Re: sem_getvalue conformance considerations

[Alexander Monakov <amonakov@ispras.ru>]

On IRC Rich noted that I've too conveniently elided cancellation, so here's how
I think cancellation handler should look like:

  val = sem->val[0];
  while (val < 0) {
    /* We believe we are a waiter that no sem_post has "seen".  */
    oldval = val;
    if ((val = a_cas(sem->val, val, val+1)) == oldval)
      return;
  }
  /* We are a waiter, and a non-negative val[0] indicates that one sem_post
   * noticed us.  Consume the corresponding wake count increment.  */
  a_dec(sem->val+1);

Alexander