linux-kernel - Re: [PATCH v16 3/8] cpu/hotplug: Add dynamic parallel bringup states before CPUHP_BRINGUP

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Message-ID: <87sfdu7a8v.ffs@tglx>
Date:   Fri, 24 Mar 2023 10:46:08 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     David Woodhouse <dwmw2@...radead.org>,
        Usama Arif <usama.arif@...edance.com>, kim.phillips@....com,
        brgerst@...il.com
Cc:     piotrgorski@...hyos.org, oleksandr@...alenko.name,
        arjan@...ux.intel.com, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, x86@...nel.org,
        pbonzini@...hat.com, paulmck@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        rcu@...r.kernel.org, mimoja@...oja.de, hewenliang4@...wei.com,
        thomas.lendacky@....com, seanjc@...gle.com, pmenzel@...gen.mpg.de,
        fam.zheng@...edance.com, punit.agrawal@...edance.com,
        simon.evans@...edance.com, liangma@...ngbit.com,
        gpiccoli@...lia.com
Subject: Re: [PATCH v16 3/8] cpu/hotplug: Add dynamic parallel bringup
 states before CPUHP_BRINGUP_CPU

On Fri, Mar 24 2023 at 02:16, Thomas Gleixner wrote:
> On Thu, Mar 23 2023 at 23:12, David Woodhouse wrote:
> So surely you could claim that up to #6 (TSC sync) nothing needs to
> synchronize.
>
> But that's simply not true because topology information is implicitely
> serialized by CPU hotplug and your attempt to serialize that in patch
> 7/8 is not cutting it at all because AP #4 (STARTING) callbacks rely
> implicitely on the immutability of the topology information and so do
> some of the later (threaded) AP callbacks too.

It's even worse. Any topology relevant change _must_ be serialized by
holding cpu_hotplug_lock write locked. So this needs fundamentally more
thought than just slapping a few misnomed states into the picture and
pretending that everything else just works.

Thanks,

        tglx