[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>

1 files changed, 74 insertions, 0 deletions

diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
new file mode 100644
index 000000000000..a23fee66064d
--- /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.