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

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Message-Id: <2cbda24e96a49c3ab7cf7039c515f9fc@kernel.crashing.org>
Date:	Wed, 15 Aug 2007 20:51:58 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	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

> 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.

> 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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