casuword
old = casuword(&umtx->u_owner, owner, UMTX_UNOWNED);
old = casuword(&umtx->u_owner, owner,
owner = casuword(&umtx->u_owner, UMTX_UNOWNED, id);
owner = casuword(&umtx->u_owner,
old = casuword(&umtx->u_owner, owner, owner | UMTX_CONTESTED);
u_long casuword(volatile u_long *p, u_long oldval, u_long newval);