linux/tools/memory-model/litmus-tests/MP+polockmbonce+poacquiresilsil.litmus
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   1C MP+polockmbonce+poacquiresilsil
   2
   3(*
   4 * Result: Never
   5 *
   6 * Do spinlocks combined with smp_mb__after_spinlock() provide order
   7 * to outside observers using spin_is_locked() to sense the lock-held
   8 * state, ordered by acquire?  Note that when the first spin_is_locked()
   9 * returns false and the second true, we know that the smp_load_acquire()
  10 * executed before the lock was acquired (loosely speaking).
  11 *)
  12
  13{}
  14
  15P0(spinlock_t *lo, int *x) // Producer
  16{
  17        spin_lock(lo);
  18        smp_mb__after_spinlock();
  19        WRITE_ONCE(*x, 1);
  20        spin_unlock(lo);
  21}
  22
  23P1(spinlock_t *lo, int *x) // Consumer
  24{
  25        int r1;
  26        int r2;
  27        int r3;
  28
  29        r1 = smp_load_acquire(x);
  30        r2 = spin_is_locked(lo);
  31        r3 = spin_is_locked(lo);
  32}
  33
  34exists (1:r1=1 /\ 1:r2=0 /\ 1:r3=1) (* Bad outcome. *)
  35