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Date:	Mon, 6 Oct 2014 10:38:29 +0400
From:	Vladimir Davydov <>
To:	Michal Hocko <>
CC:	Johannes Weiner <>, <>,
	Greg Thelen <>, Dave Hansen <>,
	<>, <>
Subject: Re: [patch 1/3] mm: memcontrol: lockless page counters

On Fri, Oct 03, 2014 at 05:41:34PM +0200, Michal Hocko wrote:
> On Fri 03-10-14 19:36:23, Vladimir Davydov wrote:
> > On Thu, Oct 02, 2014 at 08:07:48AM -0400, Johannes Weiner wrote:
> [...]
> > > The barriers are implied in change-return atomics, which is why there
> > > is an xchg.  But it's clear that this needs to be documented.  This?:
> > 
> > With the comments it looks correct to me, but I wonder if we can always
> > rely on implicit memory barriers issued by atomic ops. Are there any
> > archs where it doesn't hold?
> xchg is explcitly mentioned in Documentation/memory-barriers.txt so it
> is expected to be barrier on all archs. Besides that not all atomic ops
> imply memory barriers. Only those that "modifies some state in memory
> and returns information about the state" do.

Thank you for the info, now it's clear to me.

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