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

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Date:	Wed, 5 Jan 2011 09:51:02 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	David Rientjes <rientjes@...gle.com>
cc:	Pekka Enberg <penberg@...nel.org>, linux-kernel@...r.kernel.org,
	Bart Van Assche <bart.vanassche@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] slub: Fix sysfs circular locking dependency

On Tue, 4 Jan 2011, David Rientjes wrote:

> slub_lock protects slab_state following kmem_cache_init() if caches are
> created/destroyed prior to the sysfs slab initcall setting the global
> variable, so couldn't this leak the kobject if its initialization and add
> was deferred on kmem_cache_create() and added by slab_sysfs_init()?

Potentially.

Also note that the same lock inversion occurs in kmem_cache_create().
slub lock is taken and then sysfs operations are performed.

The problem is that slub_lock has multiple roles:

1. Protect the list of slab caches and the slab_state (thats why its
   mainly used in kmem_cache_destroy and kmem_cache_create)

2. Protect against removal and adding of new cpus and nodes
   (show_slab_objects)

If we could create another lock that protects against new cpus / nodes
being added and removed then we could take that lock in
show_slab_objects() instead.

IMHO the lock order must have the slub_lock at the top.

The other lock that would prevent adding/removal of nodes / cpu can be
taken in show_slab_objects and in the corresponding hotplug functions. We
do not need to take the lock at all in show_slab_objects cpu and memory
hotplug are not enabled. Maybe there is a hotplug specific lock that can
be used instead?

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