df locks up with 100% cpu with zfs on usb, df unkillable

[Michael Chesterton]

Last reproduced with linux-image-generic-hwe-18.04-edge 4.18.0.13.62 on bionic with cpu AMD Ryzen 7 1700 Eight-Core Processor

external usb hdd drive /dev/disk/by-id/usb-WD_My_Passport_0820_5758353xxxxx:0-part1
luks encrypted partition
zfs pool
zfs pool used only for backups using syncoid
initially after zpool import -a `df -h` works
30 minutes later `df -h` hangs, doesn't print any output at all, probably after a zfs receive
df process using 100% cpu
kill -KILL <df pid> has no change in df process, it doesn't die, it's still in R and using 100% cpu
strace -p <df pid> hangs, doesn't print any output, also can't be killed
unplugging the usb doesn't have any effect, df still in R at 100% cpu
reproducible, happens every time.

I can't reproduce with kernel 4.15.0-43-generic.

[Timo Aaltonen (tjaalton)]

assuming fixed in later versions, reopen if not