Wait-free push/Blocking pop stack: Update following Paul's review

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/urcu/rcuwfstack.h b/urcu/rcuwfstack.h
index f2906cead9058af10f133e349734866bc1202111..4bbe7c507679a07a6350ab65b8ea06f8107247c7 100644 (file)
--- a/urcu/rcuwfstack.h
+++ b/urcu/rcuwfstack.h
@@ -20,17 +20,20 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
+#include <assert.h>
+
 #if (!defined(_GNU_SOURCE) && !defined(_LGPL_SOURCE))
 #error "Dynamic loader LGPL wrappers not implemented yet"
 #endif
 
+#define RCU_WF_STACK_END ((void *)0x1UL)
+
 struct rcu_wfs_node {
        struct rcu_wfs_node *next;
 };
 
 struct rcu_wfs_stack {
        struct rcu_wfs_node *head;
-       struct rcu_wfs_node end;
 };
 
 void rcu_wfs_node_init(struct rcu_wfs_node *node)
@@ -40,14 +43,14 @@ void rcu_wfs_node_init(struct rcu_wfs_node *node)
 
 void rcu_wfs_init(struct rcu_wfs_stack *s)
 {
-       s->head = &s->end;
-       rcu_wfs_node_init(&s->end);
+       s->head = RCU_WF_STACK_END;
 }
 
 void rcu_wfs_push(struct rcu_wfs_stack *s, struct rcu_wfs_node *node)
 {
        struct rcu_wfs_node *old_head;
 
+       assert(node->next == NULL);
        /*
         * uatomic_xchg() implicit memory barrier orders earlier stores to node
         * (setting it to NULL) before publication.
@@ -61,7 +64,12 @@ void rcu_wfs_push(struct rcu_wfs_stack *s, struct rcu_wfs_node *node)
 }
 
 /*
- * The caller must wait for a grace period before freeing the returned node.
+ * The caller must wait for a grace period before:
+ * - freeing the returned node.
+ * - modifying the ->next pointer of the returned node. (be careful with unions)
+ * - passing the returned node back to push() on the same stack they got it
+ *   from.
+ *
  * Returns NULL if stack is empty.
  *
  * cmpxchg is protected from ABA races by holding a RCU read lock between
@@ -74,22 +82,25 @@ void rcu_wfs_push(struct rcu_wfs_stack *s, struct rcu_wfs_node *node)
 struct rcu_wfs_node *
 rcu_wfs_pop(struct rcu_wfs_stack *s)
 {
-       rcu_read_lock();
        for (;;) {
-               struct rcu_wfs_node *head = rcu_dereference(s->head);
+               struct rcu_wfs_node *head;
 
-               if (head != &s->end) {
+               rcu_read_lock();
+               head = rcu_dereference(s->head);
+               if (head != RCU_WF_STACK_END) {
                        struct rcu_wfs_node *next = rcu_dereference(head->next);
 
                        /* Retry while head is being set by push(). */
-                       if (!next)
+                       if (!next) {
+                               rcu_read_unlock();
                                continue;
-
+                       }
                        if (uatomic_cmpxchg(&s->head, head, next) == head) {
                                rcu_read_unlock();
                                return head;
                        } else {
                                /* Concurrent modification. Retry. */
+                               rcu_read_unlock();
                                continue;
                        }
                } else {