[Linuxarm] Re: [RFC v2] net: sched: implement TCQ_F_CAN_BYPASS for lockless qdisc

On Mon, 15 Mar 2021 11:10:18 +0800 Yunsheng Lin wrote:

@@ -606,6 +623,11 @@ static const u8 prio2band[TC_PRIO_MAX + 1] = { */ struct pfifo_fast_priv { struct skb_array q[PFIFO_FAST_BANDS];

• /* protect against data race between enqueue/dequeue and

• * qdisc->empty setting
  

• */
  

• spinlock_t lock;

};

static inline struct skb_array *band2list(struct pfifo_fast_priv *priv, @@ -623,7 +645,10 @@ static int pfifo_fast_enqueue(struct sk_buff *skb, struct Qdisc *qdisc, unsigned int pkt_len = qdisc_pkt_len(skb); int err;

• err = skb_array_produce(q, skb);

• spin_lock(&priv->lock);

• err = __ptr_ring_produce(&q->ring, skb);

• WRITE_ONCE(qdisc->empty, false);

• spin_unlock(&priv->lock);

  if (unlikely(err)) { if (qdisc_is_percpu_stats(qdisc))

@@ -642,6 +667,7 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc) struct sk_buff *skb = NULL; int band;

• spin_lock(&priv->lock); for (band = 0; band < PFIFO_FAST_BANDS && !skb; band++) { struct skb_array *q = band2list(priv, band);

@@ -655,6 +681,7 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc) } else { WRITE_ONCE(qdisc->empty, true); }

• spin_unlock(&priv->lock);

  return skb;

}

I thought pfifo was supposed to be "lockless" and this change re-introduces a lock between producer and consumer, no?