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

[Andrea Parri <parri.andrea@gmail.com>]

> 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).

  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)