lock order reversal in sys/kern/vfs_mount.c

Good day,

After updating to yesterdays -current, I got this on boot:

lock order reversal:
  1st 0xfffffe0007b04c38 ufs (ufs) _at_ /usr/src/sys/kern/vfs_mount.c:1254
  2nd 0xfffffe0007ed9478 devfs (devfs) _at_ /usr/src/sys/kern/vfs_subr.c:2158
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2a
kdb_backtrace() at kdb_backtrace+0x37
_witness_debugger() at _witness_debugger+0x2c
witness_checkorder() at witness_checkorder+0x853
__lockmgr_args() at __lockmgr_args+0x113a
vop_stdlock() at vop_stdlock+0x39
VOP_LOCK1_APV() at VOP_LOCK1_APV+0xbf
_vn_lock() at _vn_lock+0x47
vget() at vget+0x7b
devfs_allocv() at devfs_allocv+0x13f
devfs_root() at devfs_root+0x4d
dounmount() at dounmount+0x45c
vfs_unmountall() at vfs_unmountall+0x4c
kern_reboot() at kern_reboot+0x84b
sys_reboot() at sys_reboot+0x68
amd64_syscall() at amd64_syscall+0x2e0
Xfast_syscall() at Xfast_syscall+0xf7
--- syscall (55, FreeBSD ELF64, sys_reboot), rip = 0x40ebbc, rsp = 
0x7fffffffd6c8, rbp = 0x65 ---

Reverting to old kernel (that was built about a week ago) helped to 
avoid this. Any thoughts?

-- 
Regards,
Ruslan

Tinderboxing kills... the drives.