[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>
diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
index 3f60db41..451de2a 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 @@
...
}
-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 @@
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.