Re: A regression in recent 3.2 kernel: bdi_dirty_limit() divideerror

[Peter Zijlstra]

On Sun, 2012-01-08 at 10:33 +0800, Wu Fengguang wrote:
> On Sat, Jan 07, 2012 at 05:35:25PM +0100, Peter Zijlstra wrote:
> > On Sat, 2012-01-07 at 22:56 +0800, Wu Fengguang wrote:
> > > Subject: 
> > > Date: Sat Jan 07 22:50:45 CST 2012
> > > 
> > > The uninitilized shift may lead to denominator=0 in
> > > prop_fraction_percpu() and divide error in bdi_dirty_limit().
> > 
> > I'm not seeing how, only proc_change_shift() can change ->index, and it
> > does that after it writes ->pg[index]->shift.
> 
> Then I lose the clue why bdi_dirty_limit() will divide error at all.

You and me both, the weird thing is, this code hasn't been changes like
forever and I can't recall any such weirdness. 

In fact, prop_fraction_percpu() sets the denominator to period_2 +
(global_count & counter_mask). 

The only way to make that 0 is to overflow the unsigned long.. did the
crash happen on 32bit -- I never saw the initial report?

But even then, we limit PROP_MAX_SHIFT to 3*BITS_PER_LONG/4, I don't
think that could ever overflow.

> prop_change_shift() does
> 
>         change ->pg[index]->shift
>         smp_wmb()
>         change ->index
> 
> Will the read side prop_fraction_percpu() need some read memory barrier?

It actually has one, see prop_get_global()...
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