kernel/557: nullfs/unionfs lockup again/still

[Felix Schroeter <felix@mamba.pond.sub.org>]

>Number:         557
>Category:       kernel
>Synopsis:       nullfs/unionfs lockup again/still
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    bugs
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sat Aug  1 14:20:02 MDT 1998
>Last-Modified:
>Originator:     Felix Schroeter
>Organization:
net
>Release:        current CTM 670
>Environment:
	
	System      : OpenBSD 2.3
	Architecture: OpenBSD.i386
	Machine     : i386
>Description:
cd /var/tmp
mkdir a b c
echo 1 >a/1
echo 2 >b/2
mount -r -t null a c
mount -t union b c
echo 3 >c/3
rm c/1
umount c
umount c

The second umount hangs, sleeping on nullloc* (i.e. nulllock1 or nulllock2,
in /sys/miscfs/nullfs/nullfs_vnops.c, function null_lock()) uninterruptably.
The rest of the system runs fine after that, but hung processes aren't nice.
>How-To-Repeat:
>Fix:
	If I knew, the world would be a better place 8:-)
	But I don't know much about the details of locking in VFS.
	That wasn't described in the red d(a)emon book.

>Audit-Trail:
>Unformatted: