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


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