[Devel] Re: [PATCH v5 13/18] memcg/sl[au]b Track all the memcg children of a kmem_cache.

[JoonSoo Kim]

2012/10/19 Glauber Costa <glommer at parallels.com>:
> +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.