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Date:   Tue, 6 Apr 2021 09:06:59 +0200
From:   Michal Kubecek <mkubecek@suse.cz>
To:     Yunsheng Lin <linyunsheng@huawei.com>
Cc:     Jiri Kosina <jikos@kernel.org>, Hillf Danton <hdanton@sina.com>,
        John Fastabend <john.fastabend@gmail.com>,
        Cong Wang <xiyou.wangcong@gmail.com>,
        Paolo Abeni <pabeni@redhat.com>,
        Kehuan Feng <kehuan.feng@gmail.com>,
        Jike Song <albcamus@gmail.com>,
        Jonas Bonn <jonas.bonn@netrounds.com>,
        Michael Zhivich <mzhivich@akamai.com>,
        David Miller <davem@davemloft.net>,
        LKML <linux-kernel@vger.kernel.org>,
        Netdev <netdev@vger.kernel.org>, Josh Hunt <johunt@akamai.com>
Subject: Re: Packet gets stuck in NOLOCK pfifo_fast qdisc
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On Tue, Apr 06, 2021 at 08:55:41AM +0800, Yunsheng Lin wrote:
> 
> Hi, Jiri
> Do you have a reproducer that can be shared here?
> With reproducer, I can debug and test it myself too.

I'm afraid we are not aware of a simple reproducer. As mentioned in the
original discussion, the race window is extremely small and the other
thread has to do quite a lot in the meantime which is probably why, as
far as I know, this was never observed on real hardware, only in
virtualization environments. NFS may also be important as, IIUC, it can
often issue an RPC request from a different CPU right after a data
transfer. Perhaps you could cheat a bit and insert a random delay
between the empty queue check and releasing q->seqlock to make it more
likely to happen.

Other than that, it's rather just "run this complex software in a xen VM
and wait".

Michal