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Date: Fri, 17 Aug 2007 15:41:28 +1000
From: Paul Mackerras <paulus@samba.org>
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures
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To: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Stefan Richter <stefanr@s5r6.in-berlin.de>, Satyam Sharma <satyam@infradead.org>, Christoph Lameter <clameter@sgi.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Chris Snook <csnook@redhat.com>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-arch@vger.kernel.org, Linus Torvalds <torvalds@linux-foundation.org>, netdev@vger.kernel.org, Andrew Morton <akpm@linux-foundation.org>, ak@suse.de, heiko.carstens@de.ibm.com, davem@davemloft.net, schwidefsky@de.ibm.com, wensong@linux-vs.org, horms@verge.net.au, wjiang@resilience.com, cfriesen@nortel.com, zlynx@acm.org, rpjday@mindspring.com, jesper.juhl@gmail.com, segher@kernel.crashing.org
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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.