linux-kernel - Re: [PATCH 9/9] slab: remove a useless lockdep annotation

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Date:	Mon, 24 Feb 2014 13:49:18 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 9/9] slab: remove a useless lockdep annotation

On Tue, Feb 18, 2014 at 10:21:10AM -0600, Christoph Lameter wrote:
> On Mon, 17 Feb 2014, Joonsoo Kim wrote:
> 
> > > Why change the BAD_ALIEN_MAGIC?
> >
> > Hello, Christoph.
> >
> > BAD_ALIEN_MAGIC is only checked by slab_set_lock_classes(). We remove this
> > function in this patch, so returning BAD_ALIEN_MAGIC is useless.
> 
> Its not useless. The point is if there is a pointer deref then we will see
> this as a pointer value and know that it is realted to alien cache
> processing.
> 
> > And, in fact, BAD_ALIEN_MAGIC is already useless, because alloc_alien_cache()
> > can't be called on !CONFIG_NUMA. This function is called if use_alien_caches
> > is positive, but on !CONFIG_NUMA, use_alien_caches is always 0. So we don't
> > have any chance to meet this BAD_ALIEN_MAGIC in runtime.
> 
> Maybe it no longer serves a point. But note that caches may not be
> populated because processors/nodes are not up yet.

Hello,

Let me clarify about alloc_alien_cache().

alloc_alien_cache() has two definitions, one for !CONFIG_NUMA, and the other for
CONFIG_NUMA. BAD_ALIEN_MAGIC is only assigned on !CONFIG_NUMA definition. On
CONFIG_NUMA, alloc_alien_cache() doesn't use BAD_ALIEN_MAGIC. So it is sufficient
to consider just !CONFIG_NUMA case.

As I mentioned before, this function isn't called if use_alien_caches is zero
and use_alien_caches is always zero on !CONFIG_NUMA. Therefore we cannot see
BAD_ALIEN_MAGIC on any configuration. I don't know why BAD_ALIEN_MAGIC is
introduced, however, it no longer serves a point, so it is better to remove it.

There are lots of code to check whether processor/nodes are up or not and these
doesn't use BAD_ALIEN_MAGIC. Instead, it checks NULL on alien_cache of specific node.
So removing BAD_ALIEN_MAGIC doesn't harm anything here.

Thanks.
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