Documentation/RCU/rcuref.txt

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.