linux-kernel - Re: [PATCH 3/5] writeback: fix dirtied pages accounting on sub-page writes

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Date:	Tue, 22 Nov 2011 21:07:50 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Jan Kara <jack@...e.cz>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/5] writeback: fix dirtied pages accounting on
 sub-page writes

On Tue, Nov 22, 2011 at 08:57:42PM +0800, Peter Zijlstra wrote:
> On Tue, 2011-11-22 at 13:21 +0100, Jan Kara wrote:
> > > +             __get_cpu_var(bdp_ratelimits)++;
> >   I think you need preempt_disable() and preempt_enable() pair around
> > __get_cpu_var(). Otherwise a process could get rescheduled in the middle of
> > read-modify-write cycle... 
> 
> there's of course the this_cpu_inc(bdp_ratelimits); thing.
> 
> On x86 that'll turn into a single insn, on others it will add the
> required preempt_disable/enable bits.

It's good to know that. But what if we don't really care which CPU
data it's increasing, and can accept losing some increases due to the
resulted race condition?

Thanks,
Fengguang
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