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Message-Id: <c298f0e3f9e49c9dd2d326115d156084@kernel.crashing.org>
Date: Fri, 17 Aug 2007 19:41:13 +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
>> 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.
"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.
Segher
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