Add wait-free push/blocking pop stack

[urcu.git] / urcu / rcuwfstack.h

diff --git a/urcu/rcuwfstack.h b/urcu/rcuwfstack.h
new file mode 100644 (file)
index 0000000..f2906ce
--- /dev/null
+++ b/urcu/rcuwfstack.h
@@ -0,0 +1,101 @@
+/*
+ * rcuwfstack.h
+ *
+ * Userspace RCU library - RCU 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
+ */
+
+#if (!defined(_GNU_SOURCE) && !defined(_LGPL_SOURCE))
+#error "Dynamic loader LGPL wrappers not implemented yet"
+#endif
+
+struct rcu_wfs_node {
+       struct rcu_wfs_node *next;
+};
+
+struct rcu_wfs_stack {
+       struct rcu_wfs_node *head;
+       struct rcu_wfs_node end;
+};
+
+void rcu_wfs_node_init(struct rcu_wfs_node *node)
+{
+       node->next = NULL;
+}
+
+void rcu_wfs_init(struct rcu_wfs_stack *s)
+{
+       s->head = &s->end;
+       rcu_wfs_node_init(&s->end);
+}
+
+void rcu_wfs_push(struct rcu_wfs_stack *s, struct rcu_wfs_node *node)
+{
+       struct rcu_wfs_node *old_head;
+
+       /*
+        * 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);
+}
+
+/*
+ * The caller must wait for a grace period before freeing the returned node.
+ * Returns NULL if stack is empty.
+ *
+ * cmpxchg is protected from ABA races by holding a RCU read lock between
+ * s->head read and cmpxchg modifying s->head and requiring that dequeuers wait
+ * for a grace period before freeing the returned node.
+ *
+ * TODO: implement adaptative busy-wait and wait/wakeup scheme rather than busy
+ * loops. Better for UP.
+ */
+struct rcu_wfs_node *
+rcu_wfs_pop(struct rcu_wfs_stack *s)
+{
+       rcu_read_lock();
+       for (;;) {
+               struct rcu_wfs_node *head = rcu_dereference(s->head);
+
+               if (head != &s->end) {
+                       struct rcu_wfs_node *next = rcu_dereference(head->next);
+
+                       /* Retry while head is being set by push(). */
+                       if (!next)
+                               continue;
+
+                       if (uatomic_cmpxchg(&s->head, head, next) == head) {
+                               rcu_read_unlock();
+                               return head;
+                       } else {
+                               /* Concurrent modification. Retry. */
+                               continue;
+                       }
+               } else {
+                       /* Empty stack */
+                       rcu_read_unlock();
+                       return NULL;
+               }
+       }
+}