FreeBSD 11.0-ALPHA5 r302256 kernel panic in filt_proc()

My package building machine just got this panic during a poudriere run:

zipper.catspoiler.org dumped core - see /var/crash/vmcore.4

Wed Jun 29 14:28:21 PDT 2016

FreeBSD zipper.catspoiler.org 11.0-ALPHA5 FreeBSD 11.0-ALPHA5 #13 r302256: Tue Jun 28 15:00:17 PDT 2016     dl_at_zipper.catspoiler.org:/usr/obj/usr/src/sys/GENERIC  amd64

panic: 

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Unread portion of the kernel message buffer:
Kernel page fault with the following non-sleepable locks held:
exclusive sleep mutex kqueue (kqueue) r = 0 (0xfffff801b1581600) locked _at_ /usr/src/sys/kern/kern_event.c:2023
exclusive sleep mutex process lock (process lock) r = 0 (0xfffff80747ac7648) locked _at_ /usr/src/sys/kern/kern_exit.c:497
stack backtrace:
#0 0xffffffff80aa7d20 at witness_debugger+0x70
#1 0xffffffff80aa9007 at witness_warn+0x3d7
#2 0xffffffff80eb83d7 at trap_pfault+0x57
#3 0xffffffff80eb7a64 at trap+0x284
#4 0xffffffff80e98ab1 at calltrap+0x8
#5 0xffffffff80a0173b at knote+0xcb
#6 0xffffffff80a0710e at exit1+0x98e
#7 0xffffffff80a0677d at sys_sys_exit+0xd
#8 0xffffffff80eb8b2b at amd64_syscall+0x2db
#9 0xffffffff80e98d9b at Xfast_syscall+0xfb


Fatal trap 12: page fault while in kernel mode
cpuid = 2; apic id = 12
fault virtual address	= 0x40c
fault code		= supervisor read data, page not present
instruction pointer	= 0x20:0xffffffff80a02ddc
stack pointer	        = 0x28:0xfffffe085a594840
frame pointer	        = 0x28:0xfffffe085a594850
code segment		= base 0x0, limit 0xfffff, type 0x1b
			= DPL 0, pres 1, long 1, def32 0, gran 1
processor eflags	= interrupt enabled, resume, IOPL = 0
current process		= 70248 (sh)

