pkt_sched: Don't hold qdisc lock over qdisc_destroy().

Based upon reports by Denys Fedoryshchenko, and feedback
and help from Jarek Poplawski and Herbert Xu.

We always either:

1) Never made an external reference to this qdisc.

or

2) Did a dev_deactivate() which purged all asynchronous
   references.

So do not lock the qdisc when we call qdisc_destroy(),
it's illegal anyways as when we drop the lock this is
free'd memory.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_api.c

@@ -638,11 +638,8 @@ static void notify_and_destroy(struct sk_buff *skb, struct nlmsghdr *n, u32 clid
 	if (new || old)
 		qdisc_notify(skb, n, clid, old, new);
 
-	if (old) {
-		sch_tree_lock(old);
+	if (old)
 		qdisc_destroy(old);
-		sch_tree_unlock(old);
-	}
 }
 
 /* Graft qdisc "new" to class "classid" of qdisc "parent" or
@@ -1092,16 +1089,10 @@ static int tc_modify_qdisc(struct sk_buff *skb, struct nlmsghdr *n, void *arg)
 
 graft:
 	if (1) {
-		spinlock_t *root_lock;
-
 		err = qdisc_graft(dev, p, skb, n, clid, q, NULL);
 		if (err) {
-			if (q) {
-				root_lock = qdisc_root_lock(q);
-				spin_lock_bh(root_lock);
+			if (q)
 				qdisc_destroy(q);
-				spin_unlock_bh(root_lock);
-			}
 			return err;
 		}
 	}

net/sched/sch_generic.c

@@ -518,8 +518,6 @@ void qdisc_reset(struct Qdisc *qdisc)
 }
 EXPORT_SYMBOL(qdisc_reset);
 
-/* Under qdisc_lock(qdisc) and BH! */
-
 void qdisc_destroy(struct Qdisc *qdisc)
 {
 	const struct Qdisc_ops  *ops = qdisc->ops;
@@ -712,14 +710,10 @@ static void shutdown_scheduler_queue(struct net_device *dev,
 	struct Qdisc *qdisc_default = _qdisc_default;
 
 	if (qdisc) {
-		spinlock_t *root_lock = qdisc_lock(qdisc);
-
 		dev_queue->qdisc = qdisc_default;
 		dev_queue->qdisc_sleeping = qdisc_default;
 
-		spin_lock_bh(root_lock);
 		qdisc_destroy(qdisc);
-		spin_unlock_bh(root_lock);
 	}
 }