linux-kernel - Re: [PATCH] ACPI, cpu hotplug: move try_offline_node() after acpi_unmap

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Date:	Fri, 09 Aug 2013 15:28:46 +0800
From:	Tang Chen <tangchen@...fujitsu.com>
To:	Toshi Kani <toshi.kani@...com>
CC:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>,
	rafael.j.wysocki@...el.com, linux-acpi@...r.kernel.org,
	linux-kernel@...r.kernel.org, Wen Congyang <wency@...fujitsu.com>
Subject: Re: [PATCH] ACPI, cpu hotplug: move try_offline_node() after acpi_unmap_lsapic()

On 08/07/2013 12:56 AM, Toshi Kani wrote:
> On Tue, 2013-08-06 at 19:11 +0900, Yasuaki Ishimatsu wrote:
>> try_offline_node() checks that all cpus related with removed node have been
>> removed by using cpu_present_bits. If all cpus related with removed node have
>> been removed, try_offline_node() clears the node information.
>>
>> But try_offline_node() called from acpi_processor_remove() never clears
>> the node information. For disabling cpu_present_bits, acpi_unmap_lsapic()
>> need be called. But acpi_unmap_lsapic() is called after try_offline_node()
>> runs. So when try_offline_node() runs, the cpu's cpu_present_bits is always
>> set.
>>
>> This patch moves try_offline_node() after acpi_unmap_lsapic().
>>
>> Signed-off-by: Yasuaki Ishimatsu<isimatu.yasuaki@...fujitsu.com>
>
> The change looks good to me.
>
> Acked-by: Toshi Kani<toshi.kani@...com>
>
> BTW, do you know why try_offline_node() has to use stop_machine()?

try_offline_node() is used to check if the node could be hot-removed
after each memory or cpu hot-remove operation.

In memory hot-remove path, we have lock_memory_hotplug() to series all
the memory hot-remove options.

But when doing cpu hot-remove,

acpi_processor_remove()
  |->try_offline_node()

There is no lock to protect it. I think, when we are going to hot-remove
a node, others should not do any memory or cpu hotplug operation. In memory
hotplug path, we have lock_memory_hotplug(). But in cpu hotplug path, I
didn't find any lock. So we used stop_machine() to call check_cpu_on_node().
If we find any cpu still present, we return and do not remove the node.

Thanks.

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