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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>From 1fd707552a67adf869958e479910d2f70452351b Mon Sep 17 00:00:00 2001
From: Jan Kara <jack@suse.cz>
Date: Mon, 24 Sep 2012 16:17:16 +0200
Subject: [PATCH] lib: Fix corruption of denominator in flexible proportions

When racing with CPU hotplug, percpu_counter_sum() can return negative
values for the number of observed events. This confuses fprop_new_period(),
which uses unsigned type and as a result number of events is set to big
*positive* number. From that moment on, things go pear shaped and can result
e.g. in division by zero as denominator is later truncated to 32-bits.

Fix the issue by using a signed type in fprop_new_period(). That makes us
bail out from the function without doing anything (mistakenly) thinking
there are no events to age. That makes aging somewhat inaccurate but getting
accurate data would be rather hard.

Reported-by: Borislav Petkov <bp@amd64.org>
Reported-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 lib/flex_proportions.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/lib/flex_proportions.c b/lib/flex_proportions.c
index c785554..ebf3bac 100644
--- a/lib/flex_proportions.c
+++ b/lib/flex_proportions.c
@@ -62,7 +62,7 @@ void fprop_global_destroy(struct fprop_global *p)
  */
 bool fprop_new_period(struct fprop_global *p, int periods)
 {
-	u64 events;
+	s64 events;
 	unsigned long flags;
 
 	local_irq_save(flags);
-- 
1.7.1