net_sched: avoid resetting active qdisc for multiple times

Except for sch_mq and sch_mqprio, each dev queue points to the
same root qdisc, so when we reset the dev queues with
netdev_for_each_tx_queue() we end up resetting the same instance
of the root qdisc for multiple times.

Avoid this by checking the __QDISC_STATE_DEACTIVATED bit in
each iteration, so for sch_mq/sch_mqprio, we still reset all
of them like before, for the rest, we only reset it once.
Reported-by: Václav Zindulka <vaclav.zindulka@tlapnet.cz>
Tested-by: Václav Zindulka <vaclav.zindulka@tlapnet.cz>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_generic.c

@@ -1128,30 +1128,39 @@ void dev_activate(struct net_device *dev)
 }
 EXPORT_SYMBOL(dev_activate);
 
-static void dev_deactivate_queue(struct net_device *dev,
-				 struct netdev_queue *dev_queue,
-				 void *_qdisc_default)
+static void qdisc_deactivate(struct Qdisc *qdisc)
 {
-	struct Qdisc *qdisc_default = _qdisc_default;
-	struct Qdisc *qdisc;
-
-	qdisc = rtnl_dereference(dev_queue->qdisc);
-	if (qdisc) {
 	bool nolock = qdisc->flags & TCQ_F_NOLOCK;
 
+	if (qdisc->flags & TCQ_F_BUILTIN)
+		return;
+	if (test_bit(__QDISC_STATE_DEACTIVATED, &qdisc->state))
+		return;
+
 	if (nolock)
 		spin_lock_bh(&qdisc->seqlock);
 	spin_lock_bh(qdisc_lock(qdisc));
 
-		if (!(qdisc->flags & TCQ_F_BUILTIN))
 	set_bit(__QDISC_STATE_DEACTIVATED, &qdisc->state);
 
-		rcu_assign_pointer(dev_queue->qdisc, qdisc_default);
 	qdisc_reset(qdisc);
 
 	spin_unlock_bh(qdisc_lock(qdisc));
 	if (nolock)
 		spin_unlock_bh(&qdisc->seqlock);
+}
+
+static void dev_deactivate_queue(struct net_device *dev,
+				 struct netdev_queue *dev_queue,
+				 void *_qdisc_default)
+{
+	struct Qdisc *qdisc_default = _qdisc_default;
+	struct Qdisc *qdisc;
+
+	qdisc = rtnl_dereference(dev_queue->qdisc);
+	if (qdisc) {
+		qdisc_deactivate(qdisc);
+		rcu_assign_pointer(dev_queue->qdisc, qdisc_default);
 	}
 }