Can't umount /mnt/dev after calling dd(1) and with /mnt/dev is a bind mount

[Francis Moreau <francis.moro@gmail.com>]

Hello,

I'm posting here because it might be a behaviour related to the kernel
internals that I can't explain from my user point of view :)

Basically I'm doing this:

  mount -o bind /dev/ /mnt/dev &&
  chroot /mnt dd bs=440 conv=notrunc count=1 if=gptmbr.bin of=/dev/loop0
  umount /mnt/dev

but umount gives the following error: "umount: /mnt/dev: target is busy"

I tried to see if any processes were still using a file in dev with
fuser(1) but there weren't. Futhermore inserting a call to fuser(1)
right before umount fixed the issue, so it really seems a timing issue.

stracing umount showed that umount failed here:
umount("/mnt/dev", 0)                   = -1 EBUSY (Device or resource busy)

I replaced the bind mount of /dev by:
    mount -t devtmpfs none /mnt/dev
and it worked.

Could anybody tell me what I'm doing wrong ?

Thanks.