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SubjectRe: Packet gets stuck in NOLOCK pfifo_fast qdisc
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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)
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