Wait-free stack: rcu-less implementation

[urcu.git] / urcu / wfstack.h

diff --git a/urcu/wfstack.h b/urcu/wfstack.h
new file mode 100644 (file)
index 0000000..b3ee2e7
--- /dev/null
+++ b/urcu/wfstack.h
@@ -0,0 +1,129 @@
+#ifndef _URCU_WFSTACK_H
+#define _URCU_WFSTACK_H
+
+/*
+ * rcuwfstack.h
+ *
+ * Userspace RCU library - Stack with Wait-Free push, Blocking pop.
+ *
+ * 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
+
+#define WF_STACK_END                   ((void *)0x1UL)
+#define WFS_ADAPT_ATTEMPTS             10      /* Retry if being set */
+#define WFS_WAIT                       10      /* Wait 10 ms if being set */
+
+struct wfs_node {
+       struct wfs_node *next;
+};
+
+struct wfs_stack {
+       struct wfs_node *head;
+       pthread_mutex_t lock;
+};
+
+void wfs_node_init(struct wfs_node *node)
+{
+       node->next = NULL;
+}
+
+void wfs_init(struct wfs_stack *s)
+{
+       int ret;
+
+       s->head = WF_STACK_END;
+       ret = pthread_mutex_init(&s->lock, NULL);
+       assert(!ret);
+}
+
+void wfs_push(struct wfs_stack *s, struct wfs_node *node)
+{
+       struct wfs_node *old_head;
+
+       assert(node->next == NULL);
+       /*
+        * uatomic_xchg() implicit memory barrier orders earlier stores to node
+        * (setting it to NULL) before publication.
+        */
+       old_head = uatomic_xchg(&s->head, node);
+       /*
+        * At this point, dequeuers see a NULL node->next, they should busy-wait
+        * until node->next is set to old_head.
+        */
+       STORE_SHARED(node->next, old_head);
+}
+
+/*
+ * Returns NULL if stack is empty.
+ */
+struct wfs_node *
+__wfs_pop_blocking(struct wfs_stack *s)
+{
+       struct wfs_node *head, *next;
+       int attempt = 0;
+
+retry:
+       head = LOAD_SHARED(s->head);
+       if (head == WF_STACK_END)
+               return NULL;
+       /*
+        * Adaptative busy-looping waiting for push to complete.
+        */
+       while ((next = LOAD_SHARED(head->next)) == NULL) {
+               if (++attempt >= WFS_ADAPT_ATTEMPTS) {
+                       poll(NULL, 0, WFS_WAIT);        /* Wait for 10ms */
+                       attempt = 0;
+               } else
+                       cpu_relax();
+       }
+       if (uatomic_cmpxchg(&s->head, head, next) == head)
+               return head;
+       else
+               goto retry;             /* Concurrent modification. Retry. */
+}
+
+struct wfs_node *
+wfs_pop_blocking(struct wfs_stack *s)
+{
+       struct wfs_node *retnode;
+       int ret;
+
+       ret = pthread_mutex_lock(&s->lock);
+       assert(!ret);
+       retnode = __wfs_pop_blocking(s);
+       ret = pthread_mutex_unlock(&s->lock);
+       assert(!ret);
+       return retnode;
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _URCU_WFSTACK_H */