Messages in this thread | | | Date | Thu, 15 Jan 2009 12:26:40 +0100 | From | Ingo Molnar <> | Subject | Re: WARNING: at kernel/sched.c:4440 sub_preempt_count+0x81/0x95() |
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* Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> On Thursday 15 January 2009 21:00:13 Ingo Molnar wrote: > > * Nick Piggin <nickpiggin@yahoo.com.au> wrote: > > > On Tuesday 13 January 2009 23:34:25 Ingo Molnar wrote: > > > > * Michal Hocko <mhocko@suse.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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