Re: NFS client hang on attempt to do async blocking posix lock enqueue

[Jeff Layton <jlayton@redhat.com>]

On Fri, 8 Feb 2008 16:12:28 -0500
"J. Bruce Fields" <bfields@fieldses.org> wrote:

> On Fri, Feb 08, 2008 at 03:54:14PM -0500, Jeff Layton wrote:
> > Interesting. It's not clear me why the underlying filesystem would make
> > any difference there. Though now that I look, it looks like fl_grant
> > really only gets called from dlm code, and that queues up the block for
> > an immediate grant callback attempt. So perhaps that's the reason.
> 
> The asynchronous locking interface does something slightly cheesy for
> blocking locks--instead of waiting for the filesystem to respond, it
> just sends back a deny immediately (even if the lock might actually be
> available), then responds later with a granted message when it discovers
> it's available.
> 
> That works, but we should make it just wait to send the reply to the
> original lock request until we've got a real answer, as we do for
> nonblocking lock requests.  And in fact someone submitted a patch to do
> that--I just haven't gotten the time to review it.  Urp.
> 
> So anyway the effect is that on ext3 this particular lock wouldn't have
> required a grant reply, whereas on gfs2 it does.
> 
> Of course, what this means is that we'd hit the same problem on ext3 too
> if the lock request did in fact legitimately block.  So grant callbacks
> probably have never worked on ext3 over the loopback interface either.
> Oops!
> 

As best I can tell, the whole problem with rpc_pings was introduced
when we moved everything to the rpcbind stuff. Before that we generally
never did an rpc_ping when binding the client. This probably did work
until that was introduced.

> I bet nobody's ever noticed because we manage to recover by retrying the
> lock after it's available (whereas in the gfs2 case the retry hits the
> same problem).  So in practice for ext3 this probably just means
> blocking lock requests take a lot longer over loopback then they would
> otherwise.  And probably the only people that care about nlm performance
> don't usually do local mounts like that.
> 

-- 
Jeff Layton <jlayton@redhat.com>