[PATCH] RCU documentation fixes (January 2006 update)

Updates to in-tree RCU documentation based on comments over the past few
months.

Signed-off-by: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 15 insertions, 16 deletions

diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
index 3f60db41b2f0..451de2ad8329 100644
--- a/Documentation/RCU/rcuref.txt
+++ b/Documentation/RCU/rcuref.txt
@@ -1,7 +1,7 @@
-Refcounter design for elements of lists/arrays protected by RCU.
+Reference-count 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:
+Reference counting on elements of lists which are protected by traditional
+reader/writer spinlocks or semaphores are straightforward:
 
 1.                              2.
 add()                           search_and_reference()
@@ -28,12 +28,12 @@ release_referenced()			delete()
                                             ...
                                         }
 
-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
+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;
+has already been deleted from the list/array.  Use atomic_inc_not_zero()
+in this scenario as follows:
 
 1.                                      2.
 add()                                   search_and_reference()
@@ -51,17 +51,16 @@ add()					search_and_reference()
 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);       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.
-
+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.