kern/45305: umount says device busy without any process having current directory in the mount or file open

[kretschm%cs.uni-bonn.de@localhost]

>Number:         45305
>Category:       kern
>Synopsis:       umount says device busy without any process having current 
>directory in the mount or file open
>Confidential:   no
>Severity:       non-critical
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Mon Aug 29 08:30:00 +0000 2011
>Originator:     Matthias Kretschmer
>Release:        NetBSD 5.1_STABLE (July, 27 2011)
>Organization:
>Environment:
NetBSD blavet.barock.local 5.1_STABLE NetBSD 5.1_STABLE (BLAVET) #0: Sun Aug 28 
09:06:28 CEST 2011 
root@telemann.barock.local:/usr/src/sys/arch/macppc/compile/obj/BLAVET macppc
>Description:
I'm having trouble unmounting file-systems.  So far occurred with null and nfs 
mounts.  To ensure that I do not have a file open on the mount or some process 
having such a mount as current working directory, I killed every process except 
init and login, performed a clean login using root and then tried to unmount it 
without success ("Device busy").  Sending SIGHUP to init didn't help either.  
So far I was only able to unmount the file-system by performing a reboot.

I did not found out so far why this happens.  It seems only to happen, after I 
have a lot of file access, creation, or modification on the mounts.  It 
happened to the mounts of a file-system I chroot to for building packages.  I 
use three mounts, an NFS /netboot from a NetBSD 5.1_STABLE machine, 
/netboot/tmp which is a null-mount from /tmp and /netboot/wrkobjdir which is a 
null-mount from local ffs.  So far I had the problem with the NFS-mount and 
/netboot/wrkobjdir after compiling multiple packages.  Small packages don't 
seem to create such a problem.  As mentioned above I do a chroot /netboot and 
then start compiling packages from pkgsrc.  /netboot/wrkobjdir is written to 
and read from.  /netboot is primarily read from as WRKOBJDIR is 
/netboot/wrkobjdir.  As it already happened to both mount points I do not see 
that heavy write access is required for this to happen.

The kernel "BLAVET" I use on this box, is just a GENERIC kernel that includes 
FFS_EI option and is stripped by all devices not available on the iBook it runs 
on.
>How-To-Repeat:
A lot of file system access seems to cause this problem.  It seems, that 
heavy-writing is not required, as it already happened at a point where I only 
wrote a new pkgchk.conf on the not-unmountable file-system and did not need to 
require any tarball.
>Fix: