linux-kernel - Re: [RFC/PATCH] doc: volatile considered evil

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Message-ID: <4640EB9D.90409@oracle.com>
Date:	Tue, 08 May 2007 14:29:01 -0700
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Sokolovsky <pmiscml@...il.com>,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@...e.de>
Subject: Re: [RFC/PATCH] doc: volatile considered evil

Jeremy Fitzhardinge wrote:
> David Rientjes wrote:
>> Since 'volatile' has two different semantics depending on the context in 
>> which it is used, this warning should be appended to include the fact that 
>> it is legitimate to use for inline assembly.
>>   
> 
> 
> It's probably worth noting that "asm volatile (...)" doesn't mean what
> many people think it means: specifically, it *does not* prevent the asm
> from being reordered with respect to the surrounding code.  It may not
> even prevent it from being reordered with respect to other asm
> volatiles.  *All* it means is that the asm code will be emitted even if
> the compiler doesn't think its results will be used.  Note that an
> "asm()" with no outputs is implicitly "asm volatile()" - on the grounds
> that it would be otherwise useless as far as gcc can tell.
> 
> If you need to guarantee ordering of asm statements, you must do it
> explicitly, with either a "memory" clobber, or some finer-grain
> serialization variable (like the _proxy_pda stuff).  It would be useful
> if you could tell gcc "I'm passing this variable to the asm for
> serialization purposes, but there's no need to generate any explicit
> references to it", but as far as I know there's no support for that.
> 
>     J

The doc. should just be talking about "volatile" in C mostly.
Any asm volatile comments are "extra".

-- 
~Randy
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