qh_wchan
qp->qh_wchan = wchan; /* valid until queue_unlock() is called */
qrp->qr_wchan = qp->qh_wchan;
ulwp->ul_wchan = qp->qh_wchan;
ASSERT(qp->qh_wchan != NULL && ulwp->ul_wchan == qp->qh_wchan);