kern/55635: getvfsstat panic on reboot

[nia%pkgsrc.org@localhost]

>Number:         55635
>Category:       kern
>Synopsis:       getvfsstat panic on reboot
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Sep 01 14:10:00 +0000 2020
>Originator:     nia
>Release:        9.0_STABLE
>Organization:
>Environment:
amd64 GENERIC
>Description:
Reading symbols from /home/nia/netbsd-9/sys/arch/amd64/compile/obj/GENERIC/netbsd.gdb...
(gdb) target kvm /var/crash/netbsd.6.core
0xffffffff802219f5 in cpu_reboot (howto=howto@entry=256, bootstr=bootstr@entry=0x0) at /home/nia/netbsd-9/sys/arch/amd64/amd64/machdep.c:728
728                     dumpsys();
(gdb) bt
#0  0xffffffff802219f5 in cpu_reboot (howto=howto@entry=256, bootstr=bootstr@entry=0x0) at /home/nia/netbsd-9/sys/arch/amd64/amd64/machdep.c:728
#1  0xffffffff80767cb8 in db_sync_cmd (addr=<optimized out>, have_addr=<optimized out>, count=<optimized out>, modif=<optimized out>)
    at /home/nia/netbsd-9/sys/ddb/db_command.c:1435
#2  0xffffffff80768544 in db_command (last_cmdp=last_cmdp@entry=0xffffffff81445ce0 <db_last_command>) at /home/nia/netbsd-9/sys/ddb/db_command.c:933
#3  0xffffffff80768865 in db_command_loop () at /home/nia/netbsd-9/sys/ddb/db_command.c:592
#4  0xffffffff8076c295 in db_trap (type=type@entry=6, code=code@entry=0) at /home/nia/netbsd-9/sys/ddb/db_trap.c:91
#5  0xffffffff8021e3a5 in kdb_trap (type=type@entry=6, code=code@entry=0, regs=regs@entry=0xffff9680ae58bc80) at /home/nia/netbsd-9/sys/arch/amd64/amd64/db_interface.c:247
#6  0xffffffff8022309d in trap (frame=0xffff9680ae58bc80) at /home/nia/netbsd-9/sys/arch/amd64/amd64/trap.c:323
#7  0xffffffff8021c4a5 in alltraps ()
#8  0xffffffff809e05ff in fstrans_alloc_lwp_info (mp=mp@entry=0xffff924acf5ad000) at /home/nia/netbsd-9/sys/kern/vfs_trans.c:387
#9  0xffffffff809e0644 in fstrans_alloc_lwp_info (mp=<optimized out>) at /home/nia/netbsd-9/sys/kern/vfs_trans.c:395
#10 0xffffffff809e09bb in fstrans_get_lwp_info (do_alloc=true, mp=<optimized out>) at /home/nia/netbsd-9/sys/kern/vfs_trans.c:426
#11 _fstrans_start (wait=1, lock_type=FSTRANS_SHARED, mp=<optimized out>) at /home/nia/netbsd-9/sys/kern/vfs_trans.c:471
#12 fstrans_start (mp=<optimized out>) at /home/nia/netbsd-9/sys/kern/vfs_trans.c:508
#13 0xffffffff809d6a13 in _vfs_busy (wait=true, mp=0xffff924a626a0000) at /home/nia/netbsd-9/sys/kern/vfs_mount.c:326
#14 vfs_busy (mp=0xffff924a626a0000) at /home/nia/netbsd-9/sys/kern/vfs_mount.c:343
#15 _mountlist_iterator_next (wait=true, mi=0xffff924a46e807c0) at /home/nia/netbsd-9/sys/kern/vfs_mount.c:1528
#16 mountlist_iterator_next (mi=0xffff924a46e807c0) at /home/nia/netbsd-9/sys/kern/vfs_mount.c:1545
#17 0xffffffff809dd1f8 in do_sys_getvfsstat (l=0xffff924aa25084a0, sfsp=0x0, bufsize=<optimized out>, flags=2, copyfn=0xffffffff8021bc80 <copyout>,
    entry_sz=entry_sz@entry=2256, retval=0xffff9680ae58beb0) at /home/nia/netbsd-9/sys/kern/vfs_syscalls.c:1268
#18 0xffffffff809dd351 in sys_getvfsstat (l=<optimized out>, uap=<optimized out>, retval=<optimized out>) at /home/nia/netbsd-9/sys/kern/vfs_syscalls.c:1318
#19 0xffffffff8024b197 in sy_call (rval=0xffff9680ae58beb0, uap=0xffff9680ae58bf00, l=0xffff924aa25084a0, sy=0xffffffff8145afc0 <sysent+8544>)
    at /home/nia/netbsd-9/sys/sys/syscallvar.h:65
#20 sy_invoke (code=356, rval=0xffff9680ae58beb0, uap=0xffff9680ae58bf00, l=0xffff924aa25084a0, sy=0xffffffff8145afc0 <sysent+8544>)
    at /home/nia/netbsd-9/sys/sys/syscallvar.h:94
#21 syscall (frame=0xffff9680ae58bf00) at /home/nia/netbsd-9/sys/arch/x86/x86/syscall.c:138
#22 0xffffffff802086ad in handle_syscall ()

>How-To-Repeat:
1. do unknown series of nasty things with chroot null mounts with /tmp on tmpfs
2. ???
3. reboot
4. kernel panics before system halts
>Fix: