Kernel-2.6.32-573.12.1.el6_rcuref

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); }
   }

2.             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();
   } }

2.             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.