static void flush_memcg_workqueue(struct kmem_cache *s)
 {
+	/*
+	 * memcg_params.dying is synchronized using slab_mutex AND
+	 * memcg_kmem_wq_lock spinlock, because it's not always
+	 * possible to grab slab_mutex.
+	 */
 	mutex_lock(&slab_mutex);
+	spin_lock(&memcg_kmem_wq_lock);
 	s->memcg_params.dying = true;
+	spin_unlock(&memcg_kmem_wq_lock);

I would completely switch from the mutex to the new spin lock -
acquiring them both looks weird.

 	mutex_unlock(&slab_mutex);
 
 	/*

There are places where the slab_mutex is held and sleeping functions like kvzalloc() are called. I understand that taking both mutex and spinlocks look ugly, but converting all the slab_mutex critical sections to spinlock critical sections will be a major undertaking by itself. So I would suggest leaving that for now.

Cheers,
Longman