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

[Paul E. McKenney]

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)