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

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Message-ID: <18115.44462.622801.683446@cargo.ozlabs.ibm.com>
Date:	Thu, 16 Aug 2007 11:51:42 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Christoph Lameter <clameter@....com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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>
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures

Christoph Lameter writes:

> A volatile default would disable optimizations for atomic_read. 
> atomic_read without volatile would allow for full optimization by the 
> compiler. Seems that this is what one wants in many cases.

Name one such case.

An atomic_read should do a load from memory.  If the programmer puts
an atomic_read() in the code then the compiler should emit a load for
it, not re-use a value returned by a previous atomic_read.  I do not
believe it would ever be useful for the compiler to collapse two
atomic_read statements into a single load.

Paul.
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