(baseline run, DFBATCH kernel #3, captured from serial console before subsequent reboots)
DFB: === cmd='usbpf race' ===
DFB-usbpf: bdev=0xfffff8008bb7f6c0 (root unit 0)
DFB-usbpf[race]: launching tapper+destroyer; if a NULL store lands between the tap's :362 check and :488 use, the kernel panics now

Fatal trap 12: page fault while in kernel mode
cpuid = 1; lapic id = 1
fault virtual address	= 0x60
fault code		= supervisor read data, page not present
instruction pointer	= 0x8:0xffffffff80a514c7
stack pointer	        = 0x10:0xfffff80116b8ca30
frame pointer	        = 0x10:0xfffff80116b8ca90
kernel: type 12 trap, code=0
CPU1 stopping CPUs: 0x0000003d
Stopped at      usbpf_xfertap+0x447:    movq    0x60(%rax),%rdi
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