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

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

[Francis Moreau]

[resending to fs-devel mailing list hoping to get some hints]

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 ?

Oh I'm currently using the kernel shipped with F12: 2.6.32.9-67.fc12.x86_64

Thanks
--
Francis



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

[Andreas Dilger]

On 2010-03-13, at 13:30, Francis Moreau wrote:
> 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

Mounting an in-use filesystem is a bad idea.

You should just use the blkid tool, which wa written specifically for  
this purpose.  It was previously part of e2fsprogs (installed on 99%  
of systems) and in the future (possibly even FC12) it has moved over  
to util-linux (on 100% of systems).

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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

[Al Viro]

On Mon, Mar 15, 2010 at 10:04:46AM -0600, Andreas Dilger wrote:
> On 2010-03-13, at 13:30, Francis Moreau wrote:
> >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
> 
> Mounting an in-use filesystem is a bad idea.

Huh?  mount() will happily create another vfsmount refering to the same
superblock in that case.  It *is* OK to mount the same block device twice;
any fs that uses get_sb_bdev() will DTRT.

It should be safe.  The lack of error checking after mount, OTOH, is not.

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

[Francis Moreau]

On Mon, Mar 15, 2010 at 5:34 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Mon, Mar 15, 2010 at 10:04:46AM -0600, Andreas Dilger wrote:
>> On 2010-03-13, at 13:30, Francis Moreau wrote:
>> >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
>>
>> Mounting an in-use filesystem is a bad idea.
>
> Huh?  mount() will happily create another vfsmount refering to the same
> superblock in that case.  It *is* OK to mount the same block device twice;
> any fs that uses get_sb_bdev() will DTRT.
>
> It should be safe.  The lack of error checking after mount, OTOH, is not.
>

Well I omitted the error checking since it was not revelant in my
case: umount(8) fails but not mount(8).

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

[Ian Kent]

On Mon, 2010-03-15 at 16:34 +0000, Al Viro wrote:
> On Mon, Mar 15, 2010 at 10:04:46AM -0600, Andreas Dilger wrote:
> > On 2010-03-13, at 13:30, Francis Moreau wrote:
> > >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
> > 
> > Mounting an in-use filesystem is a bad idea.
> 
> Huh?  mount() will happily create another vfsmount refering to the same
> superblock in that case.  It *is* OK to mount the same block device twice;
> any fs that uses get_sb_bdev() will DTRT.
> 
> It should be safe.  The lack of error checking after mount, OTOH, is not.

I started seeing similar behavior in autofs after the pnode patches went
in. I was never able to decide if I was doing something wrong or if
there had been some latency in reducing reference counts introduced.
Also, I couldn't see anything in the kernel code that that even looked
like it would cause this so I didn't see any use in reporting it.
Instead I began using a retry loop with a short delay when umounting
mounts.

Ian

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

[Francis Moreau]

On Mon, Mar 15, 2010 at 5:04 PM, Andreas Dilger <adilger@sun.com> wrote:
> On 2010-03-13, at 13:30, Francis Moreau wrote:
>>
>> 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
>
> Mounting an in-use filesystem is a bad idea.

But it's not: /dev/sdc1 is not mounted anywhere before executing the script.

Someone else pointed out that some other processes (such as hal, gnome
stuff...) may have detected the new mount point and started to access
it hence preventing umount(8) to work.

-- 
Francis
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