From: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
Date: Tue, 13 Jul 2010 15:59:55 +0000 (-0400)
Subject: RCU lock-free queue: don't hold RCU read lock across retry
X-Git-Tag: v0.4.7~20
X-Git-Url: https://git.liburcu.org/?p=urcu.git;a=commitdiff_plain;h=23086b720b7edf8b402f22a899568bc6cbcb8235

RCU lock-free queue: don't hold RCU read lock across retry

Be nicer to grace periods.

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

diff --git a/urcu/rculfqueue.h b/urcu/rculfqueue.h
index 32fca49..09e5d41 100644
--- a/urcu/rculfqueue.h
+++ b/urcu/rculfqueue.h
@@ -72,11 +72,11 @@ 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->next to be NULL.
 		 */
@@ -97,6 +97,7 @@ 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;
 		}
 	}
@@ -113,11 +114,12 @@ void rcu_lfq_enqueue(struct rcu_lfq_queue *q, struct rcu_lfq_node *node)
 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 +127,7 @@ rcu_lfq_dequeue(struct rcu_lfq_queue *q, void (*release)(struct urcu_ref *))
 				return next;
 			} else {
 				/* Concurrently pushed, retry */
+				rcu_read_unlock();
 				continue;
 			}
 		} else {