linux-kernel - Re: [PATCH 6/24] make atomic_read() behave consistently on frv

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Message-ID: <20070815191829.GJ9645@linux.vnet.ibm.com>
Date:	Wed, 15 Aug 2007 12:18:29 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Segher Boessenkool <segher@...nel.crashing.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>, heiko.carstens@...ibm.com,
	horms@...ge.net.au, linux-kernel@...r.kernel.org,
	rpjday@...dspring.com, ak@...e.de, netdev@...r.kernel.org,
	cfriesen@...tel.com, akpm@...ux-foundation.org,
	torvalds@...ux-foundation.org, jesper.juhl@...il.com,
	linux-arch@...r.kernel.org, zlynx@....org, schwidefsky@...ibm.com,
	Chris Snook <csnook@...hat.com>, davem@...emloft.net,
	wensong@...ux-vs.org, wjiang@...ilience.com,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 6/24] make atomic_read() behave consistently on frv

On Wed, Aug 15, 2007 at 08:51:58PM +0200, Segher Boessenkool wrote:
> >Well if there is only one memory location involved, then smp_rmb() 
> >isn't
> >going to really do anything anyway, so it would be incorrect to use it.
> 
> rmb() orders *any* two reads; that includes two reads from the same
> location.

If the two reads are to the same location, all CPUs I am aware of
will maintain the ordering without need for a memory barrier.

						Thanx, Paul

> >Consider that smp_rmb basically will do anything from flushing the
> >pipeline to invalidating loads speculatively executed out of order. 
> >AFAIK
> >it will not control the visibility of stores coming from other CPUs 
> >(that
> >is up to the cache coherency).
> 
> The writer side should typically use wmb() whenever the reader side
> uses rmb(), sure.
> 
> 
> Segher
> 
> -
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