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Date:   Mon, 10 Oct 2022 13:49:18 +1000
Message-Id: <CNHXQ6R5IEHG.1D2WH96LGUA1I@bobo>
From:   "Nicholas Piggin" <npiggin@gmail.com>
To:     "Zhouyi Zhou" <zhouzhouyi@gmail.com>
Cc:     <mpe@ellerman.id.au>, <christophe.leroy@csgroup.eu>,
        <atrajeev@linux.vnet.ibm.com>, <linuxppc-dev@lists.ozlabs.org>,
        <linux-kernel@vger.kernel.org>, <lance@osuosl.org>,
        <paulmck@kernel.org>, <rcu@vger.kernel.org>
Subject: Re: [PATCH linux-next][RFC] powerpc: avoid lockdep when we are
 offline
References: <20220927014823.11439-1-zhouzhouyi@gmail.com>
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 <CAABZP2wYcNXkTo=tgX-ARziwgD2rng+-wCZ-qfQ6M30+vmLEug@mail.gmail.com>
 <CN7OZ6TMLLFS.2HER50Q3SO480@bobo>
 <CAABZP2xu5w9Ec8LmTUrF31KhY3HCcBjQYSk4xo14bdB9mxOz+A@mail.gmail.com>
In-Reply-To: <CAABZP2xu5w9Ec8LmTUrF31KhY3HCcBjQYSk4xo14bdB9mxOz+A@mail.gmail.com>
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On Thu Sep 29, 2022 at 11:48 AM AEST, Zhouyi Zhou wrote:
> On Wed, Sep 28, 2022 at 10:51 AM Nicholas Piggin <npiggin@gmail.com> wrot=
e:
> >
> > 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@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 R=
CU usage",
> > > > > I improved my fix under Paul E. McKenney's guidance:
> > > > > Link: https://lore.kernel.org/lkml/20220914021528.15946-1-zhouzho=
uyi@gmail.com/T/
> > > > >
> > > > > During the cpu offlining, the sub functions of xive_teardown_cpu =
will
> > > > > call __lock_acquire when CONFIG_LOCKDEP=3Dy. The latter function =
will
> > > > > travel RCU protected list, so "WARNING: suspicious RCU usage" wil=
l 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 thin=
k 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 the=
re.
> > > > I would say xive cleanup just needs to be done earlier. I wonder wh=
y it
> > > > is not done in __cpu_disable or thereabouts, that's where the inter=
rupt
> > > > 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)  =
=3D
> > > leads to =3D>  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!

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