[PATCH] rcu file: use atomic primitives

Use atomic_inc_not_zero for rcu files instead of special case rcuref.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Cc: "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, 40 insertions, 47 deletions

diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
index a23fee66064d..3f60db41b2f0 100644
--- a/Documentation/RCU/rcuref.txt
+++ b/Documentation/RCU/rcuref.txt
@@ -1,74 +1,67 @@
-Refcounter framework for elements of lists/arrays protected by
-RCU.
+Refcounter 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:
 
-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);	}
+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);
-					...
+    ...					    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
+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.  rcuref_lf_get_rcu takes
+has already been deleted from the list/array.  atomic_inc_not_zero 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();
+    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);
-	rcuref_dec(&el->rc, relfunc)	...
-	...				delete_element
-}					write_unlock(&list_lock);
- 					...
-					if (rcuref_dec_and_test(&el->rc))
-						call_rcu(&el->head, el_free);
-					...
+    ...					    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);
+					    ...
 					}
 
 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
+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.
-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.
+