Date:	Mon, 09 Jul 2012 16:55:32 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
CC:	linux-acpi@...r.kernel.org, lenb@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] acpi : cpu hot-remove returns error number when cpu_down()
 fails

On 07/09/2012 08:01 AM, Yasuaki Ishimatsu wrote:
> Hi Srivatsa,
> 
> Thank you for your reviewing.
> 
> 2012/07/06 18:51, Srivatsa S. Bhat wrote:
>> On 07/06/2012 08:46 AM, Yasuaki Ishimatsu wrote:
>>> Even if cpu_down() fails, acpi_processor_remove() continues to remove the cpu.
>>
>> Ouch!
>>
>>> But in this case, it should return error number since some process may run on
>>> the cpu. If the cpu has a running process and the cpu is turned the power off,
>>> the system cannot work well.
>>>
>>> Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
>>>
>>> ---
>>>   drivers/acpi/processor_driver.c |   18 ++++++++++++------
>>>   1 file changed, 12 insertions(+), 6 deletions(-)
>>>
>>> Index: linux-3.5-rc4/drivers/acpi/processor_driver.c
>>> ===================================================================
>>> --- linux-3.5-rc4.orig/drivers/acpi/processor_driver.c	2012-06-25 04:53:04.000000000 +0900
>>> +++ linux-3.5-rc4/drivers/acpi/processor_driver.c	2012-07-05 21:02:58.711285382 +0900
>>> @@ -610,7 +610,7 @@ err_free_pr:
>>>   static int acpi_processor_remove(struct acpi_device *device, int type)
>>>   {
>>>   	struct acpi_processor *pr = NULL;
>>> -
>>> +	int ret;
>>>
>>>   	if (!device || !acpi_driver_data(device))
>>>   		return -EINVAL;
>>> @@ -621,8 +621,9 @@ static int acpi_processor_remove(struct
>>>   		goto free;
>>>
>>>   	if (type == ACPI_BUS_REMOVAL_EJECT) {
>>> -		if (acpi_processor_handle_eject(pr))
>>> -			return -EINVAL;
>>> +		ret = acpi_processor_handle_eject(pr);
>>> +		if (ret)
>>> +			return ret;
>>>   	}
>>>
>>>   	acpi_processor_power_exit(pr, device);
>>> @@ -841,12 +842,17 @@ static acpi_status acpi_processor_hotadd
>>>
>>>   static int acpi_processor_handle_eject(struct acpi_processor *pr)
>>>   {
>>> -	if (cpu_online(pr->id))
>>> -		cpu_down(pr->id);
>>> +	int ret;
>>> +
>>> +	if (cpu_online(pr->id)) {
>>> +		ret = cpu_down(pr->id);
>>> +		if (ret)
>>> +			return ret;
>>> +	}
>>>
>>
>> Strictly speaking, this is not thorough enough. What prevents someone
>> from onlining that same cpu again, at this point?
>> So, IMHO, you need to wrap the contents of this function inside a
>> get_online_cpus()/put_online_cpus() block, to prevent anyone else
>> from messing with CPU hotplug at the same time.
> 
> If I understand your comment by mistake, please let me know.
> If the contents is wrapped a inside get_online_cpus()/put_online_cpus() block
> as below, cpu_down() will stop since cpu_down() calls cpu_hotplug_begin() and
> cpu_hotplug_begin() waits for cpu_hotplug.refcount to become 0.
> 

You are right. Sorry, I overlooked that.

> +	get_online_cpus()
> +	if (cpu_online(pr->id)) {
> +		ret = cpu_down(pr->id);
> +		if (ret)
> +			return ret;
> +	}
> +	put_online_cpus()
> 
> I think following patch can prevent it correctly. How about the patch?
>
> ---
>  drivers/acpi/processor_driver.c |   12 ++++++++++++
>  kernel/cpu.c                    |    8 +++++---
>  2 files changed, 17 insertions(+), 3 deletions(-)
> 
> Index: linux-3.5-rc4/drivers/acpi/processor_driver.c
> ===================================================================
> --- linux-3.5-rc4.orig/drivers/acpi/processor_driver.c	2012-07-09 09:59:01.280211202 +0900
> +++ linux-3.5-rc4/drivers/acpi/processor_driver.c	2012-07-09 11:05:34.559859236 +0900
> @@ -844,14 +844,26 @@ static int acpi_processor_handle_eject(s
>  {
>  	int ret;
> 
> +retry:
>  	if (cpu_online(pr->id)) {
>  		ret = cpu_down(pr->id);
>  		if (ret)
>  			return ret;
>  	}
> 
> +	get_online_cpus();
> +	/*
> +	 * Someone might online the cpu again at this point. So we check that
> +	 * cpu has been onlined or not. If cpu is online, we try to offline
> +	 * the cpu again.
> +	 */
> +	if (cpu_online(pr->id)) {

How about this:
	if (unlikely(cpu_online(pr->id)) {
since the probability of this happening is quite small...

> +		put_online_cpus();
> +		goto retry;
> +	}
>  	arch_unregister_cpu(pr->id);
>  	acpi_unmap_lsapic(pr->id);
> +	put_online_cpus();
>  	return ret;
>  }

This retry logic doesn't look elegant, but I don't see any better method :-(

>  #else
> Index: linux-3.5-rc4/kernel/cpu.c
> ===================================================================
> --- linux-3.5-rc4.orig/kernel/cpu.c	2012-07-09 09:59:01.280211202 +0900
> +++ linux-3.5-rc4/kernel/cpu.c	2012-07-09 09:59:02.903190965 +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;
> +	}
> +

Firstly, why is this change needed?
Secondly, if the change is indeed an improvement, then why is it
in _this_ patch? IMHO, in that case it should be part of a separate patch.

Coming back to my first point, I don't see why this hunk is needed. We
already take the cpu_add_remove_lock (cpu_maps_update_begin/end) before
checking the status of the cpu (online or present). And all hotplug
operations (cpu_up/cpu_down/disable|enable_nonboot_cpus) go through that
lock. Isn't that enough? Or am I missing something?

>  	idle = idle_thread_get(cpu);
>  	if (IS_ERR(idle)) {
>  		ret = PTR_ERR(idle);
> 
 
Regards,
Srivatsa S. Bhat

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