Message-ID: <20090115112640.GF22850@elte.hu>
Date:	Thu, 15 Jan 2009 12:26:40 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Michal Hocko <mhocko@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: WARNING: at kernel/sched.c:4440 sub_preempt_count+0x81/0x95()


* Nick Piggin <nickpiggin@...oo.com.au> wrote:

> On Thursday 15 January 2009 21:00:13 Ingo Molnar wrote:
> > * Nick Piggin <nickpiggin@...oo.com.au> wrote:
> > > On Tuesday 13 January 2009 23:34:25 Ingo Molnar wrote:
> > > > * Michal Hocko <mhocko@...e.cz> wrote:
> > > > > Hi,
> > > > > I am not sure whether someone has already reported this, but
> > > > > I can see the following early boot WARNING with the 2.6.29-rc1 kernel
> > > > > in the serial console output:
> > > > >
> > > > > ------------[ cut here ]------------
> > > > > WARNING: at kernel/sched.c:4440 sub_preempt_count+0x81/0x95()
> > > >
> > > > That one should be fixed in tip/master and it is in the to-Linus queue
> > > > of fixes:
> > > >
> > > >   http://people.redhat.com/mingo/tip.git/README
> > > >
> > > > it's this commit:
> > > >
> > > >   01e3eb8: Revert "sched: improve preempt debugging"
> > > >
> > > > if you want to cherry-pick the fix.
> > >
> > > OK, but I still don't think this is the actual problem, but there is
> > > something amiss in the init code being exposed by it.
> >
> > the warnings triggered after a softirq, and there's already preempt-leak
> > checks in the softirq code - so we can exclude that.
> >
> > a hardirq might have leaked a preempt count - but that would have quite
> > bad effects [with quick atomic check asserts in schedule()], wouldnt it?
> > So i tend to think that this is a false positive.
> >
> > One problem i can think of (and which i outlined in the revert commit log)
> > is that if a hardirq hits this window in lock_kernel():
> >
> > void __lockfunc lock_kernel(void)
> > {
> >         int depth = current->lock_depth+1;
> >                                            <-------------- HERE
> 
> current->lock_depth is not yet modified at this point.
> 
> >         if (likely(!depth))
> >                 __lock_kernel();
> 
> And here we increment preempt_count when taking the BKL, but we
> has not yet modified current->lock_depth, so preempt debugging
> will proceed with no impact of my patch at this point, regardless
> of what interrupts are taken.
> 
> So there is a race window, but it is to err on the safe side and
> not trigger a false report.
> 
> >         current->lock_depth = depth;
> > }
> 
> At this point, the BKL addition to preempt_count will be taken into
> account in underflow checking of subsequent preempt counters.

indeed, and the unlock path looks good too.

> > then we have kernel_locked() already true (it checks lock_depth), but 
> > the preempt count is not elevated yet via __lock_kernel(). So if we 
> > return from the hardirq [and run into softirqs that end with a 
> > preempt_enable() - a pure hardirq exit has no preempt debug check] 
> > we'll incorrectly think that there's a preempt leak going on.
> 
> I don't think so. And anyway BKL is taken very early in boot and not 
> released for a long time, so I don't think it is possible for these 
> kinds of races to trigger here anyway.
> 
> So I really think it still is a bug or some other misunderstanding we 
> have. I'd prefer to try to fix it or at least discover why it is 
> happening.

yes - but the thing was reported a month ago, it went then upstream and 
re-reported and re-bisected - so time ran out to debug it and we reverted 
it. We can re-apply it if you have a good theory about what happened.

	Ingo
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