linux-kernel - Re: [PATCH -rcu dev 1/2] Revert b8c17e6664c4 ("rcu: Maintain special bits at bottom of ->dynticks counter")

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Message-ID: <20190906150806.GA11355@google.com>
Date:   Fri, 6 Sep 2019 11:08:06 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Andy Lutomirski <luto@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Petr Mladek <pmladek@...e.com>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH -rcu dev 1/2] Revert b8c17e6664c4 ("rcu: Maintain special
 bits at bottom of ->dynticks counter")

On Thu, Sep 05, 2019 at 08:01:37PM -0400, Joel Fernandes wrote:
[snip] 
> > > > @@ -3004,7 +3007,7 @@ static int rcu_pending(void)
> > > >  		return 0;
> > > >  
> > > >  	/* Is the RCU core waiting for a quiescent state from this CPU? */
> > > > -	if (rdp->core_needs_qs && !rdp->cpu_no_qs.b.norm)
> > > > +	if (READ_ONCE(rdp->core_needs_qs) && !rdp->cpu_no_qs.b.norm)
> > > >  		return 1;
> > > >  
> > > >  	/* Does this CPU have callbacks ready to invoke? */
> > > > @@ -3244,7 +3247,6 @@ int rcutree_prepare_cpu(unsigned int cpu)
> > > >  	rdp->gp_seq = rnp->gp_seq;
> > > >  	rdp->gp_seq_needed = rnp->gp_seq;
> > > >  	rdp->cpu_no_qs.b.norm = true;
> > > > -	rdp->core_needs_qs = false;
> > > 
> > > How about calling the new hint-clearing function here as well? Just for
> > > robustness and consistency purposes?
> > 
> > This and the next function are both called during a CPU-hotplug online
> > operation, so there is little robustness or consistency to be had by
> > doing it twice.
> 
> Ok, sorry I missed you are clearing it below in the next function. That's
> fine with me.
> 
> This patch looks good to me and I am Ok with merging of these changes into
> the original patch with my authorship as you mentioned. Or if you wanted to
> be author, that's fine too :)

Paul, does it make sense to clear these urgency hints in rcu_qs() as well?
After all, we are clearing atleast one urgency hint there: the
rcu_read_unlock_special::need_qs bit.

thanks,

 - Joel