Re: [PATCH tip/core/locking 4/4] Documentation/memory-barriers.txt: Document ACCESS_ONCE()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Dec 10, 2013 at 08:46:28PM +0100, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:
> 
> > > So, what I don't see this statement cover (and I might be dense about 
> > > it!) is whether two ACCESS_ONCE() macros referring to different 
> > > variables are allowed to be reordered with each other.
> > > 
> > > If the compiler reorders:
> > > 
> > > 	ACCESS_ONCE(x);
> > > 	ACCESS_ONCE(y);
> > > 
> > > to:
> > > 
> > > 	ACCESS_ONCE(y);
> > > 	ACCESS_ONCE(x);
> > > 
> > > then AFAICS it still meets the "compiler need only forget the contents 
> > > of the indicated memory located" requirement that you listed, right?
> > 
> > True, but if the compiler was willing to reorder ACCESS_ONCE()'s 
> > volatile accesses, it would be really hard to write reliable device 
> > drivers. [...]
> 
> But nowhere do we link ACCESS_ONCE() to 'volatile' semantics in the 
> document, do we? (and I'm not sure we should.)

Agreed, stating ACCESS_ONCE()'s semantics separately is better.

> [ In theory a future compiler could offer a smarter, more flexible 
>   'compiler barrier' implementation - at which point we might be 
>   tempted to use that new facility to implement ACCESS_ONCE(). At that 
>   point this ambiguity might arise. ]

And they are trying to obsolete volatile, but haven't quite got there
yet.  ;-)

> > [...]  The standard says the following:
> > 
> > 	Access to volatile objects are evaluated strictly according to
> > 	the rules of the abstract machine.
> > 
> > That said, compiler writers and standards wonks will argue endlessly 
> > about exactly what that does and does not mean.  :-/
> > 
> > I added a sentence reading:
> > 
> > 	Of course, the compiler must also respect the order in which
> > 	the ACCESS_ONCE()s occur, though the CPU of course need not do so.
> > 
> > To the end of that paragraph.  Does that help?
> 
> Yeah, that looks perfect!

Very good!

							Thanx, Paul