Test IRIW+rl-o-o-rul+o-srcu-o

LISA IRIW+rl-o-o-rul+o-srcu-o (lwn.net/Articles/573497/0-RCU)
(*
 * Forbid: Implicit full barrier between end of P0 and P1's read from x.
 * This invokes something resembling A cumulativity.
 *)
{
x = 0;
y = 0;
}
 P0                 | P1           | P2          | P3          ; 
 f[rcu_read_lock]   |              |             |             ;
 r[once] r1 x       | r[once] r3 y | w[once] x 1 | w[once] y 1 ;
                    | f[sync]      |             |             ;
 r[once] r2 y       | r[once] r4 x |             |             ;
 f[rcu_read_unlock] |              |             |             ; 
Observed
    1:r4=0; 1:r3=1; 0:r2=1; 0:r1=1;
and 1:r4=0; 1:r3=0; 0:r2=0; 0:r1=1;