pkt_sched: Make sure RTNL is held in qdisc_root_lock().

It is the only legal environment in which this can be
used.

Add some commentary explaining the situation.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sch_generic.h

@@ -193,10 +193,22 @@ static inline struct Qdisc *qdisc_root(struct Qdisc *qdisc)
 	return qdisc->dev_queue->qdisc;
 }
 
+/* The qdisc root lock is a mechanism by which to top level
+ * of a qdisc tree can be locked from any qdisc node in the
+ * forest.  This allows changing the configuration of some
+ * aspect of the qdisc tree while blocking out asynchronous
+ * qdisc access in the packet processing paths.
+ *
+ * It is only legal to do this when the root will not change
+ * on us.  Otherwise we'll potentially lock the wrong qdisc
+ * root.  This is enforced by holding the RTNL semaphore, which
+ * all users of this lock accessor must do.
+ */
 static inline spinlock_t *qdisc_root_lock(struct Qdisc *qdisc)
 {
 	struct Qdisc *root = qdisc_root(qdisc);
 
+	ASSERT_RTNL();
 	return qdisc_lock(root);
 }