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From: Wu Fengguang <wfg@linux.intel.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: "Илья Тумайкин" <librarian_rus@yahoo.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org
Subject: Re: A regression in recent 3.2 kernel: bdi_dirty_limit() divide error
Date: Sun, 8 Jan 2012 10:33:05 +0800	[thread overview]
Message-ID: <20120108023305.GA5074@localhost> (raw)
In-Reply-To: <1325954125.2442.27.camel@twins>

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

  reply	other threads:[~2012-01-08  2:33 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <1325884395.57034.YahooMailClassic@web161605.mail.bf1.yahoo.com>
2012-01-07 14:56 ` A regression in recent 3.2 kernel: bdi_dirty_limit() divide error Wu Fengguang
2012-01-07 16:35   ` Peter Zijlstra
2012-01-08  2:33     ` Wu Fengguang [this message]
2012-01-08 10:19       ` Peter Zijlstra
2012-01-09  4:04         ` Wu Fengguang
     [not found]           ` <1326205945.62365.YahooMailClassic@web161603.mail.bf1.yahoo.com>
2012-01-10 14:38             ` Wu Fengguang
2012-01-09  4:55         ` Wu Fengguang
2012-01-10 14:54           ` [PATCH] lib: proportion: lower PROP_MAX_SHIFT to 32 on 64-bit kernel Wu Fengguang
2012-01-10 15:01             ` Wu Fengguang

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