[cpp-threads] SC on PPC

[Alexander Terekhov]

On 5/1/07, Hans Boehm <Hans.Boehm at hp.com> wrote:
>
>
> On Tue, 1 May 2007, Alexander Terekhov wrote:
>
> [Raul:?]
> > >
> > > This depends on how you define acquire, but if it is defined to
> > > enforce causality it is insufficient. In particular, it wouldn't
> > > support this case:
> > >
> > > P1: x=1
> > > P2: if (x.load_acquire()==1) { y.store_relaxed(1) }
> > > P3: if (y.load_acquire()==1) { Assert(x.load_relaxed()==1) }
> >
> > Well, consider that on Itanium
> >
> > P1: x.store_relaxed(1)
> > P2: if (x.load_acquire()==1) { y.store_relaxed(1) }
> > P3: if (y.load_acquire()==1) { Assert(x.load_relaxed()==1) }
> >
> > may well abort.
> >
> > I'd define *acquire* no stronger than Itanium and wouldn't impose
> > remote write atomicity for release (relax release in this respect).
> >
> I think (and hope) we're arguing about a bug in the example here.  The

I'm talking about the cost of acquire on PPC.

P1: x.store_relaxed(1)
P2: if (x.load_relaxed()==1) { y.store_release(1) }
P3: if (y.load_acquire()==1) { Assert(x.load_relaxed()==1) }

wont abort with

load_acquire:   load;"branch never taken";isync // see B.2.3 Safe
Fetch (Book II).
store_release:  lwsync;store

I don't want to have more constrained

load_acquire:   load;lwsync

regards,
alexander.