how to debug problem with "failing unmounting"

[Tomek The Messenger]

Hi
On the soc on which I work there is issue with unmouting some
partitions/directories during /sbin/reboot, here is some example:

[   OK   ]  Stopped target Local File System.
Unmouting Temporary Directory (/tmp)...
Unmouting /run/user/0...
...
[   FAILED  ]  Failed unmouting Temporary Directory (/tmp).
[   FAILED  ]  Failed unmouting  /run/user/0.
....

Unfortunately like it is typical in kernel information about why
unmouting failed doesn't appear.  So seems like searching in darkness what
is going on. Is any way how to search in darkness? Firstly I should make
sure /tmp or /run/user/0 is really a partition and was mount at linux
startup. Then maybe I should check access rights if it is rwx. What else
can be tried here?
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