memcg: fix mutex not unlocked on memcg_create_kmem_cache fail path

Commit 842e287 ("memcg: get rid of kmem_cache_dup()") introduced a
mutex for memcg_create_kmem_cache() to protect the tmp_name buffer that
holds the memcg name.  It failed to unlock the mutex if this buffer
could not be allocated.

This patch fixes the issue by appropriately unlocking the mutex if the
allocation fails.

Signed-off-by: Vladimir Davydov <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Glauber Costa <[email protected]>
Acked-by: David Rientjes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/memcontrol.c

@@ -3400,7 +3400,7 @@ void mem_cgroup_destroy_cache(struct kmem_cache *cachep)
 static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
 						  struct kmem_cache *s)
 {
-	struct kmem_cache *new;
+	struct kmem_cache *new = NULL;
 	static char *tmp_name = NULL;
 	static DEFINE_MUTEX(mutex);	/* protects tmp_name */
 
@@ -3416,7 +3416,7 @@ static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
 	if (!tmp_name) {
 		tmp_name = kmalloc(PATH_MAX, GFP_KERNEL);
 		if (!tmp_name)
-			return NULL;
+			goto out;
 	}
 
 	rcu_read_lock();
@@ -3426,12 +3426,11 @@ static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
 
 	new = kmem_cache_create_memcg(memcg, tmp_name, s->object_size, s->align,
 				      (s->flags & ~SLAB_PANIC), s->ctor, s);
-
 	if (new)
 		new->allocflags |= __GFP_KMEMCG;
 	else
 		new = s;
-
+out:
 	mutex_unlock(&mutex);
 	return new;
 }