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Message-Id: <8239c744e7ef64a6f5a15449d337ac68@kernel.crashing.org>
Date: Tue, 11 Sep 2007 04:27:22 +0200
From: Segher Boessenkool <segher@...nel.crashing.org>
To: Christoph Lameter <clameter@....com>
Cc: 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>,
"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
>> "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
Read again: I said the C "volatile" construct has nothing to do
with CPU memory access reordering.
> and may be reordered on a variety
> of processors. Writes to memory may not follow code order on several
> processors.
The _compiler_ isn't allowed to reorder things here. Yes, of course
you do need stronger barriers for many purposes, volatile isn't all
that useful you know.
Segher
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