linux-kernel - Re: [PATCH v3 6/9] x86/smpboot: Support parallel startup of secondary CPUs

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Date:   Tue, 01 Feb 2022 13:02:51 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sean Christopherson <seanjc@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "rcu@...r.kernel.org" <rcu@...r.kernel.org>,
        "mimoja@...oja.de" <mimoja@...oja.de>,
        "hewenliang4@...wei.com" <hewenliang4@...wei.com>,
        "hushiyuan@...wei.com" <hushiyuan@...wei.com>,
        "luolongjun@...wei.com" <luolongjun@...wei.com>,
        "hejingxian@...wei.com" <hejingxian@...wei.com>,
        Joerg Roedel <joro@...tes.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v3 6/9] x86/smpboot: Support parallel startup of
 secondary CPUs

On Tue, 2022-02-01 at 13:56 +0100, Borislav Petkov wrote:
> On Tue, Feb 01, 2022 at 12:39:17PM +0000, David Woodhouse wrote:
> > In the top of my git tree, you can see a half-baked 'parallel part 2'
> > commit which introduces a new x86/cpu:wait-init cpuhp state that would
> > invoke do_wait_cpu_initialized() for each CPU in turn, which *would*
> > release them all into load_ucode_bsp() at the same time and have
> > precisely the problem you're describing.
> 
> The load_ucode_bsp() is the variant that runs on the boot CPU but
> yeah...

Right. Brain not fully online today.  Sorry.

> > Hm, not sure I see how that's protecting itself from someone
> > simultaneously echoing 1 > /sys/devices/system/cpu/cpu${SIBLING}/online
> 
> So
> 
> echo 1 > ../online
> 
> means onlining the sibling.
> 
> But reload_store() grabs the CPU hotplug lock *first* and *then* runs
> check_online_cpus() to see if all CPUs are online. It doesn't do the
> update if even one CPU is missing. You can't offline any CPU for the
> duration of the update...
> 
> So I guess you'd need to explain in more detail what protection hole
> you're seeing because I might be missing something here.

No, I'd just missed cpus_read_lock() because I was looking for
something else. My fault; it looks fine. Thanks.

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