From: Cong Wang <redacted>
Date: Wed, 26 Aug 2015 15:41:26 -0700

Currently there is no check for if a qdisc is appropriate
to be used as the default qdisc. This causes we get no
error even we set the default qdisc to an inappropriate one
but an error will be shown up later. This is not good.

Also, for qdisc's like HTB, kernel will just crash when
we use it as default qdisc, because some data structures are
not even initialized yet before checking opt == NULL, the cleanup
doing ->reset() or ->destroy() on them will just crash.

Let's fail as early as we can.

Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Cong Wang <redacted>

I don't like this.

The situation is that some sophisticated qdiscs can function without
explicit parameters, some cannot.

That is the problem you need to solve.  For example, if "opts" is NULL
HTB should use a reasonable set of defaults instead of failing.

Furthermore, you can improve the behavior when this happens.

When qdisc_create_dflt() returns NULL because ops->init() fails, do
something reasonable.

I'm not applying this patch series, it papers over the issue rather
than actually addressing it properly.