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

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Message-ID: <20070817053200.GA15457@gondor.apana.org.au>
Date:	Fri, 17 Aug 2007 13:32:00 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Stefan Richter <stefanr@...6.in-berlin.de>,
	Satyam Sharma <satyam@...radead.org>,
	Christoph Lameter <clameter@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

On Fri, Aug 17, 2007 at 03:09:57PM +1000, Paul Mackerras wrote:
> Herbert Xu writes:
> 
> > Can you find an actual atomic_read code snippet there that is
> > broken without the volatile modifier?
> 
> There are some in arch-specific code, for example line 1073 of
> arch/mips/kernel/smtc.c.  On mips, cpu_relax() is just barrier(), so
> the empty loop body is ok provided that atomic_read actually does the
> load each time around the loop.

A barrier() is all you need to force the compiler to reread
the value.

The people advocating volatile in this thread are talking
about code that doesn't use barrier()/cpu_relax().

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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