netdev - Re: [PATCH net] net_sched: prio: rollback allocations if prio

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Message-ID: <1465838004.7945.167.camel@edumazet-glaptop3.roam.corp.google.com>
Date:	Mon, 13 Jun 2016 10:13:24 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net_sched: prio: rollback allocations if
 prio_init() fails

On Mon, 2016-06-13 at 09:28 -0700, Cong Wang wrote:
> On Sun, Jun 12, 2016 at 10:03 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > From: Eric Dumazet <edumazet@...gle.com>
> >
> > Now prio_init() can return -ENOMEM, it also has to make sure
> > any allocated qdisc are freed, since the caller (qdisc_create()) wont
> > call ->destroy() handler for us.
> 
> 
> But prio_tune() is called by ->change() too, so just call prio_destroy()
> inside prio_tune() ?

Because the following should work :

tc qdisc replace dev eth0 root prio bands 3

(later...)

tc qdisc change dev eth0 root prio bands 5 // memory allocation failure

We must not destroy the qdisc on a change operation if it fails.

Only the init should clean the qdisc state/memory/children since qdisc
wont be created for real.

Presumably we should commit changes on qdisc only if the whole
->change() succeeded, but I guess lot of qdisc are buggy in this
respect.