Re: divide error: bdi_dirty_limit+0x5a/0x9e

[Jan Kara <jack@suse.cz>]

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On Mon 24-09-12 22:07:37, Jan Kara wrote:
> On Mon 24-09-12 21:31:35, Borislav Petkov wrote:
> > On Tue, Sep 25, 2012 at 12:18:46AM +0530, Srivatsa S. Bhat wrote:
> > > >> Sure thing.
> > > >> Out of ~25 runs I only triggered it once, without the patch the
> > > >> trigger-rate is higher.
> > > >>
> > > >> [   55.098249] Broke affinity for irq 81
> > > >> [   55.105108] smpboot: CPU 1 is now offline
> > > >> [   55.311216] smpboot: Booting Node 0 Processor 1 APIC 0x11
> > > >> [   55.333022] LVT offset 0 assigned for vector 0x400
> > > >> [   55.545877] smpboot: CPU 2 is now offline
> > > >> [   55.753050] smpboot: Booting Node 0 Processor 2 APIC 0x12
> > > >> [   55.775582] LVT offset 0 assigned for vector 0x400
> > > >> [   55.986747] smpboot: CPU 3 is now offline
> > > >> [   56.193839] smpboot: Booting Node 0 Processor 3 APIC 0x13
> > > >> [   56.212643] LVT offset 0 assigned for vector 0x400
> > > >> [   56.423201] Got negative events: -25
> > > > 
> > > > I see it:
> > > > 
> > > > __percpu_counter_sum does for_each_online_cpu without doing
> > > > get/put_online_cpus().
> > > > 
> > > 
> > > Maybe I'm missing something, but that doesn't immediately tell me
> > > what's the exact source of the bug.. Note that there is a hotplug
> > > callback percpu_counter_hotcpu_callback() that takes the same
> > > fbc->lock before updating/resetting the percpu counters of offline
> > > CPU. So, though the synchronization is a bit weird, I don't
> > > immediately see a problematic race condition there.
> > 
> > Well, those oopses both happen when a cpu comes online.
> > 
> > According to when percpu_counter_hotcpu_callback is run (at CPU_DEAD)
> > then those percpu variables should have correctly updated values.
> > 
> > So there has to be some other case where we read garbage which is a
> > negative value - otherwise we wouldn't be seeing the debug output.
> > 
> > For example, look at the log output above: we bring down cpu 3 just to
> > bring it right back online. So there has to be something fishy along
> > that codepath...
>   Well, I think the race happens when a CPU is dying and we call
> percpu_counter_sum() after it is marked offline but before callbacks are
> run. percpu_counter_sum() then does not add died CPU's counter in the sum
> and thus total can go negative. If get/put_online_cpus() fixes this race,
> I'd be happy.
> 
>   OTOH in theory, percpu_counter_sum() can return negative values even
> without CPU hotplug when percpu_counter_sum() races with cpu local
> operations. It cannot happen with the current flexible proportion code
> but I think making the code more robust is a good idea. I'll send a patch
> for this. Still fixing the percpu counters would be nice as these races
> could cause random errors to computed proportions and that's bad for
> writeback.
  In the attachment is a fix. Fengguang, can you please merge it? Thanks!

								Honza

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