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

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Date:   Wed, 9 Nov 2016 11:27:52 -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 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.

> > 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.
> 
> -- 
> Jeff Layton <jlayton@...chiereds.net>