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