Re: Internal vs. external barriers (was: Re: Interesting LKMM litmus test)

Paul E. McKenney
2023-01-24T14:54:32Z

On Tue, Jan 24, 2023 at 12:09:48PM +0100, Andrea Parri wrote:
> > There is the one below, but I am (1) not sure that I have it right,
> > (2) not immediately certain that the Linux-kernel implementation would
> > forbid it, (3) not immediately sure that it should be forbidden.
> >
> > In the meantime, thoughts?
>
> As it stands, P0 to completion, then P1 to completion, then P2 to
> completion should meet the "exists" clause; I guess we want "x=1"
> in the clause (or the values of the stores to "x" exchanged).

OK, so I still don't have it right. ;-)

Make that x=1. I think.

Thanx, Paul

> Andrea
>
>
> > ------------------------------------------------------------------------
> >
> > C C-srcu-observed-3
> >
> > (*
> > * Result: Sometimes
> > *)
> >
> > {}
> >
> > P0(int *x, int *y, int *z, struct srcu_struct *s)
> > {
> > int r1;
> >
> > r1 = srcu_read_lock(s);
> > WRITE_ONCE(*y, 1);
> > WRITE_ONCE(*x, 1);
> > srcu_read_unlock(s, r1);
> > }
> >
> > P1(int *x, int *y, int *z, struct srcu_struct *s)
> > {
> > int r1;
> >
> > r1 = READ_ONCE(*y);
> > synchronize_srcu(s);
> > WRITE_ONCE(*z, 1);
> > }
> >
> > P2(int *x, int *y, int *z, struct srcu_struct *s)
> > {
> > WRITE_ONCE(*z, 2);
> > smp_mb();
> > WRITE_ONCE(*x, 2);
> > }
> >
> > exists (1:r1=1 /\ x=2 /\ z=2)