Documentation/RCU/rcuref.txt

   1 Refcounter design for elements of lists/arrays protected by RCU.
   2
   3 Refcounting on elements of  lists which are protected by traditional
   4 reader/writer spinlocks or semaphores are straight forward as in:
   5
   6 1.                              2.
   7 add()                           search_and_reference()
   8 {                               {
   9     alloc_object                    read_lock(&list_lock);
  10     ...                             search_for_element
  11     atomic_set(&el->rc, 1);         atomic_inc(&el->rc);
  12     write_lock(&list_lock);          ...
  13     add_element                     read_unlock(&list_lock);
  14     ...                             ...
  15     write_unlock(&list_lock);   }
  16 }
  17
  18 3.                                      4.
  19 release_referenced()                    delete()
  20 {                                       {
  21     ...                                     write_lock(&list_lock);
  22     atomic_dec(&el->rc, relfunc)            ...
  23     ...                                     delete_element
  24 }                                           write_unlock(&list_lock);
  25                                             ...
  26                                             if (atomic_dec_and_test(&el->rc))
  27                                                 kfree(el);
  28                                             ...
  29                                         }
  30
  31 If this list/array is made lock free using rcu as in changing the
  32 write_lock in add() and delete() to spin_lock and changing read_lock
  33 in search_and_reference to rcu_read_lock(), the atomic_get in
  34 search_and_reference could potentially hold reference to an element which
  35 has already been deleted from the list/array.  atomic_inc_not_zero takes
  36 care of this scenario. search_and_reference should look as;
  37
  38 1.                                      2.
  39 add()                                   search_and_reference()
  40 {                                       {
  41     alloc_object                            rcu_read_lock();
  42     ...                                     search_for_element
  43     atomic_set(&el->rc, 1);                 if (atomic_inc_not_zero(&el->rc)) {
  44     write_lock(&list_lock);                     rcu_read_unlock();
  45                                                 return FAIL;
  46     add_element                             }
  47     ...                                     ...
  48     write_unlock(&list_lock);               rcu_read_unlock();
  49 }                                       }
  50 3.                                      4.
  51 release_referenced()                    delete()
  52 {                                       {
  53     ...                                     write_lock(&list_lock);
  54     atomic_dec(&el->rc, relfunc)            ...
  55     ...                                     delete_element
  56 }                                           write_unlock(&list_lock);
  57                                             ...
  58                                             if (atomic_dec_and_test(&el->rc))
  59                                                 call_rcu(&el->head, el_free);
  60                                             ...
  61                                         }
  62
  63 Sometimes, reference to the element need to be obtained in the
  64 update (write) stream.  In such cases, atomic_inc_not_zero might be an
  65 overkill since the spinlock serialising list updates are held. atomic_inc
  66 is to be used in such cases.
  67