Test Weak-RCU-Readers-With-More-Grace

LISA Weak-RCU-Readers-With-More-Grace (*Paul says forbid*)
{
a = 0; x = 0; c = 0;
}
 P0                 | P1           | P2           | P3                 ;
 f[rcu_read_lock]   |              |              | f[rcu_read_lock]   ;
 r[once] r1 x       | r[once] r2 a | r[once] r3 c | r[once] r4 d       ;
                    | f[sync]      | f[sync]      |                    ;
 w[once] a 1        | w[once] c 1  | w[once] d 1  | w[once] x 1        ;
 f[rcu_read_unlock] |              |              | f[rcu_read_unlock] ;
Observed
    3:r4=1; 2:r3=0; 1:r2=1; 0:r1=1;
and 3:r4=1; 2:r3=1; 1:r2=0; 0:r1=1;