Thread (6 messages) 6 messages, 4 authors, 2020-10-07

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

From: Ahmad Fatoum <a.fatoum@pengutronix.de>
Date: 2020-01-20 16:06:20
Also in: netdev

Possibly related (same subject, not in this thread)

Hello Paolo,

On 1/16/20 1:40 PM, Paolo Abeni wrote:
I'm sorry for this trial & error experience. I tried to reproduce the
issue on top of the vcan virtual device, but it looks like it requires
the timing imposed by a real device, and it's missing here (TL;DR: I
can't reproduce the issue locally).
No worries. I don't mind testing.
Code wise, the 2nd patch closed a possible race, but it dumbly re-
opened the one addressed by the first attempt - the 'empty' field must
be cleared prior to the trylock operation, or we may end-up with such
field set and the queue not empty.

So, could you please try the following code?
Unfortunately, I still see observe reodering.

Thanks
Ahmad
quoted hunk
Many thanks!
---
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..41e89796cc6b 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -3624,10 +3624,23 @@ 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 (READ_ONCE(q->empty))
+				WRITE_ONCE(q->empty, false);
+			if (!qdisc_run_begin(q))
+				goto out;
+
+			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;
 }
 
-- 
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