memcg: get rid of kmem_cache_dup()

kmem_cache_dup() is only called from memcg_create_kmem_cache(). The
latter, in fact, does nothing besides this, so let's fold
kmem_cache_dup() into memcg_create_kmem_cache().

This patch also makes the memcg_cache_mutex private to
memcg_create_kmem_cache(), because it is not used anywhere else.

Signed-off-by: Vladimir Davydov <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Cc: Glauber Costa <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Balbir Singh <[e-mail]>
Cc: KAMEZAWA Hiroyuki <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/memcontrol.c

···
 	schedule_work(&cachep->memcg_params->destroy);
 }
 
-/*
- * This lock protects updaters, not readers. We want readers to be as fast as
- * they can, and they will either see NULL or a valid cache value. Our model
- * allow them to see NULL, in which case the root memcg will be selected.
- *
- * We need this lock because multiple allocations to the same cache from a non
- * will span more than one worker. Only one of them can create the cache.
- */
-static DEFINE_MUTEX(memcg_cache_mutex);
-
-/*
- * Called with memcg_cache_mutex held
- */
-static struct kmem_cache *kmem_cache_dup(struct mem_cgroup *memcg,
-					 struct kmem_cache *s)
+static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
+						  struct kmem_cache *s)
 {
 	struct kmem_cache *new;
 	static char *tmp_name = NULL;
+	static DEFINE_MUTEX(mutex);	/* protects tmp_name */
 
-	lockdep_assert_held(&memcg_cache_mutex);
+	BUG_ON(!memcg_can_account_kmem(memcg));
 
+	mutex_lock(&mutex);
 	/*
 	 * kmem_cache_create_memcg duplicates the given name and
 	 * cgroup_name for this name requires RCU context.
···
 
 	if (new)
 		new->allocflags |= __GFP_KMEMCG;
+	else
+		new = s;
 
+	mutex_unlock(&mutex);
 	return new;
-}
-
-static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
-						  struct kmem_cache *cachep)
-{
-	struct kmem_cache *new_cachep;
-
-	BUG_ON(!memcg_can_account_kmem(memcg));
-
-	mutex_lock(&memcg_cache_mutex);
-	new_cachep = kmem_cache_dup(memcg, cachep);
-	if (new_cachep == NULL)
-		new_cachep = cachep;
-	mutex_unlock(&memcg_cache_mutex);
-	return new_cachep;
 }
 
 void kmem_cache_destroy_memcg_children(struct kmem_cache *s)