[PATCH] locks: breaking read lease should not block read open

[J. Bruce Fields]

On Fri, Aug 19, 2011 at 08:08:29PM +0100, Jamie Lokier wrote:
> J. Bruce Fields wrote:
> > I'm not sure how to approach the lease code.
> > 
> > On the one hand, I've never seen any evidence that anyone outside Samba
> > and NFSv4 has ever used it, and both currently make extremely limited
> > use of it.  So we could probably get away with "fixing" the lease code
> > to do whatever both of us need.
> 
> I've never used it, but I've _nearly_ used it (project took a
> different direction), in a web application framework.
> 
> Pretty much the way CIFS/NFS use it, to keep other things (remote
> state, database state, derived files) transactionally coherent with
> changes to file contents by programs that only know about the files
> they access.
> 
> The SIGIO stuff is a horrible interface.
> I could still see me trying to use it sometime in the future.
> In which case I really don't mind if you make the semantics saner :-)
> 
> Now we have fanotify which does something very similar and could have
> generalised leases, but unfortunately fanotify came from such a
> different motivation that it's not well suited for ordinary user
> applications.

I'm not sure what you mean by that--mainly just because I'm not as
familiar with fanotify as I should be.

For my case the important difference between leases and the various
notification interfaces is that leases are synchronous--the lease-holder
is notified and has a chance to clean up before releasing its lease and
allowing the conflicting operation to continue--whereas the the various
notification interfaces tell you "tough luck, something just happened".

--b.