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

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Date:   Wed, 9 Nov 2016 15:08:52 +0000
From:   Trond Myklebust <trondmy@...marydata.com>
To:     "bfields@...ldses.org" <bfields@...ldses.org>,
        "tj@...nel.org" <tj@...nel.org>,
        "jlayton@...chiereds.net" <jlayton@...chiereds.net>
CC:     "bhaktipriya96@...il.com" <bhaktipriya96@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>
Subject: Re: [PATCH v2] fs/nfsd/nfs4callback: Remove deprecated
 create_singlethread_workqueue

On Wed, 2016-11-09 at 08:18 -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.
> 
> > 
> > Yes, there can be multiple cb_work's.
> > 
> 
> Yes, but each is effectively a separate work unit. I see no reason
> why
> we'd need to order them at all.
> 

There needs to be serialisation at the session level (i.e. the
callbacks have to respect the slot limits set by the client) however
there shouldn’t be a need for serialisation at the RPC level.

Cheers
  Trond