On Tue, Dec 10, 2013 at 08:46:28PM +0100, Ingo Molnar wrote:
>
> * Paul E. McKenney <[email protected]> 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
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