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

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Message-Id: <20070508161309.c398a913.randy.dunlap@oracle.com>
Date:	Tue, 8 May 2007 16:13:09 -0700
From:	Randy Dunlap <randy.dunlap@...cle.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Krzysztof Halasa <khc@...waw.pl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Sokolovsky <pmiscml@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC/PATCH] doc: volatile considered evil

On Tue, 8 May 2007 13:20:31 -0700 (PDT) David Rientjes wrote:

> On Tue, 8 May 2007, Krzysztof Halasa 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.
> > 
> > I think it hasn't two semantics, it's like arguing that char has two
> > semantics.
> > 
> > Volatile does one thing - prohibits C compiler from optimizing
> > accesses to the variable. Either with (volatile *) casts and with
> > volatile var;
> > 
> 
> What the meaning of an "access" to a volatile memory-mapped I/O port or a 
> variable that can be asynchronously interrupted is implementation-defined.
> 
> You're only citing qualified versions of objects.
> 
> 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.
> 
> Thus, the warning that was proposed for addition to CodingStyle should be 
> modified to explicitly state that the use of 'volatile' for asm constructs 
> is perfectly legitimate and its use as a type qualifier for objects in 
> code is inappropriate.

It's already there, isn't it?  <quote from original:>

The only acceptable uses for "volatile" are:

 - in _code_, i.e., for things like the definition of "readb()" etc, where we
   use it to force a particular access.
 - with inline asms
 - on "jiffies", for stupid legacy reasons

</quote>

or are you saying that you want to subject/header/title modified also?


---
~Randy
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