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

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Date:   Tue, 14 Nov 2017 21:04:01 -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>, 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

>> -static inline int dev_requeue_skb(struct sk_buff *skb, struct Qdisc *q)
>> +static inline int __dev_requeue_skb(struct sk_buff *skb, struct Qdisc *q)
>>  {
>
> Perhaps dev_requeue_skb_qdisc_locked is more descriptive. Or
> adding a lockdep_is_held(..) also documents that the __locked variant
> below is not just a lock/unlock wrapper around this inner function.
>
>> -       q->gso_skb = skb;
>> +       __skb_queue_head(&q->gso_skb, skb);
>>         q->qstats.requeues++;
>>         qdisc_qstats_backlog_inc(q, skb);
>>         q->q.qlen++;    /* it's still part of the queue */
>> @@ -57,6 +56,30 @@ static inline int dev_requeue_skb(struct sk_buff *skb, struct Qdisc *q)
>>         return 0;
>>  }
>>
>> +static inline int dev_requeue_skb_locked(struct sk_buff *skb, struct Qdisc *q)
>> +{
>> +       spinlock_t *lock = qdisc_lock(q);
>> +
>> +       spin_lock(lock);
>> +       __skb_queue_tail(&q->gso_skb, skb);
>
> why does this requeue at the tail, unlike __dev_requeue_skb?

I guess that requeue has to queue at the tail in the lockless case,
and it does not matter in the qdisc_locked case, as then there can
only ever be at most one outstanding gso_skb.