linux-kernel - Re: [PATCH v2] fs/nfsd/nfs4callback: Remove deprecated create_singlethread

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Message-ID: <20161109194724.GB4952@fieldses.org>
Date:   Wed, 9 Nov 2016 14:47:24 -0500
From:   "J. Bruce Fields" <bfields@...ldses.org>
To:     Jeff Layton <jlayton@...chiereds.net>
Cc:     Tejun Heo <tj@...nel.org>,
        Bhaktipriya Shridhar <bhaktipriya96@...il.com>,
        linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] fs/nfsd/nfs4callback: Remove deprecated
 create_singlethread_workqueue

On Wed, Nov 09, 2016 at 12:33:35PM -0500, Jeff Layton wrote:
> 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?

For now I wish we could just like to continue assuming the workqueue
processes only one item at a time.  Do we have that now, or do we need
to switch to (looking at workqueue.h...) alloc_ordered workqueue()?

--b.