kern/38291: panic: LOCKDEBUG, _kernel_unlock: assertion failed: nlocks <= 0, unmounting MFS /tmp

["Greg A. Woods" <woods%planix.com@localhost>]

>Number:         38291
>Category:       kern
>Synopsis:       panic: LOCKDEBUG, _kernel_unlock: assertion failed: nlocks <= 
>0, unmounting MFS /tmp
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Mar 24 21:15:00 +0000 2008
>Originator:     Greg A. Woods
>Release:        NetBSD 4.99.57 2008/03/23
>Organization:
Planix, Inc.; Toronto, Ontario; Canada
>Environment:
System: NetBSD GENERIC with "options LOCKDEBUG"
Architecture: i386
Machine: i386
>Description:

        the following crash happened when rebooting a fresh -current
        GENERIC kernel just test booted on a system that has an MFS /tmp

        previous 4.99.55 kernels rebooted cleanly on this same system

Mar 24 16:58:39 reboot: rebooted by woods
Mar 24 16:58:40 historically syslogd: Exiting on signal 15
Kernel lock error: _kernel_unlock: assertion failed: nlocks <= 0

lock address : 0x00000000c0d51a40 type     :               spin
shared holds :                  0 exclusive:                  1
shares wanted:                  0 exclusive:                  0
current cpu  :                  1 last held:                  0
current lwp  : 0x00000000cdaec460 last held: 0x00000000cc873240
last locked  : 0x00000000c066dc69 unlocked : 0x00000000c066dca3
initialized  : 0x00000000c0526d14
curcpu holds :                  0 wanted by: 000000000000000000

panic: LOCKDEBUG
Stopped in pid 156.1 (mount_mfs) at     netbsd:breakpoint+0x4:  popl    %ebp
db{1}> trace
breakpoint(cdadba58,0,cdadba8c,c0582fb7,c0a2932f) at netbsd:breakpoint+0x4
cpu_Debugger(c0a2932f,cdadba98,c36b1800,c0583e67,0) at netbsd:cpu_Debugger+0xb
panic(c0a284f0,c0583e06,c0a229ee,c0a22a1a,1f) at netbsd:panic+0x164
lockdebug_abort1(c0d54c40,c0d5e6a0,c0a229ee,c0a22a1a,1) at 
netbsd:lockdebug_abort1+0x8d
lockdebug_abort(c0d51a40,c0c92eb8,c0a229ee,c0a22a1a,cdd5552c) at 
netbsd:lockdebug_abort+0x4b
_kernel_unlock(1,cdaec460,0,c05c9e56,cdc5f3c4) at netbsd:_kernel_unlock+0x7d
VFS_START(cdc5f3c4,0,3,cdadbcb0,0) at netbsd:VFS_START+0x6f
mount_domount(cdaec460,cdadbc7c,c0c902a0,bfbfffa2,58) at 
netbsd:mount_domount+0x3f5
do_sys_mount(cdaec460,c0c902a0,8076522,bfbfffa2,58) at netbsd:do_sys_mount+0x254
compat_40_sys_mount(cdaec460,cdadbd04,cdadbcfc,cdadbd30,c064b7db) at 
netbsd:compat_40_sys_mount+0x59
syscall(cdadbd48,b3,ab,bfbf001f,1f) at netbsd:syscall+0x17b
db{1}> x/I 0x00000000c066dc69
netbsd:syscall+0x193:   movl    0xfffffff0(%ebp),%eax
db{1}> x/I 0x00000000c0526d14
netbsd:main+0x2f:       call    netbsd:uvm_init
db{1}> 

>How-To-Repeat:

        boot fresh 4.99.57 kernel on system that uses MFS /tmp then reboot

>Fix: