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SubjectRe: [PATCH 1/2] rcu/tree: Add a warning if CPU being onlined did not report QS already
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@kernel.org>
> > Cc: Neeraj Upadhyay <neeraju@codeaurora.org>
> > Suggested-by: Paul E. McKenney <paulmck@kernel.org>
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> >
> > ---
> > 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

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