On Thu, Aug 03, 2017 at 07:55:14AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 03, 2017 at 10:05:16PM +0800, Boqun Feng wrote:
> > Hi Will,
> >
> > On Wed, Aug 02, 2017 at 10:45:32AM +0100, Will Deacon wrote:
> > [...]
> > >
> > > It's worth noting that we don't have the problem with any value-returning
> > > atomics, so all flavours of xchg in this test would be forbidden on arm64
> > > too.
> > >
> > > > C C-WillDeacon-MP+o-r+ai-rmb-o.litmus
> > > >
> > > > (*
> > > > * Expected result: Never.
> > > > *
> > > > * Desired litmus test, with atomic_inc() emulated by
> > > > xchg_relaxed():
> > > > *
> > > > * WRITE_ONCE(x, 1); atomic_inc(&y);
> > > > * r0 = xchg_release(&y, 5); smp_rmb();
> > > > * r1 = READ_ONCE(x);
> > > > *
> > > > *
> > > > * WARN_ON(r0 == 0 && r1 == 0);
> > > > *)
> > > >
> > > > {
> > > > }
> > > >
> > > > P0(int *x, int *y)
> > > > {
> > > > WRITE_ONCE(*x, 1);
> > > > r0 = xchg_release(y, 5);
> > > > }
> > > >
> > > > P1(int *x, int *y)
> > > > {
> > > > r2 = xchg_relaxed(y, 1);
> > > > smp_rmb();
> > > > r1 = READ_ONCE(*x);
> > > > }
> > > >
> > > > exists
> > > > (0:r0=0 /\ 1:r1=0)
> > > >
> >
> > How about a litmus test like this?
> >
> > C C-AMO-global-transitivity.litmus
> >
> > {
> > }
> >
> > P0(int *x, int *y)
> > {
> > WRITE_ONCE(*x, 1);
> > r0 = xchg_release(y, 5);
> > }
> >
> > P1(int *y, int *z)
> > {
> > atomic_inc(y);
> > smp_mb();
>
> I am going to guess that the smp_mb() enforces the needed ordering,
> but Will will let me know. ;-)
Yup, that would be forbidden on arm64, and would also be forbidden if
you used WRITE_ONCE instead of atomic_inc (remember that we recently
became multi-copy atomic).
Will
