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Date: Sat, 22 Mar 2014 19:24:04 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Francis Moreau <francis.moro@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>
Subject: Re: Can't umount /mnt/dev after calling dd(1) and with /mnt/dev is a
 bind mount
Message-ID: <20140322192404.GG18016@ZenIV.linux.org.uk>
References: <532DBFC8.4020202@gmail.com>
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On Sat, Mar 22, 2014 at 05:52:24PM +0100, Francis Moreau wrote:
> 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"

What do you have in /proc/self/mountinfo before all that?