Lower Write service Procedure
muxlwsrv, the lower (linked) queue write service procedure is scheduled as a result of flow control subsiding downstream (it is back-enabled).
static int muxlwsrv(queue_t *q)
for (i = 0; i < mux_cnt; i++)
if (mux_mux[i].qptr && mux_mux[i].qptr->q_first)
muxlwsrv steps through all possible upper queues. If a queue is active and there are messages on the queue, then its upper write service procedure is enabled through qenable.