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

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Date:   Fri, 9 Aug 2019 08:30:57 -0700
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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, Aug 09, 2019 at 11:49:49AM +0200, Thomas Gleixner wrote:
> On Thu, 8 Aug 2019, Fenghua Yu wrote:
> > On Thu, Aug 08, 2019 at 10:32:38PM +0200, Thomas Gleixner wrote:
> > > Valdis,
> > > 
> > > On Thu, 8 Aug 2019, Valdis Klētnieks wrote:
> > > > On Thu, 08 Aug 2019 22:04:03 +0200, Thomas Gleixner said: It isn't 
> > > > clear that whatever is doing the device_initcall()s will be able to 
> > > > do any reasonable recovery if we return an error, so any error 
> > > > recovery is going to have to happen before the function returns. It 
> > > > might make sense to do an 'if (ret) return;' before going further in 
> > > > the function, but given the comment a few lines further down about 
> > > > ignoring errors, it was apparently considered more important to 
> > > > struggle through and register stuff in sysfs even if umwait was 
> > > > broken....
> > > 
> > > I missed that when going through it.
> > > 
> > > The right thing to do is to have a cpu_offline callback which clears 
> > > the umwait MSR. That covers also the failure in the cpu hotplug setup. 
> > > Then handling an error return makes sense and keeps everything in a 
> > > workable shape.
> > 
> > When cpu is offline, the MSR won't be used. We don't need to clear it, right?
> 
> 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?

Thanks.

-Fenghua