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

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, 15 May 2008, Andrew Morton wrote:

> (switched to email.  Please respond via emailed reply-to-all, not via the
> bugzilla web interface).
> 
> On Thu, 15 May 2008 03:43:36 -0700 (PDT) bugme-daemon@bugzilla.kernel.org wrote:
> 
> > http://bugzilla.kernel.org/show_bug.cgi?id=10701
> > 
> >            Summary: snd_pcsp lockdep warning
> >            Product: Drivers
> >            Version: 2.5
> >      KernelVersion: 2.6.26-rc2
> >           Platform: All
> >         OS/Version: Linux
> >               Tree: Mainline
> >             Status: NEW
> >           Severity: normal
> >           Priority: P1
> >          Component: Sound(ALSA)
> >         AssignedTo: perex@perex.cz
> >         ReportedBy: nuclearcat@nuclearcat.com
> > 
> > 
> > Latest working kernel version: unknown
> > Earliest failing kernel version: 2.6.26-rc2
> > Distribution: Gentoo
> > Hardware Environment: Intel 965 chipset, Core 2 Duo
> > Software Environment: mplayer
> > Problem Description: Just lockdep warning in dmesg
> 
> This looks like a problem in the core hrtimers code?

No, this is a problem in the sound code. It uses the hrtimer callback
mode HRTIMER_CB_IRQSAFE. This mode was never intended to provide a
lock free call mode and was created for very fast callbacks like
wakeups and the like. Also I find it pretty amazing that the pscp code
has the following comment:

/*
 * We need the hrtimer_start as a tasklet to avoid
 * the nasty locking problem. :(
 * The problem:
 * - The timer handler is called with the cpu_base->lock
 *   already held by hrtimer code.
 * - snd_pcm_period_elapsed() takes the
 *   substream->self_group.lock.
 * So far so good.
 * But the snd_pcsp_trigger() is called with the
 * substream->self_group.lock held, and it calls
 * hrtimer_start(), which takes the cpu_base->lock.
 * You see the problem. We have the code pathes
 * which take two locks in a reverse order. This
 * can deadlock and the lock validator complains.
 * The only solution I could find was to move the
 * hrtimer_start() into a tasklet. -stsp
 */

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.

Thanks,
	tglx