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

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Message-ID: <20070817010232.GK16957@linux.vnet.ibm.com>
Date:	Thu, 16 Aug 2007 18:02:32 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Chris Snook <csnook@...hat.com>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Paul Mackerras <paulus@...ba.org>,
	Satyam Sharma <satyam@...radead.org>,
	Stefan Richter <stefanr@...6.in-berlin.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.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 Thu, Aug 16, 2007 at 01:20:26PM -0700, Christoph Lameter wrote:
> On Thu, 16 Aug 2007, Chris Snook 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.

Yep.  Or you can use atomic_dec_return() instead of using a barrier.

						Thanx, Paul
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