From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [patch 00/13] vfs: add helpers to check r/o bind mounts
Date: Thu, 24 Apr 2008 18:30:13 +0100
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To: Erez Zadok <ezk@cs.sunysb.edu>
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On Thu, Apr 24, 2008 at 01:25:58PM -0400, Erez Zadok wrote:
> Al, any near-term plans for sb-level "want write" locking as we discussed
> briefly at LSF?  Being able to do so for copyup in unionfs will hopefully
> allow me to prevent concurrent topology changes.

That's pretty close to the top.

> the two directories that need locking.  This rename-locking protocol appears
> to be a special case of the sb-level "want write" idea, right?

Not really - that one doesn't provide any exclusion between writers and
that's what we want from rename/rename.

Take a look at Documentation/filesystems/directory-locking for details
of locking scheme...