--- /dev/null
+#ifndef _URCU_WFQUEUE_H
+#define _URCU_WFQUEUE_H
+
+/*
+ * wfqueue.h
+ *
+ * Userspace RCU library - Queue with Wait-Free Enqueue/Blocking Dequeue
+ *
+ * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers@efficios.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
+ */
+
+#include <pthread.h>
+#include <assert.h>
+#include <urcu/compiler.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#if (!defined(_GNU_SOURCE) && !defined(_LGPL_SOURCE))
+#error "Dynamic loader LGPL wrappers not implemented yet"
+#endif
+
+/*
+ * 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 wfq_node {
+ struct wfq_node *next;
+};
+
+struct wfq_queue {
+ struct wfq_node *head, **tail;
+ struct wfq_node dummy; /* Dummy node */
+ pthread_mutex_t lock;
+};
+
+void wfq_node_init(struct wfq_node *node)
+{
+ node->next = NULL;
+}
+
+void wfq_init(struct wfq_queue *q)
+{
+ int ret;
+
+ 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);
+}
+
+void wfq_enqueue(struct wfq_queue *q, struct wfq_node *node)
+{
+ struct 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);
+ /*
+ * 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.
+ */
+ STORE_SHARED(*old_tail, node);
+}
+
+/*
+ * 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.
+ */
+struct wfq_node *
+__wfq_dequeue_blocking(struct wfq_queue *q)
+{
+ struct wfq_node *node, *next;
+ int busy_wait = 16;
+
+ /*
+ * Queue is empty if it only contains the dummy node.
+ */
+ if (q->head == &q->dummy && LOAD_SHARED(q->tail) == &q->dummy.next)
+ return NULL;
+ node = q->head;
+
+ /*
+ * Adaptative busy-looping waiting for enqueuer to complete enqueue.
+ */
+ while ((next = LOAD_SHARED(node->next)) == NULL) {
+ if (busy_wait > 0) {
+ cpu_relax();
+ busy_wait--;
+ } else
+ poll(NULL, 0, 1); /* Wait for 1ms */
+ }
+ /*
+ * Move queue head forward.
+ */
+ q->head = next;
+ /*
+ * Requeue dummy node if we just dequeued it.
+ */
+ if (node == &q->dummy) {
+ wfq_node_init(node);
+ wfq_enqueue(q, node);
+ return __wfq_dequeue_blocking(q);
+ }
+ return node;
+}
+
+struct wfq_node *
+wfq_dequeue_blocking(struct wfq_queue *q)
+{
+ struct wfq_node *retnode;
+ int ret;
+
+ ret = pthread_mutex_lock(&q->lock);
+ assert(!ret);
+ retnode = __wfq_dequeue_blocking(q);
+ ret = pthread_mutex_unlock(&q->lock);
+ assert(!ret);
+ return retnode;
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _URCU_WFQUEUE_H */
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