RCPULWP_WANTED
old = atomic_swap_ptr(&rcpu->rcpu_prevlwp, RCPULWP_WANTED);
if (old != RCPULWP_BUSY && old != RCPULWP_WANTED) {
RCPULWP_WANTED, RCPULWP_BUSY) == RCPULWP_WANTED) {
KASSERT(old == RCPULWP_WANTED);