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	mm/slab_common: allow NULL cache pointer in kmem_cache_destroy()
kmem_cache_destroy() does not tolerate a NULL kmem_cache pointer argument
and performs a NULL-pointer dereference.  This requires additional
attention and effort from developers/reviewers and forces all
kmem_cache_destroy() callers (200+ as of 4.1) to do a NULL check
    if (cache)
        kmem_cache_destroy(cache);
Or, otherwise, be invalid kmem_cache_destroy() users.
Tweak kmem_cache_destroy() and NULL-check the pointer there.
Proposed by Andrew Morton.
Link: https://lkml.org/lkml/2015/6/8/583
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
			
			
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					@ -640,6 +640,9 @@ void kmem_cache_destroy(struct kmem_cache *s)
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	bool need_rcu_barrier = false;
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						bool need_rcu_barrier = false;
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	bool busy = false;
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						bool busy = false;
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						if (unlikely(!s))
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							return;
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	BUG_ON(!is_root_cache(s));
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						BUG_ON(!is_root_cache(s));
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	get_online_cpus();
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						get_online_cpus();
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