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

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Message-Id: <c0115c88da5d6756502e0c0499faa71f@kernel.crashing.org>
Date:	Sat, 18 Aug 2007 02:04:35 +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

>>>> 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,
>
> I don't think so.

"asm volatile" creates a side effect.  Side effects aren't
allowed to be reordered wrt sequence points.  This is exactly
the same reason as why "volatile accesses" cannot be reordered.

>> if that is
>> implemented by GCC in the "obvious" way.  Even a "plain" asm()
>> will do the same.
>
> Read the relevant GCC documentation.

I did, yes.

> [ of course, if the (latest) GCC documentation is *yet again*
>   wrong, then alright, not much I can do about it, is there. ]

There was (and is) nothing wrong about the "+m" documentation, if
that is what you are talking about.  It could be extended now, to
allow "+m" -- but that takes more than just "fixing" the documentation.


Segher

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