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Date: Mon, 3 Sep 2012 18:58:15 +0400
From: Glauber Costa <glommer@parallels.com>
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Subject: Re: C13 [08/14] Get rid of __kmem_cache_destroy
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To: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>, Joonsoo Kim <js1304@gmail.com>, linux-mm@kvack.org, David Rientjes <rientjes@google.com>

On 08/24/2012 08:12 PM, Christoph Lameter wrote:
> What is done there can be done in __kmem_cache_shutdown.
> 
> This affects RCU handling somewhat. On rcu free all slab allocators
> do not refer to other management structures than the kmem_cache structure.
> Therefore these other structures can be freed before the rcu deferred
> free to the page allocator occurs.
> 
> Reviewed-by: Joonsoo Kim <js1304@gmail.com>
> Signed-off-by: Christoph Lameter <cl@linux.com>

Here is the code for that in slab_common.c:

    if (!__kmem_cache_shutdown(s)) {
        if (s->flags & SLAB_DESTROY_BY_RCU)
            rcu_barrier();

        __kmem_cache_destroy(s);
    } ...

All that code that used to belong in __kmem_cache_destroy(), will not be
executed in kmem_cache_shutdown() without an rcu_barrier.

You need at least Paul's ack here to guarantee it is safe, but I believe
it is not. Take a look for instance at 7ed9f7e5db5, which describes a
subtle bug arising from such a situation.

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