linux-stable-rt/Documentation/RCU/rcuref.txt

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.