Re: Packet gets stuck in NOLOCK pfifo_fast qdisc
From: Jonas Bonn <hidden>
Date: 2019-10-10 06:28:03
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Hi Paolo, On 09/10/2019 21:14, Paolo Abeni wrote:
On Wed, 2019-10-09 at 08:46 +0200, Jonas Bonn wrote:quoted
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?
Yes, it's reliably reproducible. We do network latency measurements and latency spikes for these packets that get stuck in the queue.
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?
Your change looks reasonable. I'll give it a try.
quoted hunk ↗ jump to hunk
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);
Not sure this is relevant, but there's a subtle difference in the way that the underlying ptr_ring peeks at the queue head and checks whether the queue is empty. For peek it's: READ_ONCE(r->queue[r->consumer_head]); For is_empty it's: !r->queue[READ_ONCE(r->consumer_head)]; The placement of the READ_ONCE changes here. I can't get my head around whether this difference is significant or not. If it is, then perhaps an is_empty() method is needed on the qdisc_ops...??? /Jonas