linux-kernel - Re: [PATCH 0/7] Eliminate delegation self-conflicts

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Date:   Fri, 15 Feb 2019 11:35:14 -0500
From:   bfields@...ldses.org (J. Bruce Fields)
To:     "J. Bruce Fields" <bfields@...hat.com>
Cc:     Jeff Layton <jlayton@...hat.com>, linux-nfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Trond Myklebust <trondmy@...merspace.com>,
        David Howells <dhowells@...hat.com>, Tejun Heo <tj@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Shaohua Li <shli@...com>, Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 0/7] Eliminate delegation self-conflicts

On Mon, Feb 11, 2019 at 10:58:04AM -0500, J. Bruce Fields wrote:
> On Sat, Feb 09, 2019 at 07:43:54AM -0500, Jeff Layton wrote:
> > On Fri, 2019-02-08 at 15:10 -0500, J. Bruce Fields wrote:
> > > From: "J. Bruce Fields" <bfields@...hat.com>
> > > 
> > > These patches allow NFSv4 clients holding delegations to keep them when
> > > the operation that would break a delegation comes from the same client.
> > > 
> > > To do that, we somehow need to pass the identity of the
> > > delegation-breaker down through the VFS.
> > > 
> > > This series uses the tgid, a solution suggested by Trond.  To do that we
> > > need nfsd tasks to share the same tgid.  I do that by extending the
> > > kthread code slightly to allow knfsd to run the kthreadd main loop in a
> > > task of its own, and spawn its server threads off of that task.
> ...
> > Nice work! I like the basic idea, the changes seem to be well-organized, 
> > and the tgid semantics are clear and make sense.
> > 
> > Would this preclude us from moving to a workqueue-based model for knfsd
> > later? It's likely to still be worth it, but it'd be good to understand
> > the potential drawbacks.
> 
> I was wondering about that too, but I haven't looked into it yet.
> Workqueues look a lot more complicated than kthreads.

I spent some time staring, and...  I still don't really understand the
workqueue code.  But if this kthread_group[*] code is acceptable than I
can't see why it shouldn't be possible to create a workqueue whose work
items are all handled by threads spawned form the same kthread_group.

--b.

[*] Open to suggestions of better names.