netdev - Re: [Patch net-next 4/5] net_sched: forbid setting default qdisc to inappropriate ones

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Message-ID: <CAM_iQpX4VFeEe4zGj0kx7x7q6GO7Kx+kKC2FW_CRiGSrk0aZVA@mail.gmail.com>
Date:	Thu, 27 Aug 2015 15:39:12 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: [Patch net-next 4/5] net_sched: forbid setting default qdisc to
 inappropriate ones

On Thu, Aug 27, 2015 at 3:30 PM, David Miller <davem@...emloft.net> wrote:
> I don't like this.
>
> The situation is that some sophisticated qdiscs can function without
> explicit parameters, some cannot.

This is exactly what this patch tries to solve... I already mark those
with a DEFAULTABLE flag.

>
> 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.

I wish I never mention that crash, which leads you to think I am trying
to fix a crash rather than a more important issue, usability. See below.

Forget about the crash, consider the current behavior:

# echo htb > default_qdisc
# succeed without any error
(then add a root qdisc and remove it)
# failure shown here in dmesg


And compare it with the behavior after my patch:

# echo htb > default_qdisc
Invalid arguments

I think this is clearly an improvement.

Thanks.
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