linux-kernel - Re: [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads creation

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Date:   Mon, 13 Dec 2021 14:14:07 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Neeraj Upadhyay <quic_neeraju@...cinc.com>, paulmck@...nel.org,
        josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        joel@...lfernandes.org, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org, urezki@...il.com,
        boqun.feng@...il.com
Subject: Re: [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads creation

On Mon, Dec 13, 2021 at 11:28:45AM +0000, David Woodhouse wrote:
> On Mon, 2021-12-13 at 12:22 +0100, Frederic Weisbecker wrote:
> > I was about to ack the patch but, should we really add code that isn't going to
> > be necessary before a long while?
> 
> Yeah, I'm torn on that. In this case it's harmless enough and it makes
> the code reentrant in its own right instead of relying on the fact that
> the cpuhp code won't invoke it multiple times in parallel. So I think
> that's reasonable defensive programming.
> 

The thing is that RCU code is already quite complicated. Are we even at least
sure that we'll ever make CPU hotplug allow concurrent CPU onlining/offlining?

This will require much more thoughts and a new hotplug concurrency
infrastructure that we'll need to base RCU on. IMHO it's a bit early to handle
that on hotplug individual callbacks.

But anyway, let's see what Paul thinks about it...