RCPULWP_BUSY
if (atomic_cas_ptr(&rcpu->rcpu_prevlwp, l, RCPULWP_BUSY) == l) {
if (old != RCPULWP_BUSY && old != RCPULWP_WANTED) {
RCPULWP_WANTED, RCPULWP_BUSY) == RCPULWP_WANTED) {
if (old == RCPULWP_BUSY) {