From f671981162a0a84135797ba01661c5c832c5e430 Mon Sep 17 00:00:00 2001
From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Fri, 1 Jul 2011 18:50:15 -0400
Subject: [PATCH] rcu lfqueue: make dequeue lockless by helping out other
 dequeuers

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 urcu/static/rculfqueue.h | 97 +++++++++++++++++++++-------------------
 1 file changed, 51 insertions(+), 46 deletions(-)

diff --git a/urcu/static/rculfqueue.h b/urcu/static/rculfqueue.h
index 6045c18..f3df7c0 100644
--- a/urcu/static/rculfqueue.h
+++ b/urcu/static/rculfqueue.h
@@ -59,12 +59,6 @@ struct cds_lfq_node_rcu_dummy {
  * grace period.
  */
 
-static inline
-int is_dummy(struct cds_lfq_queue_rcu *q, struct cds_lfq_node_rcu *node)
-{
-	return node == CMM_LOAD_SHARED(q->dummy);
-}
-
 static inline
 struct cds_lfq_node_rcu *make_dummy(struct cds_lfq_queue_rcu *q,
 				    struct cds_lfq_node_rcu *next)
@@ -79,7 +73,7 @@ struct cds_lfq_node_rcu *make_dummy(struct cds_lfq_queue_rcu *q,
 }
 
 static inline
-void free_dummy(struct rcu_head *head)
+void free_dummy_cb(struct rcu_head *head)
 {
 	struct cds_lfq_node_rcu_dummy *dummy =
 		caa_container_of(head, struct cds_lfq_node_rcu_dummy, head);
@@ -92,7 +86,16 @@ void rcu_free_dummy(struct cds_lfq_node_rcu *node)
 	struct cds_lfq_node_rcu_dummy *dummy;
 
 	dummy = caa_container_of(node, struct cds_lfq_node_rcu_dummy, parent);
-	dummy->q->queue_call_rcu(&dummy->head, free_dummy);
+	dummy->q->queue_call_rcu(&dummy->head, free_dummy_cb);
+}
+
+static inline
+void free_dummy(struct cds_lfq_node_rcu *node)
+{
+	struct cds_lfq_node_rcu_dummy *dummy;
+
+	dummy = caa_container_of(node, struct cds_lfq_node_rcu_dummy, parent);
+	free(dummy);
 }
 
 static inline
@@ -123,9 +126,9 @@ int _cds_lfq_destroy_rcu(struct cds_lfq_queue_rcu *q)
 	struct cds_lfq_node_rcu *head;
 
 	head = rcu_dereference(q->head);
-	if (!(is_dummy(q, head) && head->next == NULL))
+	if (!(head == q->dummy && head->next == NULL))
 		return -EPERM;	/* not empty */
-	rcu_free_dummy(head);
+	free_dummy(head);
 	return 0;
 }
 
@@ -165,6 +168,21 @@ void _cds_lfq_enqueue_rcu(struct cds_lfq_queue_rcu *q,
 	}
 }
 
+static inline
+void reenqueue_dummy(struct cds_lfq_queue_rcu *q)
+{
+	struct cds_lfq_node_rcu *node;
+
+	/* We need to reallocate to protect from ABA. */
+	node = make_dummy(q, NULL);
+	if (uatomic_cmpxchg(&q->dummy, NULL, node) != NULL) {
+		/* other dequeue populated its new dummy */
+		free_dummy(node);
+		return;
+	}
+	_cds_lfq_enqueue_rcu(q, node);
+}
+
 /*
  * Should be called under rcu read lock critical section.
  *
@@ -176,52 +194,39 @@ static inline
 struct cds_lfq_node_rcu *_cds_lfq_dequeue_rcu(struct cds_lfq_queue_rcu *q)
 {
 	for (;;) {
-		struct cds_lfq_node_rcu *head, *next;
+		struct cds_lfq_node_rcu *head, *next, *dummy;
 
 		head = rcu_dereference(q->head);
 		next = rcu_dereference(head->next);
-		if (is_dummy(q, head) && next == NULL)
+		dummy = rcu_dereference(q->dummy);
+		if (head == dummy && next == NULL)
 			return NULL;	/* empty */
 		/*
 		 * We never, ever allow dequeue to get to a state where
 		 * the queue is empty (we need at least one node in the
 		 * queue). This is ensured by checking if the head next
-		 * is NULL and retry in that case (this means a
-		 * concurrent dummy node re-enqueue is in progress).
+		 * is NULL. This means a concurrent dummy node
+		 * re-enqueue is in progress. We help it reach
+		 * completion, and retry.
 		 */
-		if (next) {
-			if (uatomic_cmpxchg(&q->head, head, next) == head) {
-				if (is_dummy(q, head)) {
-					struct cds_lfq_node_rcu *node;
-					/*
-					 * Requeue dummy. We need to
-					 * reallocate to protect from
-					 * ABA.
-					 */
-					rcu_free_dummy(head);
-					node = make_dummy(q, NULL);
-					/*
-					 * We are the only thread
-					 * allowed to update dummy (we
-					 * own the old dummy). Other
-					 * dequeue threads read it
-					 * concurrently with RCU
-					 * read-lock held, which
-					 * protects from ABA.
-					 */
-					CMM_STORE_SHARED(q->dummy, node);
-					_cds_lfq_enqueue_rcu(q, node);
-					continue;	/* try again */
-				}
-				return head;
-			} else {
-				/* Concurrently pushed, retry */
-				continue;
-			}
-		} else {
-			/* Dummy node re-enqueue is in progress, retry. */
-			continue;
+		if (!next) {
+			/*
+			 * Dummy node re-enqueue is in progress. Try to
+			 * help.
+			 */
+			reenqueue_dummy(q);
+			continue;	/* try again */
+		}
+		if (uatomic_cmpxchg(&q->head, head, next) != head)
+			continue;	/* Concurrently pushed. */
+		if (head == dummy) {
+			/* Free old and requeue new dummy. */
+			rcu_set_pointer(&q->dummy, NULL);
+			rcu_free_dummy(dummy);
+			reenqueue_dummy(q);
+			continue;	/* try again */
 		}
+		return head;
 	}
 }
 
-- 
2.34.1