[lttng-dev] [RFC URCU PATCH] Introduce wfqueue v0 ABI compatibility

[Mathieu Desnoyers]

Introduce wfqueue v0 ABI compatibility
    
Preparing for v1 ABI.
    
CC: Lai Jiangshan <laijs at cn.fujitsu.com>
CC: Paul McKenney <paulmck at linux.vnet.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
---
diff --git a/wfqueue0.c b/wfqueue0.c
new file mode 100644
index 0000000..e928c0a
--- /dev/null
+++ b/wfqueue0.c
@@ -0,0 +1,191 @@
+/*
+ * wfqueue0.c
+ *
+ * Userspace RCU library - Queue with Wait-Free Enqueue/Blocking Dequeue
+ *
+ * Copyright 2010-2012 - Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
+ * Copyright 2011-2012 - Lai Jiangshan <laijs at cn.fujitsu.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/* This file is provided for ABI backward compatibility with wfqueue v0. */
+
+#include <pthread.h>
+#include <assert.h>
+#include <poll.h>
+#include <urcu/compiler.h>
+#include <urcu/uatomic.h>
+
+/*
+ * Queue with wait-free enqueue/blocking dequeue.
+ * This implementation adds a dummy head node when the queue is empty to ensure
+ * we can always update the queue locklessly.
+ *
+ * Inspired from half-wait-free/half-blocking queue implementation done by
+ * Paul E. McKenney.
+ */
+
+struct cds_wfq_node {
+	struct cds_wfq_node *next;
+};
+
+struct cds_wfq_queue {
+	struct cds_wfq_node *head, **tail;
+	struct cds_wfq_node dummy;	/* Dummy node */
+	pthread_mutex_t lock;
+};
+
+#define WFQ_ADAPT_ATTEMPTS		10	/* Retry if being set */
+#define WFQ_WAIT			10	/* Wait 10 ms if being set */
+
+static inline void _cds_wfq_node_init(struct cds_wfq_node *node)
+{
+	node->next = NULL;
+}
+
+static inline void _cds_wfq_init(struct cds_wfq_queue *q)
+{
+	int ret;
+
+	_cds_wfq_node_init(&q->dummy);
+	/* Set queue head and tail */
+	q->head = &q->dummy;
+	q->tail = &q->dummy.next;
+	ret = pthread_mutex_init(&q->lock, NULL);
+	assert(!ret);
+}
+
+static inline void _cds_wfq_enqueue(struct cds_wfq_queue *q,
+				    struct cds_wfq_node *node)
+{
+	struct cds_wfq_node **old_tail;
+
+	/*
+	 * uatomic_xchg() implicit memory barrier orders earlier stores to data
+	 * structure containing node and setting node->next to NULL before
+	 * publication.
+	 */
+	old_tail = uatomic_xchg(&q->tail, &node->next);
+	/*
+	 * At this point, dequeuers see a NULL old_tail->next, which indicates
+	 * that the queue is being appended to. The following store will append
+	 * "node" to the queue from a dequeuer perspective.
+	 */
+	CMM_STORE_SHARED(*old_tail, node);
+}
+
+/*
+ * Waiting for enqueuer to complete enqueue and return the next node
+ */
+static inline struct cds_wfq_node *
+___cds_wfq_node_sync_next(struct cds_wfq_node *node)
+{
+	struct cds_wfq_node *next;
+	int attempt = 0;
+
+	/*
+	 * Adaptative busy-looping waiting for enqueuer to complete enqueue.
+	 */
+	while ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
+		if (++attempt >= WFQ_ADAPT_ATTEMPTS) {
+			poll(NULL, 0, WFQ_WAIT);	/* Wait for 10ms */
+			attempt = 0;
+		} else
+			caa_cpu_relax();
+	}
+
+	return next;
+}
+
+/*
+ * It is valid to reuse and free a dequeued node immediately.
+ *
+ * No need to go on a waitqueue here, as there is no possible state in which the
+ * list could cause dequeue to busy-loop needlessly while waiting for another
+ * thread to be scheduled. The queue appears empty until tail->next is set by
+ * enqueue.
+ */
+static inline struct cds_wfq_node *
+___cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
+{
+	struct cds_wfq_node *node, *next;
+
+	/*
+	 * Queue is empty if it only contains the dummy node.
+	 */
+	if (q->head == &q->dummy && CMM_LOAD_SHARED(q->tail) == &q->dummy.next)
+		return NULL;
+	node = q->head;
+
+	next = ___cds_wfq_node_sync_next(node);
+
+	/*
+	 * Move queue head forward.
+	 */
+	q->head = next;
+	/*
+	 * Requeue dummy node if we just dequeued it.
+	 */
+	if (node == &q->dummy) {
+		_cds_wfq_node_init(node);
+		_cds_wfq_enqueue(q, node);
+		return ___cds_wfq_dequeue_blocking(q);
+	}
+	return node;
+}
+
+static inline struct cds_wfq_node *
+_cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
+{
+	struct cds_wfq_node *retnode;
+	int ret;
+
+	ret = pthread_mutex_lock(&q->lock);
+	assert(!ret);
+	retnode = ___cds_wfq_dequeue_blocking(q);
+	ret = pthread_mutex_unlock(&q->lock);
+	assert(!ret);
+	return retnode;
+}
+
+/*
+ * library wrappers to be used by non-LGPL compatible source code.
+ */
+
+void cds_wfq_node_init(struct cds_wfq_node *node)
+{
+	_cds_wfq_node_init(node);
+}
+
+void cds_wfq_init(struct cds_wfq_queue *q)
+{
+	_cds_wfq_init(q);
+}
+
+void cds_wfq_enqueue(struct cds_wfq_queue *q, struct cds_wfq_node *node)
+{
+	_cds_wfq_enqueue(q, node);
+}
+
+struct cds_wfq_node *cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
+{
+	return _cds_wfq_dequeue_blocking(q);
+}
+
+struct cds_wfq_node *__cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
+{
+	return ___cds_wfq_dequeue_blocking(q);
+}
-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com