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

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Message-ID: <46BFFDBD.6080804@redhat.com>
Date:	Mon, 13 Aug 2007 02:44:13 -0400
From:	Chris Snook <csnook@...hat.com>
To:	David Howells <dhowells@...hat.com>
CC:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	torvalds@...ux-foundation.org, netdev@...r.kernel.org,
	akpm@...ux-foundation.org, ak@...e.de, heiko.carstens@...ibm.com,
	davem@...emloft.net, 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
Subject: Re: [PATCH 6/24] make atomic_read() behave consistently on frv

David Howells wrote:
> Chris Snook <csnook@...hat.com> wrote:
> 
>> cpu_relax() contains a barrier, so it should do the right thing.  For non-smp
>> architectures, I'm concerned about interacting with interrupt handlers.  Some
>> drivers do use atomic_* operations.
> 
> I'm not sure that actually answers my question.  Why not smp_rmb()?
> 
> David

I would assume because we want to waste time efficiently even on non-smp 
architectures, rather than frying the CPU or draining the battery.  Certain 
looping execution patterns can cause the CPU to operate above thermal design 
power.  I have fans on my workstation that only ever come on when running 
LINPACK, and that's generally memory bandwidth-bound.  Just imagine what happens 
when you're executing the same few non-serializing instructions in a tight loop 
without ever stalling on memory fetches, or being scheduled out.

If there's another reason, I'd like to hear it too, because I'm just guessing here.

	-- Chris
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