From: Paolo Abeni <pabeni@redhat.com>
To: Jonas Bonn <jonas.bonn@netrounds.com>,
"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
"David S . Miller" <davem@davemloft.net>,
John Fastabend <john.fastabend@gmail.com>
Subject: Re: Packet gets stuck in NOLOCK pfifo_fast qdisc
Date: Wed, 09 Oct 2019 21:14:07 +0200 [thread overview]
Message-ID: <95c5a697932e19ebd6577b5dac4d7052fe8c4255.camel@redhat.com> (raw)
In-Reply-To: <d102074f-7489-e35a-98cf-e2cad7efd8a2@netrounds.com>
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)
next prev parent reply other threads:[~2019-10-09 19:14 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-09 6:46 Packet gets stuck in NOLOCK pfifo_fast qdisc Jonas Bonn
2019-10-09 19:14 ` Paolo Abeni [this message]
2019-10-10 6:27 ` Jonas Bonn
2019-10-11 0:39 ` Jonas Bonn
2020-06-23 13:42 ` Michael Zhivich
2020-06-30 19:14 ` Josh Hunt
2020-07-01 7:53 ` Jonas Bonn
2020-07-01 16:05 ` Cong Wang
2020-07-01 19:58 ` Cong Wang
2020-07-01 22:02 ` Josh Hunt
2020-07-02 6:14 ` Jonas Bonn
2020-07-02 9:45 ` Paolo Abeni
2020-07-02 18:08 ` Josh Hunt
2020-07-07 14:18 ` Paolo Abeni
2020-07-08 20:16 ` Cong Wang
2020-07-09 9:20 ` Paolo Abeni
2020-07-08 20:33 ` Zhivich, Michael
2020-08-20 7:43 ` Jike Song
2020-08-20 18:13 ` Josh Hunt
[not found] ` <20200822032800.16296-1-hdanton@sina.com>
2020-08-25 2:18 ` Fengkehuan Feng
[not found] ` <20200825032312.11776-1-hdanton@sina.com>
2020-08-25 7:14 ` Fengkehuan Feng
[not found] ` <20200825162329.11292-1-hdanton@sina.com>
2020-08-26 2:38 ` Kehuan Feng
[not found] ` <CACS=qqKptAQQGiMoCs1Zgs9S4ZppHhasy1AK4df2NxnCDR+vCw@mail.gmail.com>
[not found] ` <5f46032e.1c69fb81.9880c.7a6cSMTPIN_ADDED_MISSING@mx.google.com>
2020-08-27 6:56 ` Kehuan Feng
[not found] ` <20200827125747.5816-1-hdanton@sina.com>
2020-08-28 1:45 ` Kehuan Feng
2020-09-03 5:01 ` Cong Wang
2020-09-03 8:39 ` Paolo Abeni
2020-09-03 17:43 ` Cong Wang
2020-09-04 5:07 ` John Fastabend
2020-09-10 20:15 ` Cong Wang
2020-09-10 21:07 ` John Fastabend
2020-09-10 21:40 ` Paolo Abeni
2021-04-02 19:25 ` Jiri Kosina
2021-04-02 19:33 ` Josh Hunt
[not found] ` <20210403003537.2032-1-hdanton@sina.com>
2021-04-03 12:23 ` Jiri Kosina
2021-04-06 0:55 ` Yunsheng Lin
2021-04-06 7:06 ` Michal Kubecek
2021-04-06 10:13 ` Juergen Gross
2021-04-06 12:17 ` Yunsheng Lin
2021-04-06 1:49 ` Cong Wang
2021-04-06 2:46 ` Yunsheng Lin
2021-04-06 7:31 ` Michal Kubecek
2021-04-06 12:24 ` Yunsheng Lin
[not found] ` <20200903101957.428-1-hdanton@sina.com>
2020-09-04 3:20 ` Kehuan Feng
2020-09-10 20:19 ` Cong Wang
2020-09-14 2:10 ` Yunsheng Lin
2020-09-17 19:52 ` Cong Wang
2020-09-18 2:06 ` Kehuan Feng
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