linux-kernel - Re: A regression in recent 3.2 kernel: bdi_dirty

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Message-ID: <20120108023305.GA5074@localhost>
Date:	Sun, 8 Jan 2012 10:33:05 +0800
From:	Wu Fengguang <wfg@...ux.intel.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Илья Тумайкин 
	<librarian_rus@...oo.com>, LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: A regression in recent 3.2 kernel: bdi_dirty_limit() divide
 error

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.

prop_change_shift() does

        change ->pg[index]->shift
        smp_wmb()
        change ->index

Will the read side prop_fraction_percpu() need some read memory barrier?

Thanks,
Fengguang
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