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    Subjectunmounting multiply-mounted directories
    Hi all.

    I use devfs on my installation, even though my distribution
    (RedHat-7.3) does not endorse it. Most of the time it works fine.
    However, when I want to upgrade certain packages (for example, that
    which contains the entries in the non-devfs /dev directory) I need to
    make the original /dev directory visible to the package management
    (viz. rpm) program.

    Seeing that I cannot unmount the devfs /dev directory since many
    processes use it, I figured the best way to do that would be to
    temporarily mount the original /dev directory in its place and over
    it, and then unmount it. So I did something like this

    mount /dev/sda2 /mnt/bareroot
    mount --bind /mnt/bareroot/dev /dev
    rpm -U whichever-package-needs-to-access-dev.whatever.rpm

    This worked fine. Unfortunately, I then discovered that I couldn't
    unmount the /dev directory! I get this:

    vega root ~ # umount /dev
    umount: /dev: device is busy
    umount: /dev: device is busy

    Hum, why am I getting the error message twice? Is it because I have
    two different mounts on the same /dev mountpoint?

    Anyway, I would like to trace that "device is busy" error back to its
    causes, that is, determine which processes are preventing me from
    unmounting the topmost /dev mount. How do I do that? I tried using
    fuser but it is useless: it does not have an option to distinguish
    open files from the bottommost /dev mount and from the topmost. So
    how should I distinguish the two?

    Actually, all sorts of weird things are happening now. For example,
    the /dev/pts directory is all empty (that doesn't seem to bother xterm
    overmuch, though, even if I open new xterms). Well, I guess I'm in
    for an emergency reboot.

    But, what would be The Right Way to make the old /dev accessible to a
    program that needs to access it? I can think of running the program
    chrooted with a specially mounted / but it doesn't seem perfect
    either. Any other ideas?

    Oh, and by the way: is there some documentation to mount --bind
    somewhere? Like, something which explains the detailed semantics of
    that operation?

    Thank you for your help -

    --
    David A. Madore
    (david.madore@ens.fr,
    http://www.eleves.ens.fr:8080/home/madore/ )
    -
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