pkt_sched: Change misleading code in class delete.

While looking for a possible reason of bugzilla report on HTB oops:
http://bugzilla.kernel.org/show_bug.cgi?id=12858
I found the code in htb_delete calling htb_destroy_class on zero
refcount is very misleading: it can suggest this is a common path, and
destroy is called under sch_tree_lock. Actually, this can never happen
like this because before deletion cops->get() is done, and after
delete a class is still used by tclass_notify. The class destroy is
always called from cops->put(), so without sch_tree_lock.

This doesn't mean much now (since 2.6.27) because all vulnerable calls
were moved from htb_destroy_class to htb_delete, but there was a bug
in older kernels. The same change is done for other classful scheds,
which, it seems, didn't have similar locking problems here.
Reported-by: m0sia <m0sia@m0sia.ru>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_cbq.c

@@ -1960,8 +1960,11 @@ static int cbq_delete(struct Qdisc *sch, unsigned long arg)
 	cbq_rmprio(q, cl);
 	sch_tree_unlock(sch);
 
-	if (--cl->refcnt == 0)
-		cbq_destroy_class(sch, cl);
+	BUG_ON(--cl->refcnt == 0);
+	/*
+	 * This shouldn't happen: we "hold" one cops->get() when called
+	 * from tc_ctl_tclass; the destroy method is done from cops->put().
+	 */
 
 	return 0;
 }

net/sched/sch_drr.c

@@ -155,8 +155,11 @@ static int drr_delete_class(struct Qdisc *sch, unsigned long arg)
 	drr_purge_queue(cl);
 	qdisc_class_hash_remove(&q->clhash, &cl->common);
 
-	if (--cl->refcnt == 0)
-		drr_destroy_class(sch, cl);
+	BUG_ON(--cl->refcnt == 0);
+	/*
+	 * This shouldn't happen: we "hold" one cops->get() when called
+	 * from tc_ctl_tclass; the destroy method is done from cops->put().
+	 */
 
 	sch_tree_unlock(sch);
 	return 0;

net/sched/sch_hfsc.c

@@ -1139,8 +1139,11 @@ hfsc_delete_class(struct Qdisc *sch, unsigned long arg)
 	hfsc_purge_queue(sch, cl);
 	qdisc_class_hash_remove(&q->clhash, &cl->cl_common);
 
-	if (--cl->refcnt == 0)
-		hfsc_destroy_class(sch, cl);
+	BUG_ON(--cl->refcnt == 0);
+	/*
+	 * This shouldn't happen: we "hold" one cops->get() when called
+	 * from tc_ctl_tclass; the destroy method is done from cops->put().
+	 */
 
 	sch_tree_unlock(sch);
 	return 0;

net/sched/sch_htb.c

@@ -1275,8 +1275,11 @@ static int htb_delete(struct Qdisc *sch, unsigned long arg)
 	if (last_child)
 		htb_parent_to_leaf(q, cl, new_q);
 
-	if (--cl->refcnt == 0)
-		htb_destroy_class(sch, cl);
+	BUG_ON(--cl->refcnt == 0);
+	/*
+	 * This shouldn't happen: we "hold" one cops->get() when called
+	 * from tc_ctl_tclass; the destroy method is done from cops->put().
+	 */
 
 	sch_tree_unlock(sch);
 	return 0;