linux-kernel - Re: divide error: bdi_dirty

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Message-ID: <20120924200737.GA30997@quack.suse.cz>
Date:	Mon, 24 Sep 2012 22:07:37 +0200
From:	Jan Kara <jack@...e.cz>
To:	Borislav Petkov <bp@...64.org>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Conny Seidel <conny.seidel@....com>, Jan Kara <jack@...e.cz>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Fengguang Wu <fengguang.wu@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <jweiner@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: divide error: bdi_dirty_limit+0x5a/0x9e

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.

								Honza
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