These should
be really fast path.  Then, occasionally invoke a tasklet that will
call snd_pcm_period_elapsed(), which could be a (relatively) slow
path and lead to a spinlock mess.


thanks,

Takashi
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