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

[Ian Kent <raven@themaw.net>]

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