panic: Thread 0xffff800020b75520 cannot exit while holding sleeplocks

Stopped at      db_enter+0x18:  addq    $0x8,%rsp
    TID    PID    UID     PRFLAGS     PFLAGS  CPU  COMMAND
  65688  17938     73    0x100010          0    0  syslogd
*430576   9499      0     0x14000      0x200    1  reaper
db_enter() at db_enter+0x18 sys/arch/amd64/amd64/db_interface.c:399
panic() at panic+0x16c sys/kern/subr_prf.c:208
witness_thread_exit(e30a0e16f18bdaa) at witness_thread_exit+0x244 sys/kern/subr_witness.c:1377
reaper(0) at reaper+0x14f sys/kern/kern_exit.c:412
end trace frame: 0x0, count: 11
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ddb{1}> 
ddb{1}> set $lines = 0
ddb{1}> show panic
Thread 0xffff800020b75520 cannot exit while holding sleeplocks

ddb{1}> trace
db_enter() at db_enter+0x18 sys/arch/amd64/amd64/db_interface.c:399
panic() at panic+0x16c sys/kern/subr_prf.c:208
witness_thread_exit(e30a0e16f18bdaa) at witness_thread_exit+0x244 sys/kern/subr_witness.c:1377
reaper(0) at reaper+0x14f sys/kern/kern_exit.c:412
end trace frame: 0x0, count: -4
ddb{1}> show registers
rdi                                0
rsi                              0x1
rbp               0xffff800020b67a50
rbx               0xffff800020b67af0
rdx               0xffffffff81ec577a    cmd0646_9_tim_udma+0x16395
rcx                            0x201
rax                              0x1
r8                0xffffffff81788154    kprintf+0x174
r9                               0x1
r10               0xe4bb1caeb59d0636
r11               0x4be54ebea4369d06
r12                     0x3000000008
r13               0xffff800020b67a60
r14                            0x100
r15                              0x1
rip               0xffffffff81107618    db_enter+0x18
cs                               0x8
rflags                         0x246
rsp               0xffff800020b67a40
ss                              0x10
db_enter+0x18:  addq    $0x8,%rsp
ddb{1}> show proc
PROC (reaper) pid=430576 stat=onproc
    flags process=14000<NOZOMBIE,SYSTEM> proc=200<SYSTEM>
    pri=4, usrpri=51, nice=20
    forw=0xffffffffffffffff, list=0xffff800020b20bb8,0xffff800020b212d0
    process=0xffff800020b5a698 user=0xffff800020b62000, vmspace=0xffffffff822fc8c0
    estcpu=1, cpticks=3, pctcpu=0.3
    user=0, sys=3, intr=0
ddb{1}>