linux-kernel - Re: [PATCH] "volatile considered harmful", take 3

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Message-ID: <46455CD9.7010205@zytor.com>
Date:	Fri, 11 May 2007 23:21:13 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Satyam Sharma <satyam.sharma@...il.com>
CC:	Jonathan Corbet <corbet@....net>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, Johannes Stezenbach <js@...uxtv.org>,
	Jesper Juhl <jesper.juhl@...il.com>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Heikki Orsila <shdl@...alwe.fi>,
	jimmy bahuleyan <knight.camelot@...il.com>,
	Stefan Richter <stefanr@...6.in-berlin.de>
Subject: Re: [PATCH] "volatile considered harmful", take 3

Satyam Sharma wrote:
> 
> Because volatile is ill-defined? Or actually, *undefined* (well,
> implementation-defined is as good as that)? It's *so* _vague_,
> one doesn't _feel_ like using it at all!
> 

Sorry, that's just utter crap.  Linux isn't written in some mythical C
which only exists in standard document, it is written in a particular
subset of GNU C.  "volatile" is well enough defined in that context, it
is just frequently misused.

> We already have a complete API containing optimization barriers,
> load/store/full memory barriers. With well-defined and
> well-understood semantics. Just ... _why_ use volatile?

See below.

> It will _always_ work. In fact you can't really say the same for
> volatile. We already assume the compiler _actually_ took some
> pains to stuff meaning into C's (lack of) definition of volatile and
> implement it -- but in what sense, nobody knows (the C standard
> doesn't, so what are we).

It will always work within the context of GNU C.

>> more heavy-handed as it's disabling *all* optimization such as loop
>> invariants across the barrier.
> 
> This is a legitimate criticism, I agree.

There you have it.

	-hpa
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