kern/40472: umounting an async fs fails

[Bernd Ernesti <pr200904%veego.de@localhost>]

>Number:         40472
>Category:       kern
>Synopsis:       umounting an async fs fails
>Confidential:   no
>Severity:       critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Jan 25 09:40:00 +0000 2009
>Originator:     Bernd Ernesti
>Release:        NetBSD 5.99.7 22-01-2009
>Organization:
>Environment:
System: NetBSD 5.99.7
Architecture: i386
Machine: i386
>Description:
        I have a async mounted filesystem which can't be unmounted, even
        if it has no files on it, just after a newfs -O 2

        That fs is mounted on top of a wapbl fs.
>How-To-Repeat:

        Use the following /etc/fstab entries and boot with it:
/dev/raid0h     /source         ffs     rw,log          1       3
/dev/wd2e       /source/work    ffs     rw,async        1       3

        The root fs is also a wapbl fs.

        Execute:
                umount /source/work
        and notice that it doesn't return.

USER    PID %CPU %MEM  VSZ   RSS TTY    STAT STARTED     TIME COMMAND   UID PID 
PPID   CPU PRI NI  VSZ   RSS WCHAN    STAT TTY        TIME COMMAND
root    560 99.0  0.0  748   736 ttyp0  O+   10:01AM 19:05.90 umount      0 560 
 492 36864  76  0  748   736 -        O+   ttyp0  19:05.90 umount 

        mount shows that the filesystem is gone, but a reboot still prints that
        it tries to unmount this filesystem and hangs there.
        

>Fix: