On Thu Sep 29, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote:

On Wed, Sep 28, 2022 at 10:51 AM Nicholas Piggin [off-list ref] wrote:

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On Wed Sep 28, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote:

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Thank Nick for reviewing my patch

On Tue, Sep 27, 2022 at 12:25 PM Nicholas Piggin [off-list ref] wrote:

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On Tue Sep 27, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote:

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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/ (local)

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

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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 ;-)

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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!

I was thinking you could try it and see if it works and what you find.
If you are interested and have time to look into it?

Thanks,
Nick