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-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.