netdev - Re: [RGC PATCH v3.10] net: sched: validate that class is found in qdisc_tree_decrease

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Date:	Fri, 17 Jul 2015 17:10:51 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Alex Gartrell <agartrell@...com>
Cc:	Jamal Hadi Salim <jhs@...atatu.com>,
	netdev <netdev@...r.kernel.org>,
	David Miller <davem@...emloft.net>, kernel-team@...com
Subject: Re: [RGC PATCH v3.10] net: sched: validate that class is found in qdisc_tree_decrease_qlen

On Fri, Jul 17, 2015 at 4:27 PM, Alex Gartrell <agartrell@...com> wrote:
> To my understanding, the following situation is taking place.
>
>   tc_ctl_tclass
>    -> htb_delete
>      -> class is deleted from clhash
>    -> htb_put
>      -> qdisc_destroy
>        -> fq_codel_reset
>          -> fq_codel_dequeue
>            -> qdidsc_tree_decrease_qlen
>              -> cl = htb_get # returns NULL, removed in htb_delete
>                -> htb_qlen_notify(sch, NULL) # BOOM
>
> This patch checks cl for 0 before invoking qlen_notify and put.  The notify
> is not necessary in this case, as the parent has already been deactivated
> anyway.
>

Hmm, but in htb_delete() we do reset the leaf qdisc before removing the
class from hash:

        if (!cl->level) {
                qlen = cl->un.leaf.q->q.qlen;
                qdisc_reset(cl->un.leaf.q);
                qdisc_tree_decrease_qlen(cl->un.leaf.q, qlen);
        }

therefore, the leaf fq_codel qdisc is supposed to have 0 skb after that,
that is, the second fq_codel_reset() is supposed to return NULL immediately?
Why is qdidsc_tree_decrease_qlen() called in the second fq_codel_reset()?

Or there is a race condition between ->delete() and ->put()? In which new
skb can be enqueued?
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