linux-kernel - Re: [PATCH 1/2] rcu/tree: Add a warning if CPU being onlined did not report QS already

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Message-ID: <20200807154529.GB2865655@google.com>
Date:   Fri, 7 Aug 2020 11:45:29 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH 1/2] rcu/tree: Add a warning if CPU being onlined did not
 report QS already

On Fri, Aug 07, 2020 at 11:37:32AM -0400, Joel Fernandes wrote:
> Hi Paul,
> 
> On Thu, Jul 30, 2020 at 08:48:25PM -0700, Paul E. McKenney wrote:
> [...]
> > > And I could make the comment here as:
> > > 	/*
> > > 	 * Delete QS reporting from here, by June 2021, if the warning does not
> > >  	 * fire. Leave the warning indefinitely. Check RCU design requirements
> > > 	 * in Documentation/RCU/ about CPU hotplug requirements.
> > > 	 */
> > 
> > Rather than decide for our future selves, could we please just suggest
> > reviewing this on June 2021?  Or, given enterprise distro schedules,
> > 2024.  :-/
> 
> I am replacing it with the following, let me know if any objections, thanks:
> 
> +        * XXX: The following rcu_report_qs_rnp() is redundant. If the below
> +        * warning does not fire, consider replacing it with the "else" block,
> +        * by June 2021 or so. The rationale for this is as follows: The CPU
> +        * onlining path does not need to report QS for an offline CPU. Either
> +        * the QS should have reported during CPU offlining, or during
> +        * rcu_gp_init() if it detected a race with either CPU offlining or
> +        * task unblocking on previously offlined CPUs. To avoid deadlocks
> +        * related to waiting on timers or cpu hotplug locks, only those paths
> +        * do the QS reporting for offline CPUs.

And you did mention you still want the warn-on indefinitely, so I'll document
that in the comment as well.

Now it looks like:

        /*
         * XXX: The following rcu_report_qs_rnp() is redundant. If the below
         * warning does not fire, consider replacing it with the "else" block,
         * by June 2021 or so (while keeping the warning). The rationale for
         * this is as follows: The CPU onlining path does not need to report QS
         * for an offline CPU. Either the QS should have reported during CPU
         * offlining, or during rcu_gp_init() if it detected a race with either
         * CPU offlining or task unblocking on a node with all of its CPUs
         * previously offlined.  To avoid deadlocks related to waiting on
         * timers or cpu hotplug locks, only these paths do the QS reporting
         * for offline CPUs making the following reporting redundant.
         */

thanks,

 - Joel

> 
> thanks,
> 
>  - Joel
> 
> > 
> > 							Thanx, Paul
> > 
> > > I will post my v3 with changes to the requirements document.
> > > 
> > > Let me know any other comments, thanks,
> > > 
> > >  - Joel
> > >