From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752035AbaCVQvM (ORCPT <rfc822;w@1wt.eu>);
	Sat, 22 Mar 2014 12:51:12 -0400
Received: from mail-wi0-f171.google.com ([209.85.212.171]:53021 "EHLO
	mail-wi0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751279AbaCVQvK (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Sat, 22 Mar 2014 12:51:10 -0400
Message-ID: <532DBFC8.4020202@gmail.com>
Date: Sat, 22 Mar 2014 17:52:24 +0100
From: Francis Moreau <francis.moro@gmail.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.3.0
MIME-Version: 1.0
To: LKML <linux-kernel@vger.kernel.org>
Subject: Can't umount /mnt/dev after calling dd(1) and with /mnt/dev is a
 bind mount
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

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.