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

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Date:	Fri, 13 Jul 2012 15:24:57 +0900
From:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
CC:	<linux-acpi@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<lenb@...nel.org>, <toshi.kani@...com>
Subject: Re: [PATCH v3 2/3 RESEND] acpi : prevent cpu from becoming online

2012/07/12 21:41, Srivatsa S. Bhat wrote:
> On 07/12/2012 05:10 PM, 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@...fujitsu.com>
>>
> 
> Please consider fixing the grammar issue below (since it is a user-visible
> print statement). Other than that, everything looks fine.
> 
> Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
>   
>> ---
>>   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);
> 
> How about:
> "Failed to remove CPU %d, because some other task brought the CPU back online\n"

Looks good to me. I'll update it.

Thanks,
Yasuaki Ishimatsu

> 
> Regards,
> Srivatsa S. Bhat
> 
>> +		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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