[cpp-threads] Yet another visibility question

[Alexander Terekhov]

On 1/12/07, Paul E. McKenney <paulmck at linux.vnet.ibm.com> wrote:
> On Fri, Jan 12, 2007 at 10:30:46AM +0100, Alexander Terekhov wrote:
> > On 1/11/07, Peter Dimov <pdimov at mmltd.net> wrote:
> > >Alexander Terekhov wrote:
> > >
> > >> Yeah, and IBM does some NUMA with Intel Xeons, IIRC. That's why I've
> > >> been telling all along that lock_cmpxchg(&var, 42, 42) for loads is
> > >> the way to do SC on x86/IA32. ;-)
> > >
> > >If x86 loads do have acquire semantics as we've been assuming, wouldn't it
> > >be enough to just use lock xchg for stores?
> >
> > I don't think so. Why would that be enough? Well, actually xchg for
> > stores is still needed to ensure leading store-load fencing regarding
> > load inside cmpxchg, I think. So for SC on x86/IA32 we need cmpxchg
> > for loads and xchg for stores (both locked).
>
> Will this force all stores to independent variables from different
> processors to be ordered?  On a large NUMA configuration with multiple
> front-side busses?

Think of implementing SC  with per-variable locks on a large x86/IA32
NUMA configuration with multiple FSBs or whatever. It won't be any
better than cmpxchg for loads and xchg for stores (both locked).

regards,
alexander.