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

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Message-ID: <18117.13576.93583.222760@cargo.ozlabs.ibm.com>
Date:	Fri, 17 Aug 2007 15:41:28 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Herbert Xu <herbert@...dor.apana.org.au>
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

Herbert Xu writes:

> 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().

Did you look at it?  Here it is:

	/* Someone else is initializing in parallel - let 'em finish */
	while (atomic_read(&idle_hook_initialized) < 1000)
		;

Paul.
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