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Message-ID: <20070818215652.GD7628@linux.vnet.ibm.com>
Date: Sat, 18 Aug 2007 14:56:52 -0700
From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To: Christoph Lameter <clameter@....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, Aug 17, 2007 at 06:24:15PM -0700, Christoph Lameter wrote:
> 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
Yep. The initial reaction was in fact to close my bug as a duplicate
of 3506. But I was not asking for atomicity, but rather for smaller
code to be generated, so I reopened it.
Thanx, Paul
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