UMTX_CONTESTED
if ((owner & ~UMTX_CONTESTED) != id)
if ((owner & UMTX_CONTESTED) == 0) {
count <= 1 ? UMTX_UNOWNED : UMTX_CONTESTED);
if (owner == UMTX_CONTESTED) {
UMTX_CONTESTED, id | UMTX_CONTESTED);
if (owner == UMTX_CONTESTED)
old = casuword(&umtx->u_owner, owner, owner | UMTX_CONTESTED);