panic: softdep_deallocate_dependencies: dangling deps

Just got this panic on 

FreeBSD troutmask.apl.washington.edu 12.0-CURRENT FreeBSD 12.0-CURRENT
#0 r321800: Mon Jul 31 13:48:43 PDT 2017
kargl_at_:/data/obj/usr/src/sys/SPEW  amd64

core.txt.0 contains

panic: softdep_deallocate_dependencies: dangling deps
cpuid = 7
time = 1506549566
KDB: stack backtrace:
db_trace_self_wrapper() at db_trace_self_wrapper+0x2b/frame 0xfffffe023a281710
vpanic() at vpanic+0x19c/frame 0xfffffe023a281790
panic() at panic+0x43/frame 0xfffffe023a2817f0
softdep_deallocate_dependencies() at softdep_deallocate_dependencies+0x76/frame 0xfffffe023a281810
brelse() at brelse+0x149/frame 0xfffffe023a281870
bufwrite() at bufwrite+0x65/frame 0xfffffe023a2818b0
softdep_process_journal() at softdep_process_journal+0x7a8/frame 0xfffffe023a281950
softdep_process_worklist() at softdep_process_worklist+0x80/frame 0xfffffe023a2819b0
softdep_flush() at softdep_flush+0xff/frame 0xfffffe023a2819f0
fork_exit() at fork_exit+0x75/frame 0xfffffe023a281a30
fork_trampoline() at fork_trampoline+0xe/frame 0xfffffe023a281a30
--- trap 0, rip = 0, rsp = 0, rbp = 0 ---

__curthread () at ./machine/pcpu.h:232
232		__asm("movq %%gs:%1,%0" : "=r" (td)
(kgdb) #0  __curthread () at ./machine/pcpu.h:232
#1  doadump (textdump=1) at /usr/src/sys/kern/kern_shutdown.c:318
#2  0xffffffff805879eb in kern_reboot (howto=260)
    at /usr/src/sys/kern/kern_shutdown.c:386
#3  0xffffffff80587e66 in vpanic (fmt=<optimized out>, ap=0xfffffe023a2817d0)
    at /usr/src/sys/kern/kern_shutdown.c:779
#4  0xffffffff80587c83 in panic (fmt=<unavailable>)
    at /usr/src/sys/kern/kern_shutdown.c:710
#5  0xffffffff80787f56 in softdep_deallocate_dependencies (
    bp=0xfffffe01f008d8b8) at /usr/src/sys/ufs/ffs/ffs_softdep.c:14304
#6  0xffffffff8061dd69 in buf_deallocate (bp=0xfffffe01f008d8b8)
    at /usr/src/sys/sys/buf.h:429
#7  brelse (bp=0xfffffe01f008d8b8) at /usr/src/sys/kern/vfs_bio.c:2348
#8  0xffffffff8061b9e5 in bufwrite (bp=0xfffffe01f008d8b8)
    at /usr/src/sys/kern/vfs_bio.c:1914
#9  0xffffffff8079bec8 in softdep_process_journal (mp=<optimized out>, 
    needwk=0x0, flags=<optimized out>)
    at /usr/src/sys/ufs/ffs/ffs_softdep.c:3559
#10 0xffffffff80785dc0 in softdep_process_worklist (mp=0xfffff80007eef000, 
    full=0) at /usr/src/sys/ufs/ffs/ffs_softdep.c:1592
#11 0xffffffff807894ff in softdep_flush (addr=0xfffff80007eef000)
    at /usr/src/sys/ufs/ffs/ffs_softdep.c:1397
#12 0xffffffff80555075 in fork_exit (
    callout=0xffffffff80789400 <softdep_flush>, arg=0xfffff80007eef000, 
    frame=0xfffffe023a281a40) at /usr/src/sys/kern/kern_fork.c:1038
#13 <signal handler called>
(kgdb) 

Hmmm,

%  kgdb /usr/lib/debug/boot/kernel/kernel.debug vmcore.0
GNU gdb (GDB) 8.0 [GDB v8.0 for FreeBSD]
Copyright (C) 2017 Free Software Foundation, Inc.
Type "apropos word" to search for commands related to "word"...
Reading symbols from /usr/lib/debug/boot/kernel/kernel.debug...done.
ABI doesn't support a vmcore target

OK, so debugging is broken :-/

-- 
Steve
20170425 https://www.youtube.com/watch?v=VWUpyCsUKR4
20161221 https://www.youtube.com/watch?v=IbCHE-hONow