Date:   Mon, 13 Dec 2021 14:14:07 +0100
From:   Frederic Weisbecker <frederic@kernel.org>
To:     David Woodhouse <dwmw2@infradead.org>
Cc:     Neeraj Upadhyay <quic_neeraju@quicinc.com>, paulmck@kernel.org,
        josh@joshtriplett.org, rostedt@goodmis.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        joel@joelfernandes.org, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, urezki@gmail.com,
        boqun.feng@gmail.com
Subject: Re: [PATCH v2] rcu/nocb: Handle concurrent nocb kthreads creation
Message-ID: <20211213131407.GD782195@lothringen>
References: <20211211170139.27711-1-quic_neeraju@quicinc.com>
 <6c184b9ffc5c641736d53bb7598f814d6b4c3fe0.camel@infradead.org>
 <601ecb12-ae2e-9608-7127-c2cddc8038a6@quicinc.com>
 <20211213112246.GA782195@lothringen>
 <984a63d4c11d04e2ee8a83fc9c61006413bf209e.camel@infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <984a63d4c11d04e2ee8a83fc9c61006413bf209e.camel@infradead.org>
Precedence: bulk

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...