This commit was manufactured by cvs2svn to create branch 'vserver'.

[linux-2.6.git] / Documentation / RCU / rcuref.txt

diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
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+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.