From: Jonas Bonn <jonas.bonn@netrounds.com>
To: "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
"David S . Miller" <davem@davemloft.net>,
pabeni@redhat.com
Subject: Packet gets stuck in NOLOCK pfifo_fast qdisc
Date: Wed, 9 Oct 2019 08:46:19 +0200 [thread overview]
Message-ID: <d102074f-7489-e35a-98cf-e2cad7efd8a2@netrounds.com> (raw)
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.
What, I think, should probably happen is that Thread 1 should check that
the queue is empty again after releasing 'seqlock'. I poked at this,
but it wasn't obvious to me how to go about this given the way the
layering works here. Roughly:
qdisc_run_end() {
...
spin_unlock(seqlock);
if (!qdisc_is_empty(qdisc))
qdisc_run();
...
}
Calling qdisc_run() from qdisc_run_end() doesn't feel right!
There's a qdisc->empty property (and qdisc_is_empty() relies on it) but
it's not particularly useful in this case since there's a race in
setting this property which makes it not quite reliable.
Hope someone can shine some light on how to proceed here.
/Jonas
next reply other threads:[~2019-10-09 6:46 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-09 6:46 Jonas Bonn [this message]
2019-10-09 19:14 ` Packet gets stuck in NOLOCK pfifo_fast qdisc Paolo Abeni
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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