Reading symbols from /boot/kernel/zfs.ko...Reading symbols from /usr/lib/debug//boot/kernel/zfs.ko.debug...done.
done.
Loaded symbols for /boot/kernel/zfs.ko
Reading symbols from /boot/kernel/opensolaris.ko...Reading symbols from /usr/lib/debug//boot/kernel/opensolaris.ko.debug...done.
done.
Loaded symbols for /boot/kernel/opensolaris.ko
Reading symbols from /boot/kernel/geom_mirror.ko...Reading symbols from /usr/lib/debug//boot/kernel/geom_mirror.ko.debug...done.
done.
Loaded symbols for /boot/kernel/geom_mirror.ko
Reading symbols from /boot/kernel/netgraph.ko...Reading symbols from /usr/lib/debug//boot/kernel/netgraph.ko.debug...done.
done.
Loaded symbols for /boot/kernel/netgraph.ko
Reading symbols from /boot/kernel/ng_ether.ko...Reading symbols from /usr/lib/debug//boot/kernel/ng_ether.ko.debug...done.
done.
Loaded symbols for /boot/kernel/ng_ether.ko
Reading symbols from /boot/kernel/nullfs.ko...Reading symbols from /usr/lib/debug//boot/kernel/nullfs.ko.debug...done.
done.
Loaded symbols for /boot/kernel/nullfs.ko
Reading symbols from /boot/modules/vboxdrv.ko...done.
Loaded symbols for /boot/modules/vboxdrv.ko
Reading symbols from /boot/modules/vboxnetadp.ko...done.
Loaded symbols for /boot/modules/vboxnetadp.ko
Reading symbols from /boot/modules/vboxnetflt.ko...done.
Loaded symbols for /boot/modules/vboxnetflt.ko
Reading symbols from /boot/kernel/linux.ko...Reading symbols from /usr/lib/debug//boot/kernel/linux.ko.debug...done.
done.
Loaded symbols for /boot/kernel/linux.ko
Reading symbols from /boot/kernel/linux_common.ko...Reading symbols from /usr/lib/debug//boot/kernel/linux_common.ko.debug...done.
done.
Loaded symbols for /boot/kernel/linux_common.ko
Reading symbols from /boot/kernel/linprocfs.ko...Reading symbols from /usr/lib/debug//boot/kernel/linprocfs.ko.debug...done.
done.
Loaded symbols for /boot/kernel/linprocfs.ko
Reading symbols from /boot/kernel/tmpfs.ko...Reading symbols from /usr/lib/debug//boot/kernel/tmpfs.ko.debug...done.
done.
Loaded symbols for /boot/kernel/tmpfs.ko
Reading symbols from /boot/kernel/fdescfs.ko...Reading symbols from /usr/lib/debug//boot/kernel/fdescfs.ko.debug...done.
done.
Loaded symbols for /boot/kernel/fdescfs.ko
#0  doadump (textdump=1515799136) at pcpu.h:221
221	pcpu.h: No such file or directory.
	in pcpu.h
(kgdb) #0  doadump (textdump=1515799136) at pcpu.h:221
#1  0xffffffff80392326 in db_fncall (dummy1=<value optimized out>, 
    dummy2=<value optimized out>, dummy3=<value optimized out>, 
    dummy4=<value optimized out>) at /usr/src/sys/ddb/db_command.c:568
#2  0xffffffff80391dc9 in db_command (cmd_table=<value optimized out>)
    at /usr/src/sys/ddb/db_command.c:440
#3  0xffffffff80391b24 in db_command_loop ()
    at /usr/src/sys/ddb/db_command.c:493
#4  0xffffffff80394a5b in db_trap (type=<value optimized out>, 
    code=<value optimized out>) at /usr/src/sys/ddb/db_main.c:251
#5  0xffffffff80a89293 in kdb_trap (type=<value optimized out>, 
    code=<value optimized out>, tf=<value optimized out>)
    at /usr/src/sys/kern/subr_kdb.c:654
#6  0xffffffff80eb8331 in trap_fatal (frame=0xfffffe085a594790, eva=1036)
    at /usr/src/sys/amd64/amd64/trap.c:836
#7  0xffffffff80eb857d in trap_pfault (frame=0xfffffe085a594790, usermode=0)
    at /usr/src/sys/amd64/amd64/trap.c:691
#8  0xffffffff80eb7a64 in trap (frame=0xfffffe085a594790)
    at /usr/src/sys/amd64/amd64/trap.c:442
#9  0xffffffff80e98ab1 in calltrap ()
    at /usr/src/sys/amd64/amd64/exception.S:236
#10 0xffffffff80a02ddc in filt_proc (kn=0xfffff803c5679a80, 
    hint=<value optimized out>) at /usr/src/sys/kern/kern_event.c:473
#11 0xffffffff80a0173b in knote (list=<value optimized out>, hint=2147483648, 
    lockflags=<value optimized out>) at /usr/src/sys/kern/kern_event.c:2045
#12 0xffffffff80a0710e in exit1 (td=<value optimized out>, 
    rval=<value optimized out>, signo=<value optimized out>)
    at /usr/src/sys/kern/kern_exit.c:515
#13 0xffffffff80a0677d in sys_sys_exit (td=0xfffff803c5679a80, 
    uap=<value optimized out>) at /usr/src/sys/kern/kern_exit.c:178
#14 0xffffffff80eb8b2b in amd64_syscall (td=0xfffff80096b49500, traced=0)
    at subr_syscall.c:135
#15 0xffffffff80e98d9b in Xfast_syscall ()
    at /usr/src/sys/amd64/amd64/exception.S:396
#16 0x00000008009298ca in ?? ()
Previous frame inner to this frame (corrupt stack?)
Current language:  auto; currently minimal
(kgdb) 


The line numbers above seem to be off.  With kgdb from ports I see:

(kgdb) up
#12 filt_proc (kn=0xfffff803c5679a80, hint=<optimized out>)
    at /usr/src/sys/kern/kern_event.c:466
466				kn->kn_data = KW_EXITCODE(p->p_xexit, p->p_xsig);
(kgdb) print kn
$1 = (struct knote *) 0xfffff803c5679a80
(kgdb) print p
$2 = (struct proc *) 0x0