[Bug 195458] Hang on shutdown/root unmount after FreeBSD 10.1R upgrade

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=195458

--- Comment #55 from Glen Barber <gjb at FreeBSD.org> ---
(In reply to Konstantin Belousov from comment #53)
> (In reply to Glen Barber from comment #52)
> It is physically impossible to hang on a line which is not loop.
> 

Yes, understood.

> I suspect that it is either the buffer flush code, or softdep worker thread
> which loop and cause shutdown thread to wait.  It is good that you have
> reproducable case and willing to move it further, previous reporters only
> bother to whine.
> 
> See
> https://www.freebsd.org/doc/en_US.ISO8859-1/books/developers-handbook/
> kerneldebug-deadlocks.html for the instructions on how to configure your
> kernel and what information to get.

The test machine now has INVARIANTS, INVARIANT_SUPPORT, WITNESS, DEBUG_LOCKS,
DEBUG_VFS_LOCKS, DIAGNOSTIC, and ALT_BREAK_TO_DEBUGGER.  Unfortunately, it
panics on boot now, so I cannot proceed to the 'freebsd-update install; reboot'
phase.

Just prior to this, I left out DIAGNOSTIC and saw a lock order reversal after
the "All buffers synced." message.  (I will provide screenshots in a separate
update.)

It looks like I will need to remove DIAGNOSTIC to get the system to boot.

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