ithread_loop()
                /*
                 * Service interrupts.  If another interrupt arrives while
                 * we are running, it will set it_need to note that we
                 * should make another pass.
                 */
                while (ithd->it_need) {
                        /*
                         * This might need a full read and write barrier
                         * to make sure that this write posts before any
                         * of the memory or device accesses in the
                         * handlers.
                         */
                        atomic_store_rel_int(&ithd->it_need, 0);
                        ithread_execute_handlers(p, ie);
                }



......
intr_event_schedule_thread()
        /*
         * Set it_need to tell the thread to keep running if it is already
         * running.  Then, lock the thread and see if we actually need to
         * put it on the runqueue.
         */
        it->it_need = 1;
        thread_lock(td);
        if (TD_AWAITING_INTR(td)) {
                CTR3(KTR_INTR, "%s: schedule pid %d (%s)", __func__, p->p_pid,
                    td->td_name);
                TD_CLR_IWAIT(td);
                sched_add(td, SRQ_INTR);
        } else {
                CTR5(KTR_INTR, "%s: pid %d (%s): it_need %d, state %d",
                    __func__, p->p_pid, td->td_name, it->it_need, td->td_state);
        }
        thread_unlock(td);