Re: [Bugme-new] [Bug 10701] New: snd_pcsp lockdep warning
From: Andrew Morton
Date: Fri May 16 2008 - 13:40:28 EST
On Fri, 16 May 2008 14:46:31 +0200 (CEST) Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 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@xxxxxxxxxxxxxxxxxxx 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@xxxxxxxx
> > > ReportedBy: nuclearcat@xxxxxxxxxxxxxx
> > >
> > >
> > > 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.
You sound impressed! So what's the fix? HRTIMER_CB_SOFTIRQ?
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