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

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

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On 03/22/2014 05:52 PM, Francis Moreau wrote:
> 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.
>