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>

1 files changed, 5 insertions, 2 deletions

diff --git a/net/sched/sch_cbq.c b/net/sched/sch_cbq.c
index 9e43ed949167..d728d8111732 100644
--- a/net/sched/sch_cbq.c
+++ b/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;
 }