netdev - Re: [RFC PATCH 06/17] net: sched: explicit locking in gso

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Date:   Wed, 15 Nov 2017 12:51:00 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Michael Ma <make0818@...il.com>,
        Network Development <netdev@...r.kernel.org>,
        Jiří Pírko <jiri@...nulli.us>,
        Cong Wang <xiyou.wangcong@...il.com>
Subject: Re: [RFC PATCH 06/17] net: sched: explicit locking in gso_cpu fallback

On Wed, Nov 15, 2017 at 10:11 AM, John Fastabend
<john.fastabend@...il.com> wrote:
> On 11/14/2017 04:41 PM, Willem de Bruijn wrote:
>>>  /* use instead of qdisc->dequeue() for all qdiscs queried with ->peek() */
>>>  static inline struct sk_buff *qdisc_dequeue_peeked(struct Qdisc *sch)
>>>  {
>>> -       struct sk_buff *skb = sch->gso_skb;
>>> +       struct sk_buff *skb = skb_peek(&sch->gso_skb);
>>>
>>>         if (skb) {
>>> -               sch->gso_skb = NULL;
>>> +               skb = __skb_dequeue(&sch->gso_skb);
>>>                 qdisc_qstats_backlog_dec(sch, skb);
>>>                 sch->q.qlen--;
>>
>> In lockless qdiscs, can this race, so that __skb_dequeue returns NULL?
>> Same for its use in qdisc_peek_dequeued.
>>
>
> Yes, agree if this was used in lockless qdisc it could race. However,
> I don't think it is actually used in the lockless cases yet. For pfifo
> fast __skb_array_peek is used.

Oh right. That will be easy to miss when other qdiscs are converted
to lockless. Perhaps another location to add lockdep annotations.

Related: what happens when pfifo_fast is used as a leaf in a non-lockless
qdisc hierarchy, say htb? The individual leaves will still have
TCQ_F_NOLOCK, so will try to take the qdisc lock in dequeue_skb
and other locations, but that is already held?

Thanks for revising and resubmitting the patchset, btw!