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Date: Fri, 14 Sep 2001 16:54:12 -0400 (EDT)
From: Alexander Viro <viro@math.psu.edu>
To: Linus Torvalds <torvalds@transmeta.com>
cc: linux-kernel@vger.kernel.org
Subject: Re: [PATCH] lazy umount (1/4)
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On Fri, 14 Sep 2001, Linus Torvalds wrote:

> 
> On Fri, 14 Sep 2001, Alexander Viro wrote:
> >
> > There are only two things to take care of -
> > 	a) if we detach a parent we should do it for all children
> > 	b) we should not mount anything on "floating" vfsmounts.
> > Both are obviously staisfied for current code (presence of children
> > means that vfsmount is busy and we can't mount on something that
> > doesn't exist).
> 
> I disagree about the "we can't mount on something that doesn't exist"
> part.
> 
> If the detached mount is busy, it might be busy exactly because somebody
> has his working directory in it. Which means that
> 
> 	mount /dev/hda ./xxxx
> 
> by such a process could cause a mount within the "nonexisting" mount.

Sure, which is exactly why we need to add checks.  See part 3 - calls of
check_mnt() prevent precisely that kind of situations.

	What I mean is that adding these checks is backwards-compatible -
in absence of lazy umounts they are never triggered.