[PATCH] files: rcuref APIs
Adds a set of primitives to do reference counting for objects that are looked
up without locks using RCU.
Signed-off-by: Ravikiran Thirumalai <kiran_th@gmail.com>
Signed-off-by: Dipankar Sarma <dipankar@in.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
new file mode 100644
index 0000000..a23fee6
--- /dev/null
+++ b/Documentation/RCU/rcuref.txt
@@ -0,0 +1,74 @@
+Refcounter framework 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:
+
+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 rcuref_get in
+search_and_reference could potentially hold reference to an element which
+has already been deleted from the list/array. rcuref_lf_get_rcu takes
+care of this scenario. search_and_reference should look as;
+
+1. 2.
+add() search_and_reference()
+{ {
+ alloc_object rcu_read_lock();
+ ... search_for_element
+ atomic_set(&el->rc, 1); if (rcuref_inc_lf(&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);
+ rcuref_dec(&el->rc, relfunc) ...
+ ... delete_element
+} write_unlock(&list_lock);
+ ...
+ if (rcuref_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, rcuref_inc_lf might be an overkill
+since the spinlock serialising list updates are held. rcuref_inc
+is to be used in such cases.
+For arches which do not have cmpxchg rcuref_inc_lf
+api uses a hashed spinlock implementation and the same hashed spinlock
+is acquired in all rcuref_xxx primitives to preserve atomicity.
+Note: Use rcuref_inc api only if you need to use rcuref_inc_lf on the
+refcounter atleast at one place. Mixing rcuref_inc and atomic_xxx api
+might lead to races. rcuref_inc_lf() must be used in lockfree
+RCU critical sections only.