I have been running into this (curtin 18.1-17-gae48e86f-
0ubuntu1~16.04.1)

I think this commit basically agrees with my thoughts but I just wanted
to share them explicitly in case they are interesting

 (1) If you *unregister* the cache device from the backing device, it
first has to purge all the dirty data back to the backing device. This
may obviously take a while.

 (2) When doing that, I managed to deadlock bcache at least once on
xenial-hwe 4.15 where it was trying to reclaim memory from XFS, which I
assume was trying to write to the bcache.. traceback:
https://pastebin.canonical.com/117528/ - you can't get out of that
without a reboot

 (3) However generally I had good luck simplying "stop"ing the cache
devices (it seems perhaps that is what this bug is designed to do,
switch to stop, instead of unregister?). Specifically though I was
stopping the backing devices, and then later the cache device. It seems
like the current commit is the other way around?

** Tags added: sts

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https://bugs.launchpad.net/bugs/1796292

Title:
  Tight timeout for bcache removal causes spurious failures

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