Re: [BUG] pfifo_fast may cause out-of-order CAN frame transmission

[Paolo Abeni <pabeni@redhat.com>]

On Thu, 2020-01-09 at 18:39 +0100, Ahmad Fatoum wrote:
> Hello Paolo,
> 
> On 1/9/20 1:51 PM, Paolo Abeni wrote:
> > On Wed, 2020-01-08 at 15:55 +0100, Ahmad Fatoum wrote:
> > > I've run into an issue of CAN frames being sent out-of-order on an i.MX6 Dual
> > > with Linux v5.5-rc5. Bisecting has lead me down to this commit:
> > 
> > Thank you for the report.
> 
> Thanks for the prompt patch. :-)
> 
> > The code is only build-tested, could you please try it in your setup?
> 
> Issue still persists, albeit appears to have become much less frequent. Took 2 million
> frames till first two were swapped. What I usually saw was a swap every few thousand
> frames at least and quite often more frequent than that. Might just be noise though.

Thank you for testing. Even with the proposed patch there is still a
possible race condition: the CPU holding the seqlock can clear the
'empty' flag after that the CPU xmitting the packet enqueue it and set
the 'empty' flag.

The only option I can think of - beyond plain revert - is updating the
'empty' flag in a even a more coarse way, as in the following patch.

Again, the code only build tested and very rough, but it would be
helpful if you could give it a spin.

Thank you!

Paolo

---
diff --git a/include/net/pkt_sched.h b/include/net/pkt_sched.h
index 6a70845bd9ab..fb365fbf65f8 100644
--- a/include/net/pkt_sched.h
+++ b/include/net/pkt_sched.h
@@ -113,7 +113,7 @@ bool sch_direct_xmit(struct sk_buff *skb, struct Qdisc *q,
 		     struct net_device *dev, struct netdev_queue *txq,
 		     spinlock_t *root_lock, bool validate);
 
-void __qdisc_run(struct Qdisc *q);
+int __qdisc_run(struct Qdisc *q);
 
 static inline void qdisc_run(struct Qdisc *q)
 {
diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index fceddf89592a..df460fe0773a 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -158,7 +158,6 @@ static inline bool qdisc_run_begin(struct Qdisc *qdisc)
 	if (qdisc->flags & TCQ_F_NOLOCK) {
 		if (!spin_trylock(&qdisc->seqlock))
 			return false;
-		WRITE_ONCE(qdisc->empty, false);
 	} else if (qdisc_is_running(qdisc)) {
 		return false;
 	}
diff --git a/net/core/dev.c b/net/core/dev.c
index 0ad39c87b7fd..b6378bb7b64a 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3624,10 +3624,22 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
 end_run:
 			qdisc_run_end(q);
 		} else {
+			int quota = 0;
+
 			rc = q->enqueue(skb, q, &to_free) & NET_XMIT_MASK;
-			qdisc_run(q);
+			if (!qdisc_run_begin(q))
+				goto out;
+
+			WRITE_ONCE(q->empty, false);
+			if (likely(!test_bit(__QDISC_STATE_DEACTIVATED,
+					     &q->state)))
+				quota = __qdisc_run(q);
+			if (quota > 0)
+				WRITE_ONCE(q->empty, true);
+			qdisc_run_end(q);
 		}
 
+out:
 		if (unlikely(to_free))
 			kfree_skb_list(to_free);
 		return rc;
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 5ab696efca95..1bd2c4e9c4c2 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -376,7 +376,7 @@ static inline bool qdisc_restart(struct Qdisc *q, int *packets)
 	return sch_direct_xmit(skb, q, dev, txq, root_lock, validate);
 }
 
-void __qdisc_run(struct Qdisc *q)
+int __qdisc_run(struct Qdisc *q)
 {
 	int quota = dev_tx_weight;
 	int packets;
@@ -388,6 +388,7 @@ void __qdisc_run(struct Qdisc *q)
 			break;
 		}
 	}
+	return quota;
 }
 
 unsigned long dev_trans_start(struct net_device *dev)
@@ -649,12 +650,9 @@ static struct sk_buff *pfifo_fast_dequeue(struct Qdisc *qdisc)
 
 		skb = __skb_array_consume(q);
 	}
-	if (likely(skb)) {
-		qdisc_update_stats_at_dequeue(qdisc, skb);
-	} else {
-		WRITE_ONCE(qdisc->empty, true);
-	}
 
+	if (likely(skb))
+		qdisc_update_stats_at_dequeue(qdisc, skb);
 	return skb;
 }