kern/158231: panic on unmounting nullfs mounted over ufs on usb stick that got detached

[Paul B Mahol]

>Number:         158231
>Category:       kern
>Synopsis:       panic on unmounting nullfs mounted over ufs on usb stick that got detached
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Thu Jun 23 19:00:22 UTC 2011
>Closed-Date:
>Last-Modified:
>Originator:     Paul B Mahol
>Release:        FreeBSD CURRENT
>Organization:
>Environment:
FreeBSD home 9.0-CURRENT FreeBSD 9.0-CURRENT #0 r223183+87c92c3: Sun Jun 19 04:19:19 UTC 2011     root@:/usr/obj/usr/src/sys/kernel  i386
>Description:
panic when umounting nullfs if target is gone.
>How-To-Repeat:
Mount usb stick, da0s1a (ufs fs) on /mnt mount point. Now use nullfs to mount usb file system directory onto / subdirectory on ada0s1a, for example:

mount_nullfs /mnt/path/to/dir /usr/some/stuff_on_hard_disk

Umount (force) or detach usb stick. Force umount of nullfs, kernel will panic.
>Fix:


>Release-Note:
>Audit-Trail:
>Unformatted: