On Wed, Nov 15, 2017 at 10:11 AM, John Fastabend
<john.fastab...@gmail.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!
