[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>
diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
index a23fee6..3f60db41 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.
+