static void cancelThread(Thread t) {
  if (t == null) ret;
  ping();
  O mc = getWeakRef(cast vm_generalWeakSubMap("thread2mc").get(t));
  if (mc != null) {
    Map ping_actions = cast get(mc, 'ping_actions);
    ping_actions.put(t, "cancelled");
    set(mc, ping_anyActions := true);
  } else {
    ping_actions.put(t, "cancelled");
    ping_anyActions = true;
  }
}