linux-kernel - Re: Deleting child qdisc doesn't reset parent to default qdisc?

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Date:	Thu, 14 Apr 2016 10:49:30 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jiri Kosina <jikos@...nel.org>
Cc:	Phil Sutter <phil@....cc>, Jamal Hadi Salim <jhs@...atatu.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Deleting child qdisc doesn't reset parent to default qdisc?

On Thu, 2016-04-14 at 18:08 +0200, Jiri Kosina wrote:
> On Thu, 14 Apr 2016, Phil Sutter wrote:
> 
> > > > I've came across the behavior where adding a child qdisc and then deleting 
> > > > it again makes the networking dysfunctional (I guess that's because all of 
> > > > a sudden there is absolutely no working qdisc on the device, although 
> > > > there originally was a default one in the parent).
> > > > 
> > > > In a nutshell, is this expected behavior or bug?
> > > 
> > > This is the expected behavior.
> > 
> > OTOH some qdiscs (CBQ, DRR, DSMARK, HFSC, HTB, QFQ) assign the default
> > one upon deletion instead of noop_qdisc, hence I would describe
> > the situation using the words 'inconsistent' and 'accident' rather than
> > 'expected'. :)
> 
> Would a patch that'd unify this in a sense that all qdiscs would assign 
> the default one upon deletion acceptable?
> 

And what would be the chosen behavior ?

Relying on TBF installing a bfifo for you at delete would be hazardous.

For example CBQ got it differently than HFSC

If qdisc_create_dflt() fails in CBQ, we fail the 'delete', while HFSC
falls back to noop_qdisc, without warning the user :(

At least always using noop_qdisc is consistent. No magic there.

Doing 'unification' right now would break existing scripts.

This is too late, I am afraid.