Re: [patch] spinlocks: remove 'volatile'

["Chris Friesen" <cfriesen@nortel.com>]

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.

Only a couple months ago Dave Miller mentioned setting variables 
modified in signal handlers as "volatile", and you didn't complain.  If 
fact you added that they should be of type "sigatomic_t".

(Pardon the long URL)

http://groups.google.com/group/linux.kernel/browse_frm/thread/18a59e3c9d8f6310/84881a7e53038b0e?lnk=st&q=sigatomic_t&rnum=8&hl=en#84881a7e53038b0e


Chris

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