[RFC] Reinstate NFS exportability for JFFS2.

[Neil Brown]

On Tuesday August 5, david at fromorbit.com wrote:
> On Mon, Aug 04, 2008 at 02:19:12AM -0400, Chuck Lever wrote:
> > So, the JFFS2 locking problem is a garbage-collection issue.  I'm not
> > sure this is the case with other file systems like XFS and OCFS2.  My
> > impression was that XFS had a transaction logging deadlock,
> 
> Just to clarify - XFS has a directory buffer lock deadlock. That is,
> while reading the contents of the directory buffer it is locked to
> prevent modifications from occurring while extracting the contents.
> Looking up an entry in the directory also requires the directory
> buffer lock (for the same reason), so calling the lookup while
> already holding the directory buffer lock (i.e from the filldir
> callback) will deadlock.

How much cost would it be, do you think, to drop the lock across the
call to filldir?  Then reclaim the lock, validate pointers etc against
a 'version' counter, and restart based on the current telldir cookie
if needed?

To me, that is the generic solution to allowing filldir to call
->lookup.  I'm just not sure what it costs to be constantly dropping
and reclaiming the lock in the normal case where ->lookup isn't being
called.


NeilBrown