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

On Tue, Mar 16, 2021 at 1:35 AM Yunsheng Lin [off-list ref] 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);

Maybe it is time to fully document/explain how this can possibly work.

lockless qdisc used concurrently by multiple cpus, using
WRITE_ONCE() and READ_ONCE() ?

Just say no to this.

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?