Re: [PATCH 10/10] gfs2: nfs lock support for gfs2

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Dec 06, 2006 at 02:58:22PM -0600, David Teigland wrote:
> On Wed, Dec 06, 2006 at 02:57:22PM -0500, J. Bruce Fields wrote:
> > The more annoying problem is the need to cancel the GFS lock when
> > fl_notify fails; is that something that it's possible for GFS to do?
> > 
> > It can fail because lockd has a timeout--it waits a few seconds for the
> > callback, then gives up and returns a failure to the user.  If that
> > happens after your userspace posix lock manager acquires the lock (but
> > before fl_notify is called) then you've got to cancel it somehow.
> 
> I'd think we could just send an unlock for it at that point.  Set up an op
> with GDLM_PLOCK_OP_UNLOCK and the same fields as the lock you're removing
> and call send_op().  We probably need to flag this internal-unlock op so
> that when the result arrives, device_write() delists and frees it itself.
> 
> (I wouldn't call this "canceling", I think of cancel as trying to force a
> blocked request to return/fail prematurely.)

I call it a cancel because it should leave us in the same state we were
in if we hadn't done the lock.  An unlock doesn't do that, because the
original lock may have coalesced and/or downgraded existing locks.

We've got a similar problem with NFSv4 blocking locks.  I've got
unsubmitted patches

	http://linux-nfs.org/cgi-bin/gitweb.cgi?p=bfields-2.6.git;a=shortlog;h=fair-queueing

which introduce a new "provisional" lock type that is identical to a
posix lock in every way except that a provisional lock doesn't coalesce
or downgrade existing locks--it just sits there on the lock list
blocking conflicting requests until somebody comes along and upgrades it
to a real lock or cancels it.

Maybe there's a better solution in this case.  I can't think of anything
other than just giving up on the whole idea of timing out.  (Maybe that
wouldn't be so bad?)

--b.