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

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Date:	Sat, 18 Aug 2007 01:17:43 +0200
From:	Segher Boessenkool <segher@...nel.crashing.org>
To:	Satyam Sharma <satyam@...radead.org>
Cc:	Christoph Lameter <clameter@....com>,
	Paul Mackerras <paulus@...ba.org>, heiko.carstens@...ibm.com,
	horms@...ge.net.au, Stefan Richter <stefanr@...6.in-berlin.de>,
	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, zlynx@....org,
	Andrew Morton <akpm@...ux-foundation.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

>>> 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.
>
> If you're talking of "volatile" the type-qualifier keyword, then
> http://lkml.org/lkml/2007/8/16/231 (and sub-thread below it) shows
> otherwise.

I'm not sure what in that mail you mean, but anyway...

Yes, of course, the fact that "volatile" creates a side effect
prevents certain things from being reordered wrt the atomic_dec();
but the atomic_dec() has a side effect *already* so the volatile
doesn't change anything.

>> 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.
>
> I don't think an atomic_dec() implemented as an inline "asm volatile"
> or one that uses a "forget" macro would have the same re-ordering
> guarantees as an atomic_dec() that uses a volatile access cast.

The "asm volatile" implementation does have exactly the same
reordering guarantees as the "volatile cast" thing, if that is
implemented by GCC in the "obvious" way.  Even a "plain" asm()
will do the same.


Segher

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