Reproduceable deadlock in NFS Client

My diskless Rock64 has taken to deadlocking reproduceably whilst
building libprivatesqlite3.a as part of buildworld when running
r352792.  At the time of the deadlock, the relevant running process
is:
ar -crD libprivatesqlite3.a sqlite3.o

And those files are:
-rw-r--r--    1 root  wheel  3178496  4 Oct 01:10 libprivatesqlite3.a
-rw-r--r--    1 root  wheel  7975272  4 Oct 01:10 sqlite3.o

The "ar" reports it's in bo_wwait and, after about 30 minutes, I get:
deadlres_td_sleep_q: possible deadlock detected for 0xfffffd00012c9560, blocked for 1800613 ticks

cpuid = 2
time = 1570117920
KDB: stack backtrace:
db_trace_self() at db_trace_self_wrapper+0x28
         pc = 0xffff00000054b83c  lr = 0xffff0000000e2b08
         sp = 0xffff00004030a790  fp = 0xffff00004030a9a0

db_trace_self_wrapper() at vpanic+0x18c
         pc = 0xffff0000000e2b08  lr = 0xffff00000027fb54
         sp = 0xffff00004030a9b0  fp = 0xffff00004030aa50

vpanic() at panic+0x44
         pc = 0xffff00000027fb54  lr = 0xffff00000027f904
         sp = 0xffff00004030aa60  fp = 0xffff00004030aae0

panic() at deadlkres+0x33c
         pc = 0xffff00000027f904  lr = 0xffff00000021c19c
         sp = 0xffff00004030aaf0  fp = 0xffff00004030ab50

deadlkres() at fork_exit+0x7c
         pc = 0xffff00000021c19c  lr = 0xffff0000002404f4
         sp = 0xffff00004030ab60  fp = 0xffff00004030ab90

fork_exit() at fork_trampoline+0x10
         pc = 0xffff0000002404f4  lr = 0xffff00000056743c
         sp = 0xffff00004030aba0  fp = 0x0000000000000000


-- 
Peter Jeremy