From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Wed, 22 Oct 2014 10:54:25 +0000 (-0400)
Subject: workqueue/waitqueue: use lock-free stack for wakeup
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=7a618cf73bc9da1a6d258f361a41ed9e6f1b9f88;p=userspace-rcu.git

workqueue/waitqueue: use lock-free stack for wakeup

The case for which we try to reduce latency is the wakeup, not the
"wait", since "wait" is typically done when a thread is ready to put
itself asleep awaiting for more work.

wfstack has blocking "pop" operation, which means that if a worker
thread is delayed for an extremely long amount of time during the push
operation (add to waitqueue), it could delay execution of pop (wakeup)
for that same amount of time.

lfstack does not have this downside, at the expense of having the "wait"
operation (push) being only lock-free rather than wait-free. However,
since we don't care that much about "wait" being wait-free, it makes
sense to use lfstack here.

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

diff --git a/urcu/waitqueue-lifo.h b/urcu/waitqueue-lifo.h
index 2ccf630..fccba17 100644
--- a/urcu/waitqueue-lifo.h
+++ b/urcu/waitqueue-lifo.h
@@ -24,7 +24,7 @@
  */
 
 #include <urcu/uatomic.h>
-#include <urcu/wfstack.h>
+#include <urcu/lfstack.h>
 #include <urcu/futex.h>
 
 /*
@@ -43,7 +43,7 @@ enum urcu_wait_state {
 };
 
 struct urcu_wait_node {
-	struct cds_wfs_node node;
+	struct cds_lfs_node node;
 	int32_t state;	/* enum urcu_wait_state */
 };
 
@@ -57,11 +57,11 @@ struct urcu_wait_node {
 	struct urcu_wait_node name
 
 struct urcu_wait_queue {
-	struct __cds_wfs_stack stack;
+	struct __cds_lfs_stack stack;
 };
 
 #define URCU_WAIT_QUEUE_HEAD_INIT(name)			\
-	{ .stack.head = CDS_WFS_END, }
+	{ .stack.head = CDS_LFS_END, }
 
 #define DECLARE_URCU_WAIT_QUEUE(name)			\
 	struct urcu_wait_queue name
@@ -72,11 +72,11 @@ struct urcu_wait_queue {
 static inline
 void urcu_wait_queue_init(struct urcu_wait_queue *queue)
 {
-	__cds_wfs_init(&queue->stack);
+	__cds_lfs_init(&queue->stack);
 }
 
 struct urcu_waiters {
-	struct cds_wfs_head *head;
+	struct cds_lfs_head *head;
 };
 
 /*
@@ -88,7 +88,7 @@ static inline
 bool urcu_wait_add(struct urcu_wait_queue *queue,
 		struct urcu_wait_node *node)
 {
-	return cds_wfs_push(&queue->stack, &node->node);
+	return cds_lfs_push(&queue->stack, &node->node);
 }
 
 /*
@@ -99,7 +99,7 @@ static inline
 void urcu_move_waiters(struct urcu_waiters *waiters,
 		struct urcu_wait_queue *queue)
 {
-	waiters->head = __cds_wfs_pop_all(&queue->stack);
+	waiters->head = __cds_lfs_pop_all(&queue->stack);
 }
 
 static inline
@@ -121,7 +121,7 @@ void urcu_wait_node_init(struct urcu_wait_node *node,
 		enum urcu_wait_state state)
 {
 	urcu_wait_set_state(node, state);
-	cds_wfs_node_init(&node->node);
+	cds_lfs_node_init(&node->node);
 }
 
 /*
@@ -198,11 +198,11 @@ skip_futex_wait:
 static inline
 int urcu_dequeue_wake_single(struct urcu_wait_queue *queue)
 {
-	struct cds_wfs_node *node;
+	struct cds_lfs_node *node;
 	struct urcu_wait_node *wait_node;
 	int ret = 0;
 
-	node = __cds_wfs_pop_blocking(&queue->stack);
+	node = __cds_lfs_pop(&queue->stack);
 	if (!node)
 		return -ENOENT;
 	wait_node = caa_container_of(node, struct urcu_wait_node, node);
@@ -239,11 +239,11 @@ int urcu_dequeue_wake_n(struct urcu_wait_queue *queue, int n)
 static inline
 int urcu_wake_all_waiters(struct urcu_waiters *waiters)
 {
-	struct cds_wfs_node *iter, *iter_n;
+	struct cds_lfs_node *iter, *iter_n;
 	int nr_wakeup = 0;
 
 	/* Wake all waiters in our stack head */
-	cds_wfs_for_each_blocking_safe(waiters->head, iter, iter_n) {
+	cds_lfs_for_each_safe(waiters->head, iter, iter_n) {
 		struct urcu_wait_node *wait_node =
 			caa_container_of(iter, struct urcu_wait_node, node);
 
diff --git a/urcu/workqueue-fifo.h b/urcu/workqueue-fifo.h
index 0769acc..68e050a 100644
--- a/urcu/workqueue-fifo.h
+++ b/urcu/workqueue-fifo.h
@@ -24,7 +24,7 @@
  */
 
 #include <urcu/uatomic.h>
-#include <urcu/wfstack.h>
+#include <urcu/lfstack.h>
 #include <urcu/waitqueue-lifo.h>
 #include <urcu/wfcqueue.h>
 #include <urcu/rculist.h>
@@ -315,7 +315,7 @@ void urcu_accept_work(struct urcu_workqueue *queue,
 		 * NULL next pointer. We are therefore not in
 		 * the queue.
 		 */
-		cds_wfs_node_init(&worker->wait_node.node);
+		cds_lfs_node_init(&worker->wait_node.node);
 		/* Protect stack dequeue against ABA */
 		synchronize_rcu();
 		was_empty = !urcu_wait_add(&queue->waitqueue,