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

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Date:	Sat, 18 Aug 2007 00:08:01 +0530 (IST)
From:	Satyam Sharma <satyam@...radead.org>
To:	Segher Boessenkool <segher@...nel.crashing.org>
cc:	Christoph Lameter <clameter@....com>,
	Paul Mackerras <paulus@...ba.org>, heiko.carstens@...ibm.com,
	horms@...ge.net.au, Stefan Richter <stefanr@...6.in-berlin.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	ak@...e.de, cfriesen@...tel.com, rpjday@...dspring.com,
	Netdev <netdev@...r.kernel.org>, jesper.juhl@...il.com,
	linux-arch@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>, zlynx@....org,
	schwidefsky@...ibm.com, Chris Snook <csnook@...hat.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	wensong@...ux-vs.org, wjiang@...ilience.com
Subject: Re: [PATCH 0/24] make atomic_read() behave consistently across all
 architectures



On Fri, 17 Aug 2007, Segher Boessenkool wrote:

> > > atomic_dec() already has volatile behavior everywhere, so this is
> > > semantically
> > > okay, but this code (and any like it) should be calling cpu_relax() each
> > > iteration through the loop, unless there's a compelling reason not to.
> > > I'll
> > > allow that for some hardware drivers (possibly this one) such a compelling
> > > reason may exist, but hardware-independent core subsystems probably have
> > > no
> > > excuse.
> > 
> > No it does not have any volatile semantics. atomic_dec() can be reordered
> > at will by the compiler within the current basic unit if you do not add a
> > barrier.
> 
> "volatile" has nothing to do with reordering.

If you're talking of "volatile" the type-qualifier keyword, then
http://lkml.org/lkml/2007/8/16/231 (and sub-thread below it) shows
otherwise.

> atomic_dec() writes
> to memory, so it _does_ have "volatile semantics", implicitly, as
> long as the compiler cannot optimise the atomic variable away
> completely -- any store counts as a side effect.

I don't think an atomic_dec() implemented as an inline "asm volatile"
or one that uses a "forget" macro would have the same re-ordering
guarantees as an atomic_dec() that uses a volatile access cast.
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