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>
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index b5f40d7..c5bb130 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -193,10 +193,22 @@
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);
}