[Bug 282449] UFS deadlock with install -C during freebsd-update

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=282449

            Bug ID: 282449
           Summary: UFS deadlock with install -C during freebsd-update
           Product: Base System
           Version: Unspecified
          Hardware: amd64
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: kern
          Assignee: bugs@FreeBSD.org
          Reporter: Grau_Smue@md5hashing.net

Created attachment 254825
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=254825&action=edit
procstat -kka on the system with hung sync and install process

I have experienced deadlock requiring reset a couple of times during
freebsd-update.  The install -C command gets stuck in wdrain.  A sync executed
at this time also never returns.

I found a thread on forums.freebsd.org which seems to be the same problem,
however it also seems to be implicated in extremely slow freebsd-update when
the filesystem is ZFS. 
https://forums.freebsd.org/threads/freebsd-13-2-release-14-0-release-upgrade-stuck.91152/

I eventually was able to reproduce the problem and get a procstat -kka, which
is attached.  I can not say that it is reproducible on demand, but I can
certainly try if additional debug is needed.  This was about my 30th try, and
the last change was going sync=standard(prior disabled) on the underlying host
ZFS.

This was on 13.3-p7, going to 13.4-RELEASE.  However, it has been experienced
under other kernels earlier and later for the past year.  It seems to be
hardware independent, and I have run into it on a couple of VMs, and a VPS with
varying cpu count and disk controllers.  This particular case is VirtualBox
6.1.50, 1 cpu, ahci, on top of a ZFS with sync=standard and no hostiocache in
vbox.  Hostsystem is same kernel, on ahci supermicro with spinner.

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