linux-kernel - Re: sys_umount() returns EBUSY when doing: sh -c "mount /dev/sdc1 /mnt; umount /mnt"

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Date:	Tue, 16 Mar 2010 12:06:35 +0100
From:	Petr Uzel <petr.uzel@...e.cz>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: sys_umount() returns EBUSY when doing: sh -c "mount /dev/sdc1
 /mnt; umount /mnt"

On Mon, Mar 15, 2010 at 01:09:30PM +0100, Francis Moreau wrote:
> On Sun, Mar 14, 2010 at 5:21 PM, Robert Hancock <hancockrwd@...il.com> wrote:
> > On 03/13/2010 02:56 AM, Francis Moreau wrote:
> >> Hello
> >>
> >> I've some shell scripts which try to find out the filesystem hosted by
> >> a block device.
> >>
> >> They basically do this:
> >>
> >>     mount /dev/sdc1 /mnt
> >>     fs=$(stat -f -c %T $mount_point)
> >>     umount /mnt
> >>
> >> It happens to work but since an unknown upgrade (kernel, libs or tools
> >> upgrade), umount(8) returns -EBUSY.
> >>
> >> I found that it's actually the sys_umount() which return -EBUSY.
> >>
> >> So the question, is this expected or is this a regression ?
> >>
> >> If it's expected then which operation should I add between the
> >> mount(8) and umount(8) to make the mount operation completely finish
> >> (inside the kernel) so the next umount won't return -EBUSY ?
> >
> > If no other process were involved I would say it's likely a bug. However, my
> > guess is that some other process (HAL, something in GNOME, etc.) detects the
> > mount and decides to start accessing the drive. Then when you immediately
> > try to unmount, it fails because it's busy. I suspect if you try this in
> > single-user mode with no unnecessary processes running you won't see this.
> >

I have experienced something similar when I did addpart, immediately
followed by delpart. The delpart command failed with EBUSY, because
the first command triggered some hal subprocess that accessed the
device.

> You're right, I don't see this anymore if I'm booting in a single user mode.

You can try to put 'lsof /dev/sdc1' between mount and umount to see
what (if anything) is accessing the device.

> 
> So I need to find out how to wait until these other processes stop
> accessing the drive.

If it is hal what causes EBUSY, you could 'killall -SIGSTOP haldaemon'
before mount and SIGCONT after umount.


Petr

--
Petr Uzel, openSUSE Boosters Team
IRC: ptr_uzl @ freenode

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