$ kldload ./df5530_race.ko
RACE_LOADED
   (serial console, dfbsd-qemu/vm.sh log)
   login: DF5530: racing if_clone_create(usbus0) vs usbpf_detach() for up to 240s...
   DF5530: racing... 0s elapsed

   Fatal trap 12: page fault while in kernel mode
   cpuid = 5; lapic id = 5
   fault virtual address   = 0x5c8
   fault code              = supervisor write data, page not present
   instruction pointer     = 0x8:0xffffffff80a50a9f
   current process         = Idle
   kernel: type 12 trap, code=2
   Stopped at usbpf_clone_destroy+0xf:  movq $0,0x5c8(%rax)
   db>
(ssh died with the guest; vm.sh status -> down)