panic: Thread 0xffff800020b75778 cannot exit while holding sleeplocks

Stopped at      db_enter+0x18:  addq    $0x8,%rsp
    TID    PID    UID     PRFLAGS     PFLAGS  CPU  COMMAND
 304240  81959     73    0x100010          0    0  syslogd
*141867  19154      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(364de562cec9e2b2) 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 0xffff800020b75778 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(364de562cec9e2b2) 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               0xffff800020b67370
rbx               0xffff800020b67410
rdx               0xffffffff81f2ea76    apollo_pio_rec+0x9bcf
rcx                            0x201
rax                              0x1
r8                0xffffffff810e4f24    kprintf+0x174
r9                               0x1
r10               0xd34759b5eaf9cdcd
r11               0x38f9f047577a0ec3
r12                     0x3000000008
r13               0xffff800020b67380
r14                            0x100
r15                              0x1
rip               0xffffffff8171e6c8    db_enter+0x18
cs                               0x8
rflags                         0x246
rsp               0xffff800020b67360
ss                              0x10
db_enter+0x18:  addq    $0x8,%rsp
ddb{1}>