linux-kernel - Re: [PATCH 04/12] ia64: Replace cpu_down with freeze_secondary

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Date:   Tue, 19 Nov 2019 23:59:10 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Qais Yousef <qais.yousef@....com>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 04/12] ia64: Replace cpu_down with
 freeze_secondary_cpus

On Tue, 19 Nov 2019, Qais Yousef wrote:
> On 11/19/19 23:21, Thomas Gleixner wrote:
> > On Wed, 30 Oct 2019, Qais Yousef wrote:
> > >  void machine_shutdown(void)
> > >  {
> > >  #ifdef CONFIG_HOTPLUG_CPU
> > > -	int cpu;
> > > -
> > > -	for_each_online_cpu(cpu) {
> > > -		if (cpu != smp_processor_id())
> > > -			cpu_down(cpu);
> > > -	}
> > > +	/* TODO: Can we use disable_nonboot_cpus()? */
> > > +	freeze_secondary_cpus(smp_processor_id());
> > 
> > freeze_secondary_cpus() is only available for CONFIG_PM_SLEEP_SMP=y and
> > disable_nonboot_cpus() is a NOOP for CONFIG_PM_SLEEP_SMP=n :)
> 
> I thought I replied to this :-(
> 
> My plan was to simply make freeze_secondary_cpus() available and protected by
> CONFIG_SMP only instead.
> 
> Good plan?

No. freeze_secondary_cpus() is really for hibernation. Look at the exit
conditions there.

So you really want a seperate function which depends on CONFIG_HOTPLUG_CPU
and provides an inline stub for the CONFIG_HOTPLUG_CPU=n case.

But I have a hard time to see how that stuff works at all on
ia64:

  cpu_down() might sleep, i.e. it must be called from preemptible
  context. smp_processor_id() is invalid from preemtible context.

As this is obviously broken and ia64 is in keep compile mode only, it
should just go away.

Thanks,

	tglx