Refcounter design for elements of lists/arrays protected by RCU. Refcounting on elements of lists which are protected by traditional reader/writer spinlocks or semaphores are straight forward as in: 1. 2. add() search_and_reference() { { alloc_object read_lock(&list_lock); ... search_for_element atomic_set(&el->rc, 1); atomic_inc(&el->rc); write_lock(&list_lock); ... add_element read_unlock(&list_lock); ... ... write_unlock(&list_lock); } } 3. 4. release_referenced() delete() { { ... write_lock(&list_lock); atomic_dec(&el->rc, relfunc) ... ... delete_element } write_unlock(&list_lock); ... if (atomic_dec_and_test(&el->rc)) kfree(el); ... } If this list/array is made lock free using rcu as in changing the write_lock in add() and delete() to spin_lock and changing read_lock in search_and_reference to rcu_read_lock(), the atomic_get in search_and_reference could potentially hold reference to an element which has already been deleted from the list/array. atomic_inc_not_zero takes care of this scenario. search_and_reference should look as; 1. 2. add() search_and_reference() { { alloc_object rcu_read_lock(); ... search_for_element atomic_set(&el->rc, 1); if (atomic_inc_not_zero(&el->rc)) { write_lock(&list_lock); rcu_read_unlock(); return FAIL; add_element } ... ... write_unlock(&list_lock); rcu_read_unlock(); } } 3. 4. release_referenced() delete() { { ... write_lock(&list_lock); atomic_dec(&el->rc, relfunc) ... ... delete_element } write_unlock(&list_lock); ... if (atomic_dec_and_test(&el->rc)) call_rcu(&el->head, el_free); ... } Sometimes, reference to the element need to be obtained in the update (write) stream. In such cases, atomic_inc_not_zero might be an overkill since the spinlock serialising list updates are held. atomic_inc is to be used in such cases.