add missing comment to wfstack.c

[urcu.git] / urcu / rculfqueue.h

diff --git a/urcu/rculfqueue.h b/urcu/rculfqueue.h
index f9b14fe93657343c5e1917ab77afcab4dc087a0d..d5c74288831d32e810a045fc068c668e7d60347f 100644 (file)
--- a/urcu/rculfqueue.h
+++ b/urcu/rculfqueue.h
@@ -1,3 +1,6 @@
+#ifndef _URCU_RCULFQUEUE_H
+#define _URCU_RCULFQUEUE_H
+
 /*
  * rculfqueue.h
  *
@@ -23,6 +26,10 @@
 #include <urcu/urcu_ref.h>
 #include <assert.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #if (!defined(_GNU_SOURCE) && !defined(_LGPL_SOURCE))
 #error "Dynamic loader LGPL wrappers not implemented yet"
 #endif
@@ -72,13 +79,13 @@ void rcu_lfq_enqueue(struct rcu_lfq_queue *q, struct rcu_lfq_node *node)
         * node before publication.
         */
 
-       rcu_read_lock();
        for (;;) {
-               struct rcu_lfq_node *tail = rcu_dereference(q->tail);
-               struct rcu_lfq_node *next;
+               struct rcu_lfq_node *tail, *next;
 
+               rcu_read_lock();
+               tail = rcu_dereference(q->tail);
                /*
-                * Typically expect tail to be NULL.
+                * Typically expect tail->next to be NULL.
                 */
                next = uatomic_cmpxchg(&tail->next, NULL, node);
                if (next == NULL) {
@@ -97,6 +104,7 @@ void rcu_lfq_enqueue(struct rcu_lfq_queue *q, struct rcu_lfq_node *node)
                         * further and retry.
                         */
                        uatomic_cmpxchg(&q->tail, tail, next);
+                       rcu_read_unlock();
                        continue;
                }
        }
@@ -105,19 +113,22 @@ void rcu_lfq_enqueue(struct rcu_lfq_queue *q, struct rcu_lfq_node *node)
 /*
  * The entry returned by dequeue must be taken care of by doing a urcu_ref_put,
  * which calls the release primitive when the reference count drops to zero. A
- * grace period must be waited before performing the actual memory reclamation
- * in the release primitive.
- * The entry lfq node returned by dequeue must no be re-used before the
- * reference count reaches zero.
+ * grace period must be waited after execution of the release callback before
+ * performing the actual memory reclamation or modifying the rcu_lfq_node
+ * structure.
+ * In other words, the entry lfq node returned by dequeue must not be
+ * modified/re-used/freed until the reference count reaches zero and a grace
+ * period has elapsed (after the refcount reached 0).
  */
 struct rcu_lfq_node *
 rcu_lfq_dequeue(struct rcu_lfq_queue *q, void (*release)(struct urcu_ref *))
 {
-       rcu_read_lock();
        for (;;) {
-               struct rcu_lfq_node *head = rcu_dereference(q->head);
-               struct rcu_lfq_node *next = rcu_dereference(head->next);
+               struct rcu_lfq_node *head, *next;
 
+               rcu_read_lock();
+               head = rcu_dereference(q->head);
+               next = rcu_dereference(head->next);
                if (next) {
                        if (uatomic_cmpxchg(&q->head, head, next) == head) {
                                rcu_read_unlock();
@@ -125,6 +136,7 @@ rcu_lfq_dequeue(struct rcu_lfq_queue *q, void (*release)(struct urcu_ref *))
                                return next;
                        } else {
                                /* Concurrently pushed, retry */
+                               rcu_read_unlock();
                                continue;
                        }
                } else {
@@ -134,3 +146,9 @@ rcu_lfq_dequeue(struct rcu_lfq_queue *q, void (*release)(struct urcu_ref *))
                }
        }
 }
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _URCU_RCULFQUEUE_H */