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

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Message-ID: <alpine.LFD.0.999.0708161953310.30176@woody.linux-foundation.org>
Date:	Thu, 16 Aug 2007 20:03:55 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Paul Mackerras <paulus@...ba.org>
cc:	Christoph Lameter <clameter@....com>,
	Chris Snook <csnook@...hat.com>,
	Ilpo J?rvinen <ilpo.jarvinen@...sinki.fi>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Satyam Sharma <satyam@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org, Netdev <netdev@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>, ak@...e.de,
	heiko.carstens@...ibm.com, David Miller <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, 17 Aug 2007, Paul Mackerras wrote:
>
> Volatile doesn't mean it can't be reordered; volatile means the
> accesses can't be eliminated.

It also does limit re-ordering. 

Of course, since *normal* accesses aren't necessarily limited wrt 
re-ordering, the question then becomes one of "with regard to *what* does 
it limit re-ordering?".

A C compiler that re-orders two different volatile accesses that have a 
sequence point in between them is pretty clearly a buggy compiler. So at a 
minimum, it limits re-ordering wrt other volatiles (assuming sequence 
points exists). It also means that the compiler cannot move it 
speculatively across conditionals, but other than that it's starting to 
get fuzzy.

In general, I'd *much* rather we used barriers. Anything that "depends" on 
volatile is pretty much set up to be buggy. But I'm certainly also willing 
to have that volatile inside "atomic_read/atomic_set()" if it avoids code 
that would otherwise break - ie if it hides a bug.

		Linus
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