linux/tools/memory-model/litmus-tests/Z6.0+pooncerelease+poacquirerelease+fencembonceonce.litmus
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   1C Z6.0+pooncerelease+poacquirerelease+fencembonceonce
   2
   3(*
   4 * Result: Sometimes
   5 *
   6 * This litmus test shows that a release-acquire chain, while sufficient
   7 * when there is but one non-reads-from (AKA non-rf) link, does not suffice
   8 * if there is more than one.  Of the three processes, only P1() reads from
   9 * P0's write, which means that there are two non-rf links: P1() to P2()
  10 * is a write-to-write link (AKA a "coherence" or just "co" link) and P2()
  11 * to P0() is a read-to-write link (AKA a "from-reads" or just "fr" link).
  12 * When there are two or more non-rf links, you typically will need one
  13 * full barrier for each non-rf link.  (Exceptions include some cases
  14 * involving locking.)
  15 *)
  16
  17{}
  18
  19P0(int *x, int *y)
  20{
  21        WRITE_ONCE(*x, 1);
  22        smp_store_release(y, 1);
  23}
  24
  25P1(int *y, int *z)
  26{
  27        int r0;
  28
  29        r0 = smp_load_acquire(y);
  30        smp_store_release(z, 1);
  31}
  32
  33P2(int *x, int *z)
  34{
  35        int r1;
  36
  37        WRITE_ONCE(*z, 2);
  38        smp_mb();
  39        r1 = READ_ONCE(*x);
  40}
  41
  42exists (1:r0=1 /\ z=2 /\ 2:r1=0)
  43