linux-kernel - Re: [PATCH] mm/hotplug: Remove stop_machine() from try_offline

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Date:	Wed, 14 Aug 2013 21:21:08 -0400
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	Toshi Kani <toshi.kani@...com>
CC:	akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org,
	rjw@...k.pl, kosaki.motohiro@...fujitsu.com,
	kamezawa.hiroyu@...fujitsu.com, tangchen@...fujitsu.com,
	isimatu.yasuaki@...fujitsu.com, liwanp@...ux.vnet.ibm.com,
	kosaki.motohiro@...il.com
Subject: Re: [PATCH] mm/hotplug: Remove stop_machine() from try_offline_node()

(8/12/13 3:34 PM), Toshi Kani wrote:
> lock_device_hotplug() serializes hotplug & online/offline operations.
> The lock is held in common sysfs online/offline interfaces and ACPI
> hotplug code paths.
> 
> try_offline_node() off-lines a node if all memory sections and cpus
> are removed on the node.  It is called from acpi_processor_remove()
> and acpi_memory_remove_memory()->remove_memory() paths, both of which
> are in the ACPI hotplug code.
> 
> try_offline_node() calls stop_machine() to stop all cpus while checking
> all cpu status with the assumption that the caller is not protected from
> CPU hotplug or CPU online/offline operations.  However, the caller is
> always serialized with lock_device_hotplug().  Also, the code needs to
> be properly serialized with a lock, not by stopping all cpus at a random
> place with stop_machine().
> 
> This patch removes the use of stop_machine() in try_offline_node() and
> adds comments to try_offline_node() and remove_memory() that
> lock_device_hotplug() is required.

This patch need more verbose explanation. check_cpu_on_node() traverse cpus
and cpu hotplug seems to use cpu_hotplug_driver_lock() instead of lock_device_hotplug().

That said, the race is not happen against another memeory happen. It's likely happen
another cpu hotplug. So commenting remove_memory() doesn't make much sense.

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