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

[Joel Fernandes]

On Thu, Jul 30, 2020 at 09:21:59AM -0700, Paul E. McKenney wrote:
> On Wed, Jul 29, 2020 at 11:02:20PM -0400, Joel Fernandes (Google) wrote:
> > Add a warning if CPU being onlined did not report QS already. This is to
> > simplify the code in the CPU onlining path and also to make clear about
> > where QS is reported. The act of QS reporting in CPU onlining path is
> > is likely unnecessary as shown by code reading and testing with
> > rcutorture's TREE03 and hotplug parameters.
> 
> How about something like this for the commit log?
> 
> ------------------------------------------------------------------------
> 
> Currently, rcu_cpu_starting() checks to see if the RCU core expects a
> quiescent state from the incoming CPU.  However, the current interaction
> between RCU quiescent-state reporting and CPU-hotplug operations should
> mean that the incoming CPU never needs to report a quiescent state.
> First, the outgoing CPU reports a quiescent state if needed.  Second,
> the race where the CPU is leaving just as RCU is initializing a new
> grace period is handled by an explicit check for this condition.  Third,
> the CPU's leaf rcu_node structure's ->lock serializes these checks.
> 
> This means that if rcu_cpu_starting() ever feels the need to report
> a quiescent state, then there is a bug somewhere in the CPU hotplug
> code or the RCU grace-period handling code.  This commit therefore
> adds a WARN_ON_ONCE() to bring that bug to everyone's attention.
> 
> ------------------------------------------------------------------------
> 
> > Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> > Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > 
> > ---
> >  kernel/rcu/tree.c | 14 +++++++++++++-
> >  1 file changed, 13 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 65e1b5e92319..1e51962b565b 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -3996,7 +3996,19 @@ void rcu_cpu_starting(unsigned int cpu)
> >  	rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */
> >  	rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq);
> >  	rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags);
> > -	if (rnp->qsmask & mask) { /* RCU waiting on incoming CPU? */
> > +
> > +	/*
> > +	 * Delete QS reporting from here, by June 2021, if warning does not
> > +	 * fire. Let us make the rules for reporting QS for an offline CPUs
> > +	 * more explicit. 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. Note
> > +	 * that the FQS loop also does not report QS for an offline CPU any
> > +	 * longer (unless it splats due to an offline CPU blocking the GP for
> > +	 * too long).
> > +	 */
> 
> Let's leave at least the WARN_ON_ONCE() indefinitely.  If you don't
> believe me, remove this code in your local tree, have someone give you
> several branches, some with bugs injected, and then try to figure out
> which have the bugs and then try to find those bugs.
> 
> This is not a fastpath, so the overhead of the check is not a concern.
> Believe me, the difficulty of bug location without this check is a very
> real concern!  ;-)
> 
> On the other hand, I fully agree with the benefits of documenting the
> design rules.  But is this really the best place to do that from the
> viewpoint of someone who is trying to figure out how RCU works?

I can move this comment to: "Hotplug CPU" section in
Documentation/RCU/Design/Requirements/Requirements.rst

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.
	 */

I will post my v3 with changes to the requirements document.

Let me know any other comments, thanks,

 - Joel