Subversion Repositories test_project

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_get 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)) {
    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);
    if (atomic_dec_and_test(&el->rc))       ...
        call_rcu(&el->head, el_free);       delete_element
    ...                                     write_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.