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

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Message-ID: <Pine.LNX.4.64.0708171817050.15427@schroedinger.engr.sgi.com>
Date:	Fri, 17 Aug 2007 18:24:15 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Paul Mackerras <paulus@...ba.org>,
	Segher Boessenkool <segher@...nel.crashing.org>,
	heiko.carstens@...ibm.com, horms@...ge.net.au,
	linux-kernel@...r.kernel.org, rpjday@...dspring.com, ak@...e.de,
	netdev@...r.kernel.org, cfriesen@...tel.com,
	akpm@...ux-foundation.org, jesper.juhl@...il.com,
	linux-arch@...r.kernel.org, zlynx@....org, satyam@...radead.org,
	schwidefsky@...ibm.com, Chris Snook <csnook@...hat.com>,
	davem@...emloft.net, 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, Paul E. McKenney wrote:

> On Sat, Aug 18, 2007 at 08:09:13AM +0800, Herbert Xu wrote:
> > On Fri, Aug 17, 2007 at 04:59:12PM -0700, Paul E. McKenney wrote:
> > >
> > > gcc bugzilla bug #33102, for whatever that ends up being worth.  ;-)
> > 
> > I had totally forgotten that I'd already filed that bug more
> > than six years ago until they just closed yours as a duplicate
> > of mine :)
> > 
> > Good luck in getting it fixed!
> 
> Well, just got done re-opening it for the third time.  And a local
> gcc community member advised me not to give up too easily.  But I
> must admit that I am impressed with the speed that it was identified
> as duplicate.
> 
> Should be entertaining!  ;-)

Right. ROTFL... volatile actually breaks atomic_t instead of making it 
safe. x++ becomes a register load, increment and a register store. Without 
volatile we can increment the memory directly. It seems that volatile 
requires that the variable is loaded into a register first and then 
operated upon. Understandable when you think about volatile being used to 
access memory mapped I/O registers where a RMW operation could be 
problematic.

See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=3506
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