Re: [PATCH] rcu: remove unnecessary check cpu_no_qs.norm on rcu_report_qs_rdp

[Yun Levi]

Hi Z qiang!

Thanks for replying. But I'm pinned on something wrong..!

> For built with CONFIG_RCU_STRICT_GRACE_PERIOD=y and CONFIG_PREEMPT=n kernels
> Consider the following scenario:
>
> __rcu_read_unlock()
>    -> rcu_read_unlock_strict()
>         ->rdp = this_cpu_ptr(&rcu_data);
>         ->rdp->cpu_no_qs.b.norm = false;
>
>                  by interrupt and return invoke rcu_core():
>                  ->rcu_check_quiescent_state()
>                       ->rdp = raw_cpu_ptr(&rcu_data);
>                       -> rcu_check_quiescent_state(rdp);
>                             ->note_gp_changes(rdp);
>                                 -> __note_gp_changes(rnp, rdp)
>                                 start new gp
>                                 ->rdp->cpu_no_qs.b.norm = true;
>
>         ->rcu_report_qs_rdp(rdp);
>            ->if (rdp->cpu_no_qs.b.norm || ...)

I've already seen this scenario, But I think something is missing in my view.
What I couldn't catch is

                  ->rcu_check_quiescent_state()
                       ->rdp = raw_cpu_ptr(&rcu_data);
                       -> rcu_check_quiescent_state(rdp);
                             ->note_gp_changes(rdp);
                                 -> __note_gp_changes(rnp, rdp)
                                 start new gp

the new gp is started.

to set cpu_no_qs.b.norm as true, below condition should be true

1201 >---if (rcu_seq_new_gp(rdp->gp_seq, rnp->gp_seq) ||
  1202 >---    unlikely(READ_ONCE(rdp->gpwrap))) {

Here,
How rcu_seq_new_gp could return true and new gp already started via
rcu_gp_kthread.
IIUC, because rcu_gp_fqs_loop couldn't see the root rnp->qsmask is
zero, it couldn't call rcu_gp_init.

Sorry to make noise, but would you correct me what I'm thinking wrong?

Many thanks..!

-----------
Sincerely,
Levi.

On Mon, Jul 24, 2023 at 4:21 AM Z qiang <qiang.zhang1211@xxxxxxxxx> wrote:
>
> >
> > Thanks for replying to reply Paul :)
> >
> > > And try testing with CONFIG_RCU_STRICT_GRACE_PERIOD=y and CONFIG_PREEMPT=n.
> > > Though there might be better Kconfig options to use.  Those two come
> > > immediately to mind.
> >
> > I've tested with this option via rcu torture.
> > and it doesn't report any problems.
> > and after commit 6d60ea03ac2d3 ("rcu: Report QS for outermost
> > PREEMPT=n rcu_read_unlock() for strict GPs")
> > it always makes cpu_no_qs.b.norm false whenever it calls
> > rcu_report_qs_rdp in rcu_read_unlock.
> >
> > > But one critical piece is that softirq handlers, including the RCU_SOFTIRQ
> > > handler rcu_core_si(), can be invoked upon return from interrupts.
> >
> > I think in that case, no problem because if it reports qs already,
> > rcu_check_quiescent_state wouldn't call rcu_report_qs_rdp again.
> > And if RCU_SOFTIRQ is called as soon as RCU interrupt is finished,
> > it seems the fastest one to notify qs to related tree.
> >
> > > Another critical piece is that if a CPU is idle during any part of a
> > > grace period, the grace-period kthread can report a quiescent state on
> > > its behalf.
> >
> > I think
> >     1) If timer interrupt is still programed,
> >           - when rcu_sched_clock_irq first reports qs, no problem
> >           - If qs is reported via grace period thread first,
> > note_gp_chagned in rcu interrupt
> >             will recognize this situation by setting core_needs_qs as false,
> >             it doesn't call rcu_report_qs_rdp thou cpu_no_qs.b.norm is true.
> >
> >      2) If the timer interrupt isn't programmed,
> >           - rcu_gp_kthreaad reports its qs, no problem.
> >
> > So, I think there's no problem removing
> >       "rdp->cpu_no_qs.b.norm" check in rcu_report_qs_rdp.
> > or wrap this condition check as WARN_ON_ONCE.
> >
> > > Does that help?
> > Many thanks always :)
> >
>
>
> Hi Levi
>
> For built with CONFIG_RCU_STRICT_GRACE_PERIOD=y and CONFIG_PREEMPT=n kernels
> Consider the following scenario:
>
> __rcu_read_unlock()
>    -> rcu_read_unlock_strict()
>         ->rdp = this_cpu_ptr(&rcu_data);
>         ->rdp->cpu_no_qs.b.norm = false;
>
>                  by interrupt and return invoke rcu_core():
>                  ->rcu_check_quiescent_state()
>                       ->rdp = raw_cpu_ptr(&rcu_data);
>                       -> rcu_check_quiescent_state(rdp);
>                             ->note_gp_changes(rdp);
>                                 -> __note_gp_changes(rnp, rdp)
>                                 start new gp
>                                 ->rdp->cpu_no_qs.b.norm = true;
>
>         ->rcu_report_qs_rdp(rdp);
>            ->if (rdp->cpu_no_qs.b.norm || ...)
>
>
> Thanks
> Zqiang
>
>
> >
> > --------
> > SIncerely,
> > Levi.