netdev - Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

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Message-Id: <20070821.000404.39159401.davem@davemloft.net>
Date:	Tue, 21 Aug 2007 00:04:04 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	torvalds@...ux-foundation.org
Cc:	csnook@...hat.com, piggin@...erone.com.au, satyam@...radead.org,
	herbert@...dor.apana.org.au, paulus@...ba.org, clameter@....com,
	ilpo.jarvinen@...sinki.fi, paulmck@...ux.vnet.ibm.com,
	stefanr@...6.in-berlin.de, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, netdev@...r.kernel.org,
	akpm@...ux-foundation.org, ak@...e.de, heiko.carstens@...ibm.com,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, cfriesen@...tel.com, zlynx@....org,
	rpjday@...dspring.com, jesper.juhl@...il.com,
	segher@...nel.crashing.org
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures

From: Linus Torvalds <torvalds@...ux-foundation.org>
Date: Mon, 20 Aug 2007 22:46:47 -0700 (PDT)

> Ie a "barrier()" is likely _cheaper_ than the code generation downside 
> from using "volatile".

Assuming GCC were ever better about the code generation badness
with volatile that has been discussed here, I much prefer
we tell GCC "this memory piece changed" rather than "every
piece of memory has changed" which is what the barrier() does.

I happened to have been scanning a lot of assembler lately to
track down a gcc-4.2 miscompilation on sparc64, and the barriers
do hurt quite a bit in some places.  Instead of keeping unrelated
variables around cached in local registers, it reloads everything.
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