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From:   Thomas Gleixner <tglx@linutronix.de>
To:     David Woodhouse <dwmw2@infradead.org>,
        Usama Arif <usama.arif@bytedance.com>, kim.phillips@amd.com,
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Subject: Re: [PATCH v16 3/8] cpu/hotplug: Add dynamic parallel bringup
 states before CPUHP_BRINGUP_CPU
In-Reply-To: <87v8ir6j96.ffs@tglx>
References: <20230321194008.785922-1-usama.arif@bytedance.com>
 <20230321194008.785922-4-usama.arif@bytedance.com> <874jqb8588.ffs@tglx>
 <871qlf83wj.ffs@tglx>
 <8dff6ae5ffaebfbcc55a01c04420fd478070b830.camel@infradead.org>
 <87v8ir6j96.ffs@tglx>
Date:   Fri, 24 Mar 2023 10:46:08 +0100
Message-ID: <87sfdu7a8v.ffs@tglx>
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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