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

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Message-ID: <Pine.LNX.4.64.0708241009120.20501@schroedinger.engr.sgi.com>
Date:	Fri, 24 Aug 2007 10:15:47 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Denys Vlasenko <vda.linux@...glemail.com>
cc:	Paul Mackerras <paulus@...ba.org>,
	Satyam Sharma <satyam@...radead.org>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Chris Snook <csnook@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	netdev@...r.kernel.org, Andrew Morton <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,
	segher@...nel.crashing.org,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures

On Fri, 24 Aug 2007, Denys Vlasenko wrote:

> On Thursday 16 August 2007 00:22, Paul Mackerras wrote:
> > Satyam Sharma writes:
> > In the kernel we use atomic variables in precisely those situations
> > where a variable is potentially accessed concurrently by multiple
> > CPUs, and where each CPU needs to see updates done by other CPUs in a
> > timely fashion.  That is what they are for.  Therefore the compiler
> > must not cache values of atomic variables in registers; each
> > atomic_read must result in a load and each atomic_set must result in a
> > store.  Anything else will just lead to subtle bugs.
> 
> Amen.

A "timely" fashion? One cannot rely on something like that when coding. 
The visibility of updates is insured by barriers and not by some fuzzy 
notion of "timeliness".
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