As discussed on IRC earlier today.
```
$ reboot
...
lock order reversal:
1st 0xfffff80003999830 ufs (ufs, lockmgr) @ /usr/home/conrad/src/freebsd/sys/kern/vfs_mount.c:1696
2nd 0xfffff8000399c640 devfs (devfs, lockmgr) @ /usr/home/conrad/src/freebsd/sys/kern/vfs_subr.c:2892
Lock order devfs -> ufs established at:
#0 0xffffffff80c2cbc0 at witness_checkorder+0x460
#1 0xffffffff80b925b6 at lockmgr_lock_flags+0x186
#2 0xffffffff80e6e681 at ffs_lock+0x71
#3 0xffffffff80cb3174 at _vn_lock+0x54
#4 0xffffffff80c934f4 at vfs_domount+0xdc4
#5 0xffffffff80c91cb5 at vfs_donmount+0x855
#6 0xffffffff80c95c27 at kernel_mount+0x57
#7 0xffffffff80c98231 at parse_mount+0x491
#8 0xffffffff80c96775 at vfs_mountroot+0x575
#9 0xffffffff80b54558 at start_init+0x28
#10 0xffffffff80b7db20 at fork_exit+0x80
#11 0xffffffff80fc868e at fork_trampoline+0xe
Lock order ufs -> devfs attempted at:
#0 0xffffffff80c2d4f9 at witness_checkorder+0xd99
#1 0xffffffff80b93ff5 at lockmgr_xlock+0x55
#2 0xffffffff80cb3174 at _vn_lock+0x54
#3 0xffffffff80c9c46f at vget_finish+0x4f
#4 0xffffffff80a6842d at devfs_allocv+0xbd
#5 0xffffffff80a67bf3 at devfs_root+0x43
#6 0xffffffff80ca1918 at vfs_cache_root_fallback+0x118
#7 0xffffffff80c9dae8 at vflush+0x48
#8 0xffffffff80a67afe at devfs_unmount+0x3e
#9 0xffffffff80c945cd at dounmount+0x43d
#10 0xffffffff80c9f241 at vfs_unmountall+0xc1
#11 0xffffffff80c75f6a at bufshutdown+0x2ba
#12 0xffffffff80bc0c22 at kern_reboot+0x212
#13 0xffffffff80bc09b9 at sys_reboot+0x399
#14 0xffffffff80ff1c70 at amd64_syscall+0x140
#15 0xffffffff80fc7f5e at fast_syscall_common+0xf8
```