1.00-pre1 problem: can't umount oldroot after pivot_root&chroot

Hi,

I use busybox in my embedded system, and use cramfs as root filesystem
in flash. To update the root filesystem without reboot the board, I
wrote some scripts to do the following things:

1. Copy rootfilesystem image file to /tmp(tmpfs)
2. Restart init, this step will kill all processes, umount all
filesystem except /(cramfs) and /tmp(tmpfs), then init exec my script.
3. Here is my first script:

        #!/bin/sh

        # Copy all system file to new root
        cp -a /bin /sbin /etc /lib /dev /tmp

        cd /tmp
        mkdir old_root proc tmp

        mount -t proc none proc

        # Change root device
        pivot_root . old_root

        #chroot to tmpfs
        exec sbin/chroot . sbin/flash.sh <dev/console >dev/console 2>&1

These steps work well in both 0.60.5 and 1.00-pre1.

In my second script (flash.sh), I first do an umount on old_root:

        umount old_root

this works in busybox-0.60.5, but fails in busybox-1.00-pre1 with the
error "Device or resource is busy". By adding a

        mount

command before above line, I found the difference: in 0.60.5, the device
on /old_root is the real device:

        /dev/mtd_blk2 on /old_root type cramfs (rw)

but in 1.00-pre1, the device changes to /dev/root:

        /dev/root on /old_root type cramfs (rw)

I wonder whether this change cause the error? Is this a bug?