linux-kernel - Re: [PATCH] slub: Fix sysfs circular locking dependency

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Date:	Tue, 11 Jan 2011 10:29:11 +0200
From:	Pekka Enberg <penberg@...nel.org>
To:	David Rientjes <rientjes@...gle.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <cl@...ux.com>,
	Bart Van Assche <bvanassche@....org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] slub: Fix sysfs circular locking dependency

On 1/11/11 10:24 AM, David Rientjes wrote:
> On Tue, 11 Jan 2011, Pekka Enberg wrote:
>
>>>> New patch that just covers the slub changes.
>>>>
>>>> Subject: slub: Avoid use of slub_lock in show_slab_objects()
>>>>
>>>> The purpose of the locking is to prevent removal and additions
>>>> of nodes when statistics are gathered for a slab cache. So we
>>>> need to avoid racing with memory hotplug functionality.
>>>>
>>>> It is enough to take the memory hotplug locks there instead
>>>> of the slub_lock.
>>>>
>>>> online_pages() currently does not acquire the memory_hotplug
>>>> lock. Another patch will be submitted by the memory hotplug
>>>> authors to take the memory hotplug lock and describe the
>>>> uses of the memory hotplug lock to protect against
>>>> adding and removal of nodes from non hotplug data structures.
>>>>
>>>> Signed-off-by: Christoph Lameter<cl@...ux.com>
>>> Acked-by: David Rientjes<rientjes@...gle.com>
>> Is this safe to be applied without the other hotplug parts?
> It's safe, but not protecting anything since it can race with memory
> hot-add and cause inconsistent information to be displayed (since we
> iterate over N_NORMAL_MEMORY several times in show_slab_objects() and the
> memory hotplug code modifies it).  I'm hoping Andrew can push Kame's patch
> to add lock_memory_hotplug() to online_pages() either during the merge
> window or during -rc1 (it has good justification -- it can race with
> memory hot-remove) and this can also be pushed during the rc series (to
> fix the lockdep warning).

I think the two patches should go in the same batch. Andrew, do you want 
to pick up the slab patch as well or do you want me to pick up Kame's patch?

             Pekka

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