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

[Yang Xiaopeng]

Erik Andersen wrote:

>ok.  And /proc is mounted?  I did not see you unmounting it
>and mounting it within the new root...
>
> -Erik
>
>--
>Erik B. Andersen             http://codepoet-consulting.com/
>--This message was written using 73% post-consumer electrons--
>
>
>
Old /proc is unmounted when init restart, before executing my scripts,
all user processes are also killed in that step. When my scripts run,
proc filesystem is remounted to /tmp/proc before pivot_root&chroot to /tmp.
after chroot, its real mount point should be /proc in the new root.

Here is the output of `mount` just after pivot_root&chroot to /tmp:

/dev/root on /old_root type cramfs (rw)
none on / type tmpfs (rw)
none on /proc type proc (rw)

and the `ps` output:

  PID  Uid     VmSize Stat Command
    1 root        616 S   /bin/sh sbin/flash.sh
    2 root            SW  [keventd]
    3 root            SWN [ksoftirqd_CPU0]
    4 root            SW  [kswapd]
    5 root            SW  [bdflush]
    6 root            SW  [kupdated]
    7 root            SW  [keyd]
    8 root            RW  [ledtimed]
    9 root            SW  [mtdblockd]
   10 root            Z   [khubd]
  181 root        676 R   ps

I'm sure that no user process use old root now,  but when run "umount 
/old_root", it says:
    umount: /old_root: Device or resource busy

I have tried to remount /proc within the new root *after* chroot, but 
get the same result.