1 Refcounter design for elements of lists/arrays protected by RCU.
3 Refcounting on elements of lists which are protected by traditional
4 reader/writer spinlocks or semaphores are straight forward as in:
7 add() search_and_reference()
9 alloc_object read_lock(&list_lock);
10 ... search_for_element
11 atomic_set(&el->rc, 1); atomic_inc(&el->rc);
12 write_lock(&list_lock); ...
13 add_element read_unlock(&list_lock);
15 write_unlock(&list_lock); }
19 release_referenced() delete()
21 ... write_lock(&list_lock);
22 atomic_dec(&el->rc, relfunc) ...
24 } write_unlock(&list_lock);
26 if (atomic_dec_and_test(&el->rc))
31 If this list/array is made lock free using rcu as in changing the
32 write_lock in add() and delete() to spin_lock and changing read_lock
33 in search_and_reference to rcu_read_lock(), the atomic_get in
34 search_and_reference could potentially hold reference to an element which
35 has already been deleted from the list/array. atomic_inc_not_zero takes
36 care of this scenario. search_and_reference should look as;
39 add() search_and_reference()
41 alloc_object rcu_read_lock();
42 ... search_for_element
43 atomic_set(&el->rc, 1); if (atomic_inc_not_zero(&el->rc)) {
44 write_lock(&list_lock); rcu_read_unlock();
48 write_unlock(&list_lock); rcu_read_unlock();
51 release_referenced() delete()
53 ... write_lock(&list_lock);
54 atomic_dec(&el->rc, relfunc) ...
56 } write_unlock(&list_lock);
58 if (atomic_dec_and_test(&el->rc))
59 call_rcu(&el->head, el_free);
63 Sometimes, reference to the element need to be obtained in the
64 update (write) stream. In such cases, atomic_inc_not_zero might be an
65 overkill since the spinlock serialising list updates are held. atomic_inc
66 is to be used in such cases.