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

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Message-ID: <20070910205434.GF11801@linux.vnet.ibm.com>
Date:	Mon, 10 Sep 2007 13:54:34 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Segher Boessenkool <segher@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>, heiko.carstens@...ibm.com,
	horms@...ge.net.au, Stefan Richter <stefanr@...6.in-berlin.de>,
	Satyam Sharma <satyam@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Miller <davem@...emloft.net>,
	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 Mon, Sep 10, 2007 at 11:59:29AM -0700, Christoph Lameter wrote:
> On Fri, 17 Aug 2007, Segher Boessenkool wrote:
> 
> > "volatile" has nothing to do with reordering.  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.
> 
> Stores can be reordered. Only x86 has (mostly) implicit write ordering. So 
> no atomic_dec has no volatile semantics and may be reordered on a variety 
> of processors. Writes to memory may not follow code order on several 
> processors.

The one exception to this being the case where process-level code is
communicating to an interrupt handler running on that same CPU -- on
all CPUs that I am aware of, a given CPU always sees its own writes
in order.

							Thanx, Paul
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