kernel panic in genfs_deadunlock

[coypu%sdf.org@localhost]

hi folks,
while testing a very recent kernel, and waiting for it to reboot, I got this:

Crash version 8.99.37, image version 8.99.37.
System panicked: lock error: Reader / writer lock: rw_vector_exit,454: assertion failed: RW_COUNT(rw) != 0: lock 0xffffed5bd50116b0 cpu 3 lwp 0xffffed5f70a20ae0
Backtrace from time of crash is available.
crash> bt
_KERNEL_OPT_NARCNET() at 0
ostype() at ostype+0xb7290
vpanic() at vpanic+0x169
snprintf() at snprintf
lockdebug_abort() at lockdebug_abort+0xe7
rw_vector_exit() at rw_vector_exit+0xce
genfs_deadunlock() at genfs_deadunlock+0x14
VOP_UNLOCK() at VOP_UNLOCK+0x51
cnclose() at cnclose+0x7e
cdev_close() at cdev_close+0xbc
spec_close() at spec_close+0x199
VOP_CLOSE() at VOP_CLOSE+0x4c
vn_close() at vn_close+0x34
closef() at closef+0x6d
fd_close() at fd_close+0x1f4
sys_close() at sys_close+0x20
syscall() at syscall+0x173
--- syscall (number 6) ---
755abfa42bea:


If it helps anyone. I have no idea :-)