linux-kernel - Re: [PATCH 08/10] rcu: Allow to deactivate nocb on a CPU

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Message-ID: <20200514225520.GC4071@lenoir>
Date:   Fri, 15 May 2020 00:55:21 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH 08/10] rcu: Allow to deactivate nocb on a CPU

On Thu, May 14, 2020 at 03:47:35PM -0700, Paul E. McKenney wrote:
> On Fri, May 15, 2020 at 12:30:23AM +0200, Frederic Weisbecker wrote:
> > On Thu, May 14, 2020 at 08:47:07AM -0700, Paul E. McKenney wrote:
> > > On Thu, May 14, 2020 at 12:45:26AM +0200, Frederic Weisbecker wrote:
> > > This last seems best to me.  The transition from CBLIST_NOT_OFFLOADED
> > > to CBLIST_OFFLOADING of course needs to be on the CPU in question with
> > > at least bh disabled.  Probably best to be holding rcu_nocb_lock(),
> > > but that might just be me being overly paranoid.
> > 
> > So that's in the case of offloading, right? Well, I don't think we'd
> > need to even disable bh nor lock nocb. We just need the current CPU
> > to see the local update of cblist->offloaded = CBLIST_OFFLOADING
> > before the kthread is unparked:
> > 
> >     cblist->offloaded = CBLIST_OFFLOADING;
> >     /* Make sure subsequent softirq lock nocb */
> >     barrier();
> >     kthread_unpark(rdp->nocb_cb_thread);
> > 
> > Now, although that guarantees that nocb_cb will see CBLIST_OFFLOADING
> > upon unparking, it's not guaranteed that the nocb_gp will see it on its
> > next round. Ok so eventually you're right, I should indeed lock nocb...
> 
> I suspect that our future selves would hate us much less if we held
> that lock.  ;-)

Also, taking the decision to hold that lock could teach a lesson to our
past selves. Win-win! Let us become that most welcome time bridge!