Re: [PATCH] arch/x86/kernel/cpu/umwait.c - remove unused variable

[Thomas Gleixner]

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