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Date: Tue, 6 Apr 2021 09:06:59 +0200 From: Michal Kubecek <mkubecek@...e.cz> To: Yunsheng Lin <linyunsheng@...wei.com> Cc: Jiri Kosina <jikos@...nel.org>, Hillf Danton <hdanton@...a.com>, John Fastabend <john.fastabend@...il.com>, Cong Wang <xiyou.wangcong@...il.com>, Paolo Abeni <pabeni@...hat.com>, Kehuan Feng <kehuan.feng@...il.com>, Jike Song <albcamus@...il.com>, Jonas Bonn <jonas.bonn@...rounds.com>, Michael Zhivich <mzhivich@...mai.com>, David Miller <davem@...emloft.net>, LKML <linux-kernel@...r.kernel.org>, Netdev <netdev@...r.kernel.org>, Josh Hunt <johunt@...mai.com> Subject: Re: Packet gets stuck in NOLOCK pfifo_fast qdisc 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
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