[PATCH linux-next][RFC] powerpc: avoid lockdep when we are offline

[Zhouyi Zhou]

On Wed, Sep 28, 2022 at 10:51 AM Nicholas Piggin <npiggin at gmail.com> wrote:
>
> On Wed Sep 28, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote:
> > Thank Nick for reviewing my patch
> >
> > On Tue, Sep 27, 2022 at 12:25 PM Nicholas Piggin <npiggin at gmail.com> wrote:
> > >
> > > On Tue Sep 27, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote:
> > > > This is second version of my fix to PPC's  "WARNING: suspicious RCU usage",
> > > > I improved my fix under Paul E. McKenney's guidance:
> > > > Link: https://lore.kernel.org/lkml/20220914021528.15946-1-zhouzhouyi@gmail.com/T/
> > > >
> > > > During the cpu offlining, the sub functions of xive_teardown_cpu will
> > > > call __lock_acquire when CONFIG_LOCKDEP=y. The latter function will
> > > > travel RCU protected list, so "WARNING: suspicious RCU usage" will be
> > > > triggered.
> > > >
> > > > Avoid lockdep when we are offline.
> > >
> > > I don't see how this is safe. If RCU is no longer watching the CPU then
> > > the memory it is accessing here could be concurrently freed. I think the
> > > warning is valid.
> > Agree
> > >
> > > powerpc's problem is that cpuhp_report_idle_dead() is called before
> > > arch_cpu_idle_dead(), so it must not rely on any RCU protection there.
> > > I would say xive cleanup just needs to be done earlier. I wonder why it
> > > is not done in __cpu_disable or thereabouts, that's where the interrupt
> > > controller is supposed to be stopped.
> > Yes, I learn flowing events sequence from kgdb debugging
> > __cpu_disable -> pseries_cpu_disable -> set_cpu_online(cpu, false)  =
> > leads to =>  do_idle: if (cpu_is_offline(cpu) -> arch_cpu_idle_dead
> > so xive cleanup should be done in pseries_cpu_disable.
>
> It's a good catch and a reasonable approach to the problem.
Thank Nick for your encouragement ;-)
>
> > But as a beginner, I afraid that I am incompetent to do above
> > sophisticated work without error although I am very like to,
> > Could any expert do this for us?
>
> This will be difficult for anybody, it's tricky code. I'm not an
> expert at it.
>
> It looks like the interrupt controller disable split has been there
> since long before xive. I would try just move them together than see
> if that works.
Yes, I use "git blame" (I learned "git blame" from Paul E. McKenny ;-)
) to see the same.
and anticipate your great works!
>
> Documentation/core-api/cpu_hotplug.rst says that __cpu_disable should
> shut down the interrupt handler. So if there is a complication it
> would probably be from powerpc-specific CPU hotplug  or interrupt
> code.
Thank Nick for your guidance! I studied
Documentation/core-api/cpu_hotplug.rst this morning.
I also found X86 shut down the interrupt handler in __cpu_disable
according to above document.

Many Thanks
Zhouyi
>
> Thanks,
> Nick
>