linux-kernel - Re: [PATCH] arch/x86/kernel/cpu/umwait.c

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Message-ID: <alpine.DEB.2.21.1908092136510.21433@nanos.tec.linutronix.de>
Date:   Fri, 9 Aug 2019 21:37:31 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
cc:     Valdis Klētnieks <valdis.kletnieks@...edu>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arch/x86/kernel/cpu/umwait.c - remove unused variable

On Fri, 9 Aug 2019, Fenghua Yu wrote:
> On Fri, Aug 09, 2019 at 11:49:49AM +0200, Thomas Gleixner wrote:
> > Groan. If soemthing goes wrong when registering the hotplug callback, what
> > undoes the MSR setup which might have happened and what takes care of it on
> > cpus coming online later? Exactly nothing. Then you have a non-consistent
> > behaviour.
> > 
> > Make stuff symmmetric and correct and not optimized for the sunshine case.
> 
> I see.
> 
> Just want to make sure I understand it correctly:
> 
> sysfs_create_group() should not be called if cpuhp_setup_state() has
>  error.
> 
> Otherwise, the sysadmin can change the MSR through the sysfs interface.
> After that, a CPU is online and its MSR is not updated because cpu_online
> is not installed. Then this online CPU has different MSR value from
> the other CPUs.
> 
> Is that right?

Yes. You need to enforce safe and consistent behaviour.

Thanks,

	tglx