From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: sys_umount() returns EBUSY when doing: sh -c "mount /dev/sdc1
 /mnt;	umount /mnt"
Date: Mon, 15 Mar 2010 16:34:54 +0000
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Cc: Francis Moreau <francis.moro@gmail.com>,
	linux-fsdevel@vger.kernel.org
To: Andreas Dilger <adilger@sun.com>
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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.