Re: [PATCH v2] fs/nfsd/nfs4callback: Remove deprecated create_singlethread_workqueue

[Jeff Layton]

On Wed, 2016-11-09 at 11:27 -0500, J. Bruce Fields wrote:
> On Wed, Nov 09, 2016 at 08:18:08AM -0500, Jeff Layton wrote:
> > 
> > On Tue, 2016-11-08 at 20:27 -0500, J. Bruce Fields wrote:
> > > 
> > > On Tue, Nov 08, 2016 at 05:52:21PM -0500, Tejun Heo wrote:
> > > > 
> > > > 
> > > > Hello, Bruce.
> > > > 
> > > > On Tue, Nov 08, 2016 at 04:39:11PM -0500, J. Bruce Fields wrote:
> > > > > 
> > > > > 
> > > > > Apologies, just cleaning out old mail and finding some I should have
> > > > > responded to long ago:
> > > > > 
> > > > > On Wed, Aug 31, 2016 at 02:23:48AM +0530, Bhaktipriya Shridhar wrote:
> > > > > > 
> > > > > > 
> > > > > > The workqueue "callback_wq" queues a single work item &cb->cb_work per
> > > > > > nfsd4_callback instance and thus, it doesn't require execution ordering.
> > > > > 
> > > > > What's "execution ordering"?
> > > > > 
> > 
> > AIUI, it means that jobs are always run in the order queued and are
> > serialized.
> > 
> > > 
> > > > 
> > > > > 
> > > > > We definitely do depend on the fact that at most one of these is running
> > > > > at a time.
> > > > 
> > 
> > We do?
> > 
> > > 
> > > > 
> > > > If there can be multiple cb's and thus cb->cb_work's per callback_wq,
> > > > it'd need explicit ordering.  Is that the case?
> > > 
> > 
> > These are basically client RPC tasks, and the cb_work just handles the
> > submission into the client RPC state machine. Just because we're running
> > several callbacks at the same time doesn't mean that they need to be
> > strictly ordered. The client state machine can certainly handle running
> > these in parallel.
> 
> I'm not worried about the rpc calls themselves, I'm worried about the
> other stuff in nfsd4_run_cb_work(), especially
> nfsd4_process_cb_update().
> 
> It's been a while since I thought about it and maybe it'd be OK with a
> little bit of extra locking.
> 
> --b.
> 

Ahh good point there. nfsd4_process_cb_update is a bit of a special
case, and I hadn't considered that.

I think we could use the cl_lock to protect most of the fields that are
affected there.

I'm not sure how to handle setup_callback_client though. Should we
serialize those calls so that we're only constructing one at a time and
have other threads wait on it? We could use a cl_flags bit for a
NFSD4_CLIENT_CB_CONSTRUCTING flag and serialize on those?

-- 
Jeff Layton <jlayton@xxxxxxxxxxxxxxx>