memcg: initialize kmem-cache destroying work earlier

Fix a warning from lockdep caused by calling cancel_work_sync() for
uninitialized struct work.  This path has been triggered by destructon
kmem-cache hierarchy via destroying its root kmem-cache.

  cache ffff88003c072d80
  obj ffff88003b410000 cache ffff88003c072d80
  obj ffff88003b924000 cache ffff88003c20bd40
  INFO: trying to register non-static key.
  the code is fine but needs lockdep annotation.
  turning off the locking correctness validator.
  Pid: 2825, comm: insmod Tainted: G           O 3.9.0-rc1-next-20130307+ #611
  Call Trace:
    __lock_acquire+0x16a2/0x1cb0
    lock_acquire+0x8a/0x120
    flush_work+0x38/0x2a0
    __cancel_work_timer+0x89/0xf0
    cancel_work_sync+0xb/0x10
    kmem_cache_destroy_memcg_children+0x81/0xb0
    kmem_cache_destroy+0xf/0xe0
    init_module+0xcb/0x1000 [kmem_test]
    do_one_initcall+0x11a/0x170
    load_module+0x19b0/0x2320
    SyS_init_module+0xc6/0xf0
    system_call_fastpath+0x16/0x1b

Example module to demonstrate:

  #include <linux/module.h>
  #include <linux/slab.h>
  #include <linux/mm.h>
  #include <linux/workqueue.h>

  int __init mod_init(void)
  {
  	int size = 256;
  	struct kmem_cache *cache;
  	void *obj;
  	struct page *page;

  	cache = kmem_cache_create("kmem_cache_test", size, size, 0, NULL);
  	if (!cache)
  		return -ENOMEM;

  	printk("cache %p\n", cache);

  	obj = kmem_cache_alloc(cache, GFP_KERNEL);
  	if (obj) {
  		page = virt_to_head_page(obj);
  		printk("obj %p cache %p\n", obj, page->slab_cache);
  		kmem_cache_free(cache, obj);
  	}

  	flush_scheduled_work();

  	obj = kmem_cache_alloc(cache, GFP_KERNEL);
  	if (obj) {
  		page = virt_to_head_page(obj);
  		printk("obj %p cache %p\n", obj, page->slab_cache);
  		kmem_cache_free(cache, obj);
  	}

  	kmem_cache_destroy(cache);

  	return -EBUSY;
  }

  module_init(mod_init);
  MODULE_LICENSE("GPL");
Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Glauber Costa <glommer@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -3012,6 +3012,8 @@ void memcg_update_array_size(int num)
 		memcg_limited_groups_array_size = memcg_caches_array_size(num);
 }
 
+static void kmem_cache_destroy_work_func(struct work_struct *w);
+
 int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
 {
 	struct memcg_cache_params *cur_params = s->memcg_params;
@@ -3031,6 +3033,8 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
 			return -ENOMEM;
 		}
 
+		INIT_WORK(&s->memcg_params->destroy,
+				kmem_cache_destroy_work_func);
 		s->memcg_params->is_root_cache = true;
 
 		/*
@@ -3078,6 +3082,8 @@ int memcg_register_cache(struct mem_cgroup *memcg, struct kmem_cache *s,
 	if (!s->memcg_params)
 		return -ENOMEM;
 
+	INIT_WORK(&s->memcg_params->destroy,
+			kmem_cache_destroy_work_func);
 	if (memcg) {
 		s->memcg_params->memcg = memcg;
 		s->memcg_params->root_cache = root_cache;
@@ -3358,8 +3364,6 @@ static void mem_cgroup_destroy_all_caches(struct mem_cgroup *memcg)
 	list_for_each_entry(params, &memcg->memcg_slab_caches, list) {
 		cachep = memcg_params_to_cache(params);
 		cachep->memcg_params->dead = true;
-		INIT_WORK(&cachep->memcg_params->destroy,
-				  kmem_cache_destroy_work_func);
 		schedule_work(&cachep->memcg_params->destroy);
 	}
 	mutex_unlock(&memcg->slab_caches_mutex);