Comment 9 for bug 1801349

Additionally, after the umount, the jbd kernel thread is still running for the block device, which is why it can't be claimed.

$ sudo ps ax | grep jbd
  333 ? S 0:00 [jbd2/vda1-8]
  824 ? S 0:00 [jbd2/vdb-8]
                                 ^^^^^
                                 This shouldn't be running.

However, nothing, anywhere, *seems* to be holding it. Kernel bug?

sudo grep -h vdb /proc/*/task/*/mountinfo

gives no output.