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

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Message-Id: <20070508160908.ed1824b4.randy.dunlap@oracle.com>
Date:	Tue, 8 May 2007 16:09:08 -0700
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	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

On Tue, 8 May 2007 14:27:33 -0700 (PDT) David Rientjes wrote:

> On Tue, 8 May 2007, Jeremy Fitzhardinge wrote:
> 
> > 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.
> > 

Well, the document is really about "volatile" in C, not in gcc asm
extensions.
But if you want to add paragraphs(s) to the file, that's OK too.

> Ok, so let's take your second paragraph and my email of an hour ago:
> 
> 	In an asm construct, if all your input operands are modified and 
> 	specified as output operands as well, volatile must be added so 
> 	that the entire construct is not optimized away.  Additionally, 
> 	it must be added if your construct modifies memory that is neither 
> 	listed in inputs nor outputs to the construct so that it is known 
> 	to have at least one side-effect.  Then, the compiler cannot 
> 	delete your construct if it is reachable because it may produce 
> 	such side-effects.
> 
> and add it to any proposed change to CodingStyle that suggests against the 
> 'volatile' keyword since there exists a distinct difference in behavior 
> between using the keyword as a type qualifier for an object and as a 
> qualifier for an asm construct.


---
~Randy
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