Re: Packet gets stuck in NOLOCK pfifo_fast qdisc

[Paolo Abeni]

On Wed, 2019-10-09 at 08:46 +0200, Jonas Bonn wrote:
> Hi,
> 
> The lockless pfifo_fast qdisc has an issue with packets getting stuck in 
> the queue.  What appears to happen is:
> 
> i)  Thread 1 holds the 'seqlock' on the qdisc and dequeues packets.
> ii)  Thread 1 dequeues the last packet in the queue.
> iii)  Thread 1 iterates through the qdisc->dequeue function again and 
> determines that the queue is empty.
> 
> iv)  Thread 2 queues up a packet.  Since 'seqlock' is busy, it just 
> assumes the packet will be dequeued by whoever is holding the lock.
> 
> v)  Thread 1 releases 'seqlock'.
> 
> After v), nobody will check if there are packets in the queue until a 
> new packet is enqueued.  Thereby, the packet enqueued by Thread 2 may be 
> delayed indefinitely.

I think you are right.

It looks like this possible race is present since the initial lockless
implementation - commit 6b3ba9146fe6 ("net: sched: allow qdiscs to
handle locking")

Anyhow the racing windows looks quite tiny - I never observed that
issue in my tests. Do you have a working reproducer?

Something alike the following code - completely untested - can possibly
address the issue, but it's a bit rough and I would prefer not adding
additonal complexity to the lockless qdiscs, can you please have a spin
a it?

Thanks,

Paolo
---
diff --git a/include/net/pkt_sched.h b/include/net/pkt_sched.h
index 6a70845bd9ab..65a1c03330d6 100644
--- a/include/net/pkt_sched.h
+++ b/include/net/pkt_sched.h
@@ -113,18 +113,23 @@ 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)
 {
+	int quota = 0;
+
 	if (qdisc_run_begin(q)) {
 		/* NOLOCK qdisc must check 'state' under the qdisc seqlock
 		 * to avoid racing with dev_qdisc_reset()
 		 */
 		if (!(q->flags & TCQ_F_NOLOCK) ||
 		    likely(!test_bit(__QDISC_STATE_DEACTIVATED, &q->state)))
-			__qdisc_run(q);
+			quota = __qdisc_run(q);
 		qdisc_run_end(q);
+
+		if (quota > 0 && q->flags & TCQ_F_NOLOCK && q->ops->peek(q))
+			__netif_schedule(q);
 	}
 }
 
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index 17bd8f539bc7..013480f6a794 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;
@@ -390,9 +390,10 @@ void __qdisc_run(struct Qdisc *q)
 		quota -= packets;
 		if (quota <= 0 || need_resched()) {
 			__netif_schedule(q);
-			break;
+			return 0;
 		}
 	}
+	return quota;
 }
 
 unsigned long dev_trans_start(struct net_device *dev)