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Date:   Fri, 15 May 2020 00:30:23 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        Josh Triplett <josh@joshtriplett.org>
Subject: Re: [PATCH 08/10] rcu: Allow to deactivate nocb on a CPU
Message-ID: <20200514223021.GA4071@lenoir>
References: <20200513164714.22557-1-frederic@kernel.org>
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 <20200513183831.GV2869@paulmck-ThinkPad-P72>
 <20200513224525.GA18303@lenoir>
 <20200514154707.GL2869@paulmck-ThinkPad-P72>
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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...

> 
> > > > +static long rcu_nocb_rdp_deoffload(void *arg)
> > > > +{
> > > > +	struct rcu_data *rdp = arg;
> > > > +
> > > > +	WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
> > > > +	__rcu_nocb_rdp_deoffload(rdp);
> > > > +
> > > > +	return 0;
> > > > +}
> > > 
> > > For example, is the problem caused by invocations of this
> > > rcu_nocb_rdp_deoffload() function?
> > 
> > How so?
> 
> It looked to me like it wasn't excluding either rcu_barrier() or CPU
> hotplug.  It might also not have been pinning onto the CPU in question,
> but that might just be me misremembering.  Then again, I didn't see a
> call to it, so maybe its callers set things up appropriately.
> 
> OK, I will bite...  What is the purpose of rcu_nocb_rdp_deoffload()?  ;-)

Ah it's called using work_on_cpu() which launch a workqueue on the
target and waits for completion. And that whole thing is protected
inside the barrier mutex and hotplug.

> Agreed!  And I do believe that concurrent callback execution will
> prove better than a possibly indefinite gap in callback execution.

Mutual agreement! :-)

Thanks.