pkt_sched: Add 'deactivated' state.

This new state lets dev_deactivate() mark a qdisc as having been
deactivated.

dev_queue_xmit() and ing_filter() check for this bit and do not
try to process the qdisc if the bit is set.

dev_deactivate() polls the qdisc after setting the bit, waiting
for both __QDISC_STATE_RUNNING and __QDISC_STATE_SCHED to clear.

This isn't perfect yet, but subsequent changesets will make it so.
This part is just one piece of the puzzle.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sch_generic.h

@@ -27,6 +27,7 @@ enum qdisc_state_t
 {
 	__QDISC_STATE_RUNNING,
 	__QDISC_STATE_SCHED,
+	__QDISC_STATE_DEACTIVATED,
 };
 
 struct qdisc_size_table {

net/core/dev.c

@@ -1800,6 +1800,12 @@ int dev_queue_xmit(struct sk_buff *skb)
 
 		spin_lock(root_lock);
 
+		if (unlikely(test_bit(__QDISC_STATE_DEACTIVATED, &q->state))) {
+			spin_unlock(root_lock);
+			rc = NET_XMIT_DROP;
+			goto out_kfree_skb;
+		}
+
 		rc = qdisc_enqueue_root(skb, q);
 		qdisc_run(q);
 
@@ -2084,6 +2090,7 @@ static int ing_filter(struct sk_buff *skb)
 	q = rxq->qdisc;
 	if (q != &noop_qdisc) {
 		spin_lock(qdisc_lock(q));
+		if (likely(!test_bit(__QDISC_STATE_DEACTIVATED, &q->state)))
 			result = qdisc_enqueue_root(skb, q);
 		spin_unlock(qdisc_lock(q));
 	}

net/sched/sch_generic.c

@@ -597,6 +597,9 @@ static void transition_one_qdisc(struct net_device *dev,
 	struct Qdisc *new_qdisc = dev_queue->qdisc_sleeping;
 	int *need_watchdog_p = _need_watchdog;
 
+	if (!(new_qdisc->flags & TCQ_F_BUILTIN))
+		clear_bit(__QDISC_STATE_DEACTIVATED, &new_qdisc->state);
+
 	rcu_assign_pointer(dev_queue->qdisc, new_qdisc);
 	if (need_watchdog_p && new_qdisc != &noqueue_qdisc)
 		*need_watchdog_p = 1;
@@ -640,6 +643,9 @@ static void dev_deactivate_queue(struct net_device *dev,
 	if (qdisc) {
 		spin_lock_bh(qdisc_lock(qdisc));
 
+		if (!(qdisc->flags & TCQ_F_BUILTIN))
+			set_bit(__QDISC_STATE_DEACTIVATED, &qdisc->state);
+
 		dev_queue->qdisc = qdisc_default;
 		qdisc_reset(qdisc);