> Here is the output of the kernel when the "unregister_netdevice" bug
was hit

Thanks!  And just to verify, that's the full output of all those lines
before the next "waiting..." line, right?  If so, it looks like the
problem in your case is not a kernel socket holding the netns open, it
looks like a good-old dst leak, or something else holding a dst, so I'll
need to add debug tracking those now...

Also, when you reproduce the problem, if you leave it for a while (like,
5 minutes or so), does it resolve itself?  Or stay hung and printing the
"waiting..." messages forever?  Make sure to check the netns addr in the
"waiting..." message to verify it's stuck waiting for the *same* netns
to close forever, not just reproducing the problem over and over with
different netns.

I'll get another kernel ready with additional dst debug.  I think there
is at least one dst leak patch upstream too, I'll check for those.

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https://bugs.launchpad.net/bugs/1711407

Title:
  unregister_netdevice: waiting for lo to become free

Status in linux package in Ubuntu:
  In Progress
Status in linux source package in Trusty:
  In Progress
Status in linux source package in Xenial:
  In Progress
Status in linux source package in Zesty:
  In Progress
Status in linux source package in Artful:
  In Progress
Status in linux source package in Bionic:
  In Progress

Bug description:
  This is a "continuation" of bug 1403152, as that bug has been marked
  "fix released" and recent reports of failure may (or may not) be a new
  bug.  Any further reports of the problem should please be reported
  here instead of that bug.

  --

  [Impact]

  When shutting down and starting containers the container network
  namespace may experience a dst reference counting leak which results
  in this message repeated in the logs:

      unregister_netdevice: waiting for lo to become free. Usage count =
  1

  This can cause issues when trying to create net network namespace and
  thus block a user from creating new containers.

  [Test Case]

  See comment 16, reproducer provided at https://github.com/fho/docker-
  samba-loop

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