On 2021/3/16 8:35, Yunsheng Lin wrote:

On 2021/3/16 2:53, Jakub Kicinski wrote:

quoted

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

quoted

@@ -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?

Yes, the lock breaks the "lockless" of the lockless qdisc for now
I do not how to solve the below data race locklessly:

	CPU1:					CPU2:
      dequeue skb				 .
	  .				    	 .	
	  .				    enqueue skb
	  .					 .
	  .			 WRITE_ONCE(qdisc->empty, false);
	  .					 .
	  .					 .
WRITE_ONCE(qdisc->empty, true);

If the above happens, the qdisc->empty is true even if the qdisc has some
skb, which may cuase out of order or packet stuck problem.

It seems we may need to update ptr_ring' status(empty or not) while
enqueuing/dequeuing atomically in the ptr_ring implementation.

Any better idea?

It seems we can use __ptr_ring_empty() within the qdisc->seqlock protection,
because qdisc->seqlock is clearly served as r->consumer_lock.

quoted

.

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