From mboxrd@z Thu Jan  1 00:00:00 1970
From: Wu Fengguang <fengguang.wu@intel.com>
Subject: Re: [PATCH 3/5] writeback: fix dirtied pages accounting on
 sub-page writes
Date: Tue, 22 Nov 2011 21:07:50 +0800
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Cc: Jan Kara <jack@suse.cz>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	Christoph Hellwig <hch@lst.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	LKML <linux-kernel@vger.kernel.org>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
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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