[PATCH v2 4/7] memcg, slab: unregister cache from memcg before starting to destroy it

[Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>]

Currently, memcg_unregister_cache(), which deletes the cache being
destroyed from the memcg_slab_caches list, is called after
__kmem_cache_shutdown() (see kmem_cache_destroy()), which starts to
destroy the cache. As a result, one can access a partially destroyed
cache while traversing a memcg_slab_caches list, which can have deadly
consequences (for instance, cache_show() called for each cache on a
memcg_slab_caches list from mem_cgroup_slabinfo_read() will dereference
pointers to already freed data).

To fix this, let's move memcg_unregister_cache() before the cache
destruction process beginning, issuing memcg_register_cache() on
failure.

Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
---
 mm/memcontrol.c  |   12 ++++++------
 mm/slab_common.c |    3 ++-
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index b94d9917707c..6a059e73212c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3247,6 +3247,7 @@ int memcg_alloc_cache_params(struct kmem_cache *s,
 		s->memcg_params->root_cache = root_cache;
 		INIT_WORK(&s->memcg_params->destroy,
 				kmem_cache_destroy_work_func);
+		css_get(&memcg->css);
 	} else
 		s->memcg_params->is_root_cache = true;
 
@@ -3255,6 +3256,10 @@ int memcg_alloc_cache_params(struct kmem_cache *s,
 
 void memcg_free_cache_params(struct kmem_cache *s)
 {
+	if (!s->memcg_params)
+		return;
+	if (!s->memcg_params->is_root_cache)
+		css_put(&s->memcg_params->memcg->css);
 	kfree(s->memcg_params);
 }
 
@@ -3277,9 +3282,6 @@ void memcg_register_cache(struct kmem_cache *s)
 	memcg = s->memcg_params->memcg;
 	id = memcg_cache_id(memcg);
 
-	css_get(&memcg->css);
-
-
 	/*
 	 * Since readers won't lock (see cache_from_memcg_idx()), we need a
 	 * barrier here to ensure nobody will see the kmem_cache partially
@@ -3328,10 +3330,8 @@ void memcg_unregister_cache(struct kmem_cache *s)
 	 * after removing it from the memcg_slab_caches list, otherwise we can
 	 * fail to convert memcg_params_to_cache() while traversing the list.
 	 */
-	VM_BUG_ON(!root->memcg_params->memcg_caches[id]);
+	VM_BUG_ON(root->memcg_params->memcg_caches[id] != s);
 	root->memcg_params->memcg_caches[id] = NULL;
-
-	css_put(&memcg->css);
 }
 
 /*
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 3314fb3ead7f..4dff4bb66f19 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -310,9 +310,9 @@ void kmem_cache_destroy(struct kmem_cache *s)
 	s->refcount--;
 	if (!s->refcount) {
 		list_del(&s->list);
+		memcg_unregister_cache(s);
 
 		if (!__kmem_cache_shutdown(s)) {
-			memcg_unregister_cache(s);
 			mutex_unlock(&slab_mutex);
 			if (s->flags & SLAB_DESTROY_BY_RCU)
 				rcu_barrier();
@@ -322,6 +322,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
 			kmem_cache_free(kmem_cache, s);
 		} else {
 			list_add(&s->list, &slab_caches);
+			memcg_register_cache(s);
 			mutex_unlock(&slab_mutex);
 			printk(KERN_ERR "kmem_cache_destroy %s: Slab cache still has objects\n",
 				s->name);
-- 
1.7.10.4

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