Re: [patch] spinlocks: remove 'volatile'

[Arjan van de Ven <arjan@infradead.org>]

On Thu, 2006-07-06 at 13:33 -0600, Chris Friesen wrote:
> Linus Torvalds wrote:
> 
> > On Thu, 6 Jul 2006, Mark Lord wrote:
> 
> >> A volatile declaration may be used to describe an object corresponding
> >> to a memory-mapped input/output port or an object accessed by an
> >> aysnchronously interrupting function.  Actions on objects so declared
> >> shall not be "optimized out" by an implementation or reordered except
> >> as permitted by the rules for evaluating expressions.
> > 
> > 
> > Note that the "reordered" is totally pointless.
> > 
> > The _hardware_ will re-order accesses. Which is the whole point. 
> > "volatile" is basically never sufficient in itself.
> 
> The "reordered" thing really only matters on SMP machines, no?  In which 
> case (for userspace) the locking mechanisms (mutexes, etc.) should do 
> The Right Thing to ensure visibility between cpus.
> 
> The C standard requires the use of volatile for signal handlers and setjmp.
> 
> For userspace at least the whole discussion of "barriers" is sort of 
> moot--there are no memory barriers defined in the C language, which 
> makes it kind of hard to write portable code that uses them.

You're falling into RBJ's trap. I did not say *MEMORY BARRIER*. While
for some uses of "volatile" that is the right substitute, for others it
is *optimization barrier* which matters.