pkt_sched: Use qdisc_lock() on already sampled root qdisc.

Based upon a bug report by Jeff Kirsher.

Don't use qdisc_root_lock() in these cases as the root
qdisc could have been changed, and we'd thus lock the
wrong object.

Tested by Emil S Tantilov who confirms that this seems
to fix the problem.
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -1796,7 +1796,7 @@ int dev_queue_xmit(struct sk_buff *skb)
 	skb->tc_verd = SET_TC_AT(skb->tc_verd,AT_EGRESS);
 #endif
 	if (q->enqueue) {
-		spinlock_t *root_lock = qdisc_root_lock(q);
+		spinlock_t *root_lock = qdisc_lock(q);
 
 		spin_lock(root_lock);
 
@@ -1995,7 +1995,7 @@ static void net_tx_action(struct softirq_action *h)
 			smp_mb__before_clear_bit();
 			clear_bit(__QDISC_STATE_SCHED, &q->state);
 
-			root_lock = qdisc_root_lock(q);
+			root_lock = qdisc_lock(q);
 			if (spin_trylock(root_lock)) {
 				qdisc_run(q);
 				spin_unlock(root_lock);

net/sched/sch_generic.c

@@ -29,7 +29,7 @@
 /* Main transmission queue. */
 
 /* Modifications to data participating in scheduling must be protected with
- * qdisc_root_lock(qdisc) spinlock.
+ * qdisc_lock(qdisc) spinlock.
  *
  * The idea is the following:
  * - enqueue, dequeue are serialized via qdisc root lock
@@ -126,7 +126,7 @@ static inline int qdisc_restart(struct Qdisc *q)
 	if (unlikely((skb = dequeue_skb(q)) == NULL))
 		return 0;
 
-	root_lock = qdisc_root_lock(q);
+	root_lock = qdisc_lock(q);
 
 	/* And release qdisc */
 	spin_unlock(root_lock);
@@ -507,7 +507,7 @@ struct Qdisc * qdisc_create_dflt(struct net_device *dev,
 }
 EXPORT_SYMBOL(qdisc_create_dflt);
 
-/* Under qdisc_root_lock(qdisc) and BH! */
+/* Under qdisc_lock(qdisc) and BH! */
 
 void qdisc_reset(struct Qdisc *qdisc)
 {
@@ -543,7 +543,7 @@ static void __qdisc_destroy(struct rcu_head *head)
 	kfree((char *) qdisc - qdisc->padded);
 }
 
-/* Under qdisc_root_lock(qdisc) and BH! */
+/* Under qdisc_lock(qdisc) and BH! */
 
 void qdisc_destroy(struct Qdisc *qdisc)
 {
@@ -659,7 +659,7 @@ static bool some_qdisc_is_running(struct net_device *dev, int lock)
 
 		dev_queue = netdev_get_tx_queue(dev, i);
 		q = dev_queue->qdisc;
-		root_lock = qdisc_root_lock(q);
+		root_lock = qdisc_lock(q);
 
 		if (lock)
 			spin_lock_bh(root_lock);
@@ -735,7 +735,7 @@ static void shutdown_scheduler_queue(struct net_device *dev,
 	struct Qdisc *qdisc_default = _qdisc_default;
 
 	if (qdisc) {
-		spinlock_t *root_lock = qdisc_root_lock(qdisc);
+		spinlock_t *root_lock = qdisc_lock(qdisc);
 
 		dev_queue->qdisc = qdisc_default;
 		dev_queue->qdisc_sleeping = qdisc_default;