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Date: Mon, 10 Sep 2012 10:35:41 -0400
From: "J. Bruce Fields" <bfields@fieldses.org>
To: Guo Chao <yan@linux.vnet.ibm.com>
Cc: "J. Bruce Fields" <bfields@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>,
        linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [RFC PATCH 05/13] vfs: take i_mutex on renamed file
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On Mon, Sep 10, 2012 at 10:40:51AM +0800, Guo Chao wrote:
> On Fri, Sep 07, 2012 at 05:39:01PM -0400, J. Bruce Fields wrote:
> > On Fri, Sep 07, 2012 at 10:27:05AM +0800, Guo Chao wrote:
> > > One script ...
> > > 
> > > #!/bin/bash
> > > while true
> > > do
> > > 	mount -t sysfs nodev mnt && umount mnt
> > > done
> > > 
> > > 
> > > 
> > > The other ...
> > > 
> > > #!/bin/bash
> > > while true
> > > do
> > > 	mv mnt mnt2 && mv mnt2 mnt
> > > done
> > > 
> > > 
> > > 
> > > Run them simultaneously in two consoles. When mount keeps reporting 
> > > 'mount point mnt does not exist', stop them, then you will see the 
> > > familiar sysfs under mnt2.
> > 
> > Oh, thanks, for some reason I assumed it would be more difficult to
> > reproduce.
> > 
> > I think we can do this--I don't think it even requires any care to the
> > locking order of the renamed vs the victim directory, though I can't
> > completely convince myself of that.
> > 
> > Is it necessary to fix this, though?  Does it cause any problems other
> > than unexpected behavior?
> > 
> > --b.
> > --
> 
> Hard to say whether it's a bug or what's problems of being able to rename 
> mountpoint.
> 
> Anyway, this patch closes this race when mountpoint is a file. Thus we get 
> different behaviour when deal with files and directories. It's apparently 
> not well-defined, but again, is it a problem? Not sure ... ...

OK.  I'm not seeing a strong argument to change the current behavior, so
inclined to leave it alone for now.

--b.