Reference-count design for elements of lists/arrays protected by RCU. Reference counting on elements of lists which are protected by traditional reader/writer spinlocks or semaphores are straightforward: 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_inc() in search_and_reference() could potentially hold reference to an element which has already been deleted from the list/array. Use atomic_inc_not_zero() in this scenario as follows: 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)) { spin_lock(&list_lock); rcu_read_unlock(); return FAIL; add_element } ... ... spin_unlock(&list_lock); rcu_read_unlock(); } } 3. 4. release_referenced() delete() { { ... spin_lock(&list_lock); if (atomic_dec_and_test(&el->rc)) ... call_rcu(&el->head, el_free); delete_element ... spin_unlock(&list_lock); } ... if (atomic_dec_and_test(&el->rc)) call_rcu(&el->head, el_free); ... } Sometimes, a reference to the element needs to be obtained in the update (write) stream. In such cases, atomic_inc_not_zero() might be overkill, since we hold the update-side spinlock. One might instead use atomic_inc() in such cases.