Re: [PATCH v3 2/3 RESEND] acpi : prevent cpu from becoming online

[Yasuaki Ishimatsu]

Hi Toshi,

2012/07/13 1:49, Toshi Kani wrote:
> On Thu, 2012-07-12 at 20:40 +0900, Yasuaki Ishimatsu wrote:
> > Even if acpi_processor_handle_eject() offlines cpu, there is a chance
> > to online the cpu after that. So the patch closes the window by using
> > get/put_online_cpus().
> >
> > Why does the patch change _cpu_up() logic?
> >
> > The patch cares the race of hot-remove cpu and _cpu_up(). If the patch
> > does not change it, there is the following race.
> >
> > hot-remove cpu                         |  _cpu_up()
> > ------------------------------------- ------------------------------------
> > call acpi_processor_handle_eject()     |
> >       call cpu_down()                   |
> >       call get_online_cpus()            |
> >                                         | call cpu_hotplug_begin() and stop here
> >       call arch_unregister_cpu()        |
> >       call acpi_unmap_lsapic()          |
> >       call put_online_cpus()            |
> >                                         | start and continue _cpu_up()
> >       return acpi_processor_remove()    |
> > continue hot-remove the cpu            |
> >
> > So _cpu_up() can continue to itself. And hot-remove cpu can also continue
> > itself. If the patch changes _cpu_up() logic, the race disappears as below:
> >
> > hot-remove cpu                         | _cpu_up()
> > -----------------------------------------------------------------------
> > call acpi_processor_handle_eject()     |
> >       call cpu_down()                   |
> >       call get_online_cpus()            |
> >                                         | call cpu_hotplug_begin() and stop here
> >       call arch_unregister_cpu()        |
> >       call acpi_unmap_lsapic()          |
> >            cpu's cpu_present is set     |
> >            to false by set_cpu_present()|
> >       call put_online_cpus()            |
> >                                         | start _cpu_up()
> >                                         | check cpu_present() and return -EINVAL
> >       return acpi_processor_remove()    |
> > continue hot-remove the cpu            |
> >
> > Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
> >
> > ---
> >   drivers/acpi/processor_driver.c |   14 ++++++++++++++
> >   kernel/cpu.c                    |    8 +++++---
> >   2 files changed, 19 insertions(+), 3 deletions(-)
> >
> > Index: linux-3.5-rc6/drivers/acpi/processor_driver.c
> > ===================================================================
> > --- linux-3.5-rc6.orig/drivers/acpi/processor_driver.c	2012-07-12 20:34:29.438289841 +0900
> > +++ linux-3.5-rc6/drivers/acpi/processor_driver.c	2012-07-12 20:39:29.190542257 +0900
> > @@ -850,8 +850,22 @@ static int acpi_processor_handle_eject(s
> >   			return ret;
> >   	}
> >
> > +	get_online_cpus();
> > +	/*
> > +	 * The cpu might become online again at this point. So we check whether
> > +	 * the cpu has been onlined or not. If the cpu became online, it means
> > +	 * that someone wants to use the cpu. So acpi_processor_handle_eject()
> > +	 * returns -EAGAIN.
> > +	 */
> > +	if (unlikely(cpu_online(pr->id))) {
> > +		put_online_cpus();
> > +		printk(KERN_WARNING "Failed to remove CPU %d, "
> > +		       "since someone onlines the cpu\n" , pr->id);
>
> pr_warn() should be used per the recent checkpatch change.

O.K. I'll update it.

Thanks,
Yasuaki Ishimatsu

> Thanks,
> -Toshi
>
> > +		return -EAGAIN;
> > +	}
> >   	arch_unregister_cpu(pr->id);
> >   	acpi_unmap_lsapic(pr->id);
> > +	put_online_cpus();
> >   	return ret;
> >   }
> >   #else
> > Index: linux-3.5-rc6/kernel/cpu.c
> > ===================================================================
> > --- linux-3.5-rc6.orig/kernel/cpu.c	2012-07-12 20:34:29.438289841 +0900
> > +++ linux-3.5-rc6/kernel/cpu.c	2012-07-12 20:34:35.040219535 +0900
> > @@ -343,11 +343,13 @@ static int __cpuinit _cpu_up(unsigned in
> >   	unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
> >   	struct task_struct *idle;
> >
> > -	if (cpu_online(cpu) || !cpu_present(cpu))
> > -		return -EINVAL;
> > -
> >   	cpu_hotplug_begin();
> >
> > +	if (cpu_online(cpu) || !cpu_present(cpu)) {
> > +		ret =  -EINVAL;
> > +		goto out;
> > +	}
> > +
> >   	idle = idle_thread_get(cpu);
> >   	if (IS_ERR(idle)) {
> >   		ret = PTR_ERR(idle);
>
>
> --
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