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

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Date:	Wed, 15 Aug 2007 21:46:55 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	paulmck@...ux.vnet.ibm.com
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,
	Nick Piggin <nickpiggin@...oo.com.au>,
	linux-arch@...r.kernel.org, jesper.juhl@...il.com, 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.
>
> 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.

That's true of course, although there is no real guarantee for that.


Segher

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