Re: [RFC PATCH 0/4] nfsd: add CB_NOTIFY_LOCK support

[Jeff Layton <jlayton@redhat.com>]

On Tue, 2016-08-02 at 16:38 -0400, J. Bruce Fields wrote:
> On Tue, Aug 02, 2016 at 02:15:27PM -0400, Jeff Layton wrote:
> > 
> > A small set of patches that should add CB_NOTIFY_LOCK support for knfsd.
> > The basic idea is to use FL_SLEEP to set blocks when a lock is contended,
> > and then queue a callback to issue a CB_NOTIFY_LOCK in the lm_notify op.
> > 
> > Per the RFC, we take no steps to reserve the lock for the client. This is
> > a simple notification to tell the client that it may want to poll for it
> > again.
> > 
> > It also takes steps to clean out old, abandoned blocks when the client
> > loses interest in obtaining the lock.
> 
> I had to double-check the spec language here: ok, 5661 9.6 does
> explicitly say that clients can't depend on the notify, so they do need
> to continue polling--so that cleanup doesn't risk leaving clients
> blocking indefinitely.  Might be worth a comment referencing that
> language.
> 

Yep, exactly. This is 100% a best-effort kind of thing. The clients
have to poll regardless of whether we send a notification.

> > Looks like you're not attempting any sort of fair queueing, so clients
> that get a notify just race for the lock?  That's fine.
> 

Yep. I don't think that we can reasonably do anything more
sophisticated. Hopefully the delay between callbacks will be enough to
prevent the herd from thundering too badly.

> > Thanks for looking at this!
> 
> --b.
> 

No problem. It has been on my to-do list for a while now...

> > 
> > 
> > Only lightly tested so far, but it seems to do the right thing.
> > The client-side piece is the next step.
> > 
> > Jeff Layton (4):
> >   nfsd: plumb in a CB_NOTIFY_LOCK operation
> >   nfsd: have nfsd4_lock use blocking locks for v4.1+ locks
> >   nfsd: add a LRU list for blocked locks
> >   nfsd: set the MAY_NOTIFY_LOCK flag in OPEN replies
> > 
> >  fs/nfsd/netns.h           |   1 +
> >  fs/nfsd/nfs4callback.c    |  57 +++++++++++++
> >  fs/nfsd/nfs4state.c       | 209 ++++++++++++++++++++++++++++++++++++++++++----
> >  fs/nfsd/state.h           |  21 ++++-
> >  fs/nfsd/xdr4cb.h          |   9 ++
> >  include/uapi/linux/nfs4.h |   5 +-
> >  6 files changed, 281 insertions(+), 21 deletions(-)
> > 
> > -- 
> > 2.7.4

-- 
Jeff Layton <jlayton@redhat.com>