162306a36Sopenharmony_ciC RCU+sync+free 262306a36Sopenharmony_ci 362306a36Sopenharmony_ci(* 462306a36Sopenharmony_ci * Result: Never 562306a36Sopenharmony_ci * 662306a36Sopenharmony_ci * This litmus test demonstrates that an RCU reader can never see a write that 762306a36Sopenharmony_ci * follows a grace period, if it did not see writes that precede that grace 862306a36Sopenharmony_ci * period. 962306a36Sopenharmony_ci * 1062306a36Sopenharmony_ci * This is a typical pattern of RCU usage, where the write before the grace 1162306a36Sopenharmony_ci * period assigns a pointer, and the writes following the grace period destroy 1262306a36Sopenharmony_ci * the object that the pointer used to point to. 1362306a36Sopenharmony_ci * 1462306a36Sopenharmony_ci * This is one implication of the RCU grace-period guarantee, which says (among 1562306a36Sopenharmony_ci * other things) that an RCU read-side critical section cannot span a grace period. 1662306a36Sopenharmony_ci *) 1762306a36Sopenharmony_ci 1862306a36Sopenharmony_ci{ 1962306a36Sopenharmony_ciint x = 1; 2062306a36Sopenharmony_ciint *y = &x; 2162306a36Sopenharmony_ciint z = 1; 2262306a36Sopenharmony_ci} 2362306a36Sopenharmony_ci 2462306a36Sopenharmony_ciP0(int *x, int *z, int **y) 2562306a36Sopenharmony_ci{ 2662306a36Sopenharmony_ci int *r0; 2762306a36Sopenharmony_ci int r1; 2862306a36Sopenharmony_ci 2962306a36Sopenharmony_ci rcu_read_lock(); 3062306a36Sopenharmony_ci r0 = rcu_dereference(*y); 3162306a36Sopenharmony_ci r1 = READ_ONCE(*r0); 3262306a36Sopenharmony_ci rcu_read_unlock(); 3362306a36Sopenharmony_ci} 3462306a36Sopenharmony_ci 3562306a36Sopenharmony_ciP1(int *x, int *z, int **y) 3662306a36Sopenharmony_ci{ 3762306a36Sopenharmony_ci rcu_assign_pointer(*y, z); 3862306a36Sopenharmony_ci synchronize_rcu(); 3962306a36Sopenharmony_ci WRITE_ONCE(*x, 0); 4062306a36Sopenharmony_ci} 4162306a36Sopenharmony_ci 4262306a36Sopenharmony_ciexists (0:r0=x /\ 0:r1=0) 43