On 10/29/2012 07:26 PM, JoonSoo Kim wrote:

2012/10/19 Glauber Costa [off-list ref]:

quoted

+void kmem_cache_destroy_memcg_children(struct kmem_cache *s)
+{
+       struct kmem_cache *c;
+       int i;
+
+       if (!s->memcg_params)
+               return;
+       if (!s->memcg_params->is_root_cache)
+               return;
+
+       /*
+        * If the cache is being destroyed, we trust that there is no one else
+        * requesting objects from it. Even if there are, the sanity checks in
+        * kmem_cache_destroy should caught this ill-case.
+        *
+        * Still, we don't want anyone else freeing memcg_caches under our
+        * noses, which can happen if a new memcg comes to life. As usual,
+        * we'll take the set_limit_mutex to protect ourselves against this.
+        */
+       mutex_lock(&set_limit_mutex);
+       for (i = 0; i < memcg_limited_groups_array_size; i++) {
+               c = s->memcg_params->memcg_caches[i];
+               if (c)
+                       kmem_cache_destroy(c);
+       }
+       mutex_unlock(&set_limit_mutex);
+}

It may cause NULL deref.
Look at the following scenario.

1. some memcg slab caches has remained object.
2. start to destroy memcg.
3. schedule_delayed_work(kmem_cache_destroy_work_func, @delay 60hz)
4. all remained object is freed.
5. start to destroy root cache.
6. kmem_cache_destroy makes 's->memcg_params->memcg_caches[i]" NULL!!
7. Start delayed work function.
8. cachep in kmem_cache_destroy_work_func() may be NULL

Thanks.

Thanks for spotting. This is the same problem we have in
memcg_cache_destroy(),
which I solved by not respawning the worker.

In here, I believe it should be possible to just cancel all remaining
pending work, since we are now in the process of deleting the caches
ourselves.