On 2021/6/2 4:48, Jakub Kicinski wrote:

On Tue, 1 Jun 2021 16:18:54 +0800 Yunsheng Lin wrote:

quoted

quoted

I see, thanks! That explains the need. Perhaps we can rephrase the
comment? Maybe:

+			/* Retest nolock_qdisc_is_empty() within the protection
+			 * of q->seqlock to protect from racing with requeuing.
+			 */  

Yes if we still decide to preserve the nolock_qdisc_is_empty() rechecking
under q->seqlock.

Sounds good.

quoted

quoted

quoted

--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c

@@ -38,6 +38,15 @@ EXPORT_SYMBOL(default_qdisc_ops);
 static void qdisc_maybe_clear_missed(struct Qdisc *q,
                                     const struct netdev_queue *txq)
 {
+       set_bit(__QDISC_STATE_DRAINING, &q->state);
+
+       /* Make sure DRAINING is set before clearing MISSED
+        * to make sure nolock_qdisc_is_empty() always return
+        * false for aoviding transmitting a packet directly
+        * bypassing the requeued packet.
+        */
+       smp_mb__after_atomic();
+
        clear_bit(__QDISC_STATE_MISSED, &q->state);

        /* Make sure the below netif_xmit_frozen_or_stopped()

@@ -52,8 +61,6 @@ static void qdisc_maybe_clear_missed(struct Qdisc *q,
         */
        if (!netif_xmit_frozen_or_stopped(txq))
                set_bit(__QDISC_STATE_MISSED, &q->state);
-       else
-               set_bit(__QDISC_STATE_DRAINING, &q->state);
 }  

But this would not be enough because we may also clear MISSING 
in pfifo_fast_dequeue()?  

For the MISSING clearing in pfifo_fast_dequeue(), it seems it
looks like the data race described in RFC v3 too?

      CPU1                 CPU2               CPU3
qdisc_run_begin(q)          .                  .
        .              MISSED is set           .
  MISSED is cleared         .                  .
    q->dequeue()            .                  .
        .              enqueue skb1     check MISSED # true
qdisc_run_end(q)            .                  .
        .                   .         qdisc_run_begin(q) # true
        .            MISSED is set      send skb2 directly

Not sure what you mean.

       CPU1                 CPU2               CPU3
 qdisc_run_begin(q)          .                  .
         .              MISSED is set           .
   MISSED is cleared         .                  .
   another dequeuing         .                  .
         .                   .                  .
         .              enqueue skb1  nolock_qdisc_is_empty() # true
 qdisc_run_end(q)            .                  .
         .                   .         qdisc_run_begin(q) # true
         .                   .          send skb2 directly
         .               MISSED is set          .

As qdisc is indeed empty at the point when MISSED is clear and
another dequeue is retried by CPU1, MISSED setting is not under
q->seqlock, so it seems retesting MISSED under q->seqlock does not
seem to make any difference? and it seems like the case that does
not need handling as we agreed previously?

.