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

[Thomas Gleixner <tglx@linutronix.de>]

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.

Thanks,
	tglx