Messages in this thread Patch in this message | | | Subject | Re: Packet gets stuck in NOLOCK pfifo_fast qdisc | From | Paolo Abeni <> | Date | Wed, 09 Oct 2019 21:14:07 +0200 |
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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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