+#ifndef _URCU_STATIC_LFSTACK_H
+#define _URCU_STATIC_LFSTACK_H
+
+/*
+ * urcu/static/lfstack.h
+ *
+ * Userspace RCU library - Lock-Free Stack
+ *
+ * Copyright 2010-2012 - Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
+ *
+ * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See rculfstack.h for linking
+ * dynamically with the userspace rcu library.
+ *
+ * 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 <urcu/uatomic.h>
+#include <urcu-pointer.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+static inline
+void _cds_lfs_node_init(struct cds_lfs_node *node)
+{
+}
+
+static inline
+void _cds_lfs_init(struct cds_lfs_stack *s)
+{
+ s->head = NULL;
+}
+
+/*
+ * cds_lfs_push: push a node into the stack.
+ *
+ * Does not require any synchronization with other push nor pop.
+ *
+ * Lock-free stack push is not subject to ABA problem, so no need to
+ * take the RCU read-side lock. Even if "head" changes between two
+ * uatomic_cmpxchg() invocations here (being popped, and then pushed
+ * again by one or more concurrent threads), the second
+ * uatomic_cmpxchg() invocation only cares about pushing a new entry at
+ * the head of the stack, ensuring consistency by making sure the new
+ * node->next is the same pointer value as the value replaced as head.
+ * It does not care about the content of the actual next node, so it can
+ * very well be reallocated between the two uatomic_cmpxchg().
+ *
+ * We take the approach of expecting the stack to be usually empty, so
+ * we first try an initial uatomic_cmpxchg() on a NULL old_head, and
+ * retry if the old head was non-NULL (the value read by the first
+ * uatomic_cmpxchg() is used as old head for the following loop). The
+ * upside of this scheme is to minimize the amount of cacheline traffic,
+ * always performing an exclusive cacheline access, rather than doing
+ * non-exclusive followed by exclusive cacheline access (which would be
+ * required if we first read the old head value). This design decision
+ * might be revisited after more throrough benchmarking on various
+ * platforms.
+ *
+ * Returns 0 if the stack was empty prior to adding the node.
+ * Returns non-zero otherwise.
+ */
+static inline
+int _cds_lfs_push(struct cds_lfs_stack *s,
+ struct cds_lfs_node *node)
+{
+ struct cds_lfs_node *head = NULL;
+
+ for (;;) {
+ struct cds_lfs_node *old_head = head;
+
+ /*
+ * node->next is still private at this point, no need to
+ * perform a _CMM_STORE_SHARED().
+ */
+ node->next = head;
+ /*
+ * uatomic_cmpxchg() implicit memory barrier orders earlier
+ * stores to node before publication.
+ */
+ head = uatomic_cmpxchg(&s->head, old_head, node);
+ if (old_head == head)
+ break;
+ }
+ return (int) !!((unsigned long) head);
+}
+
+/*
+ * cds_lfs_pop: pop a node from the stack.
+ *
+ * Returns NULL if stack is empty.
+ *
+ * cds_lfs_pop needs to be synchronized using one of the following
+ * techniques:
+ *
+ * 1) Calling cds_lfs_pop under rcu read lock critical section. The
+ * caller must wait for a grace period to pass before freeing the
+ * returned node or modifying the cds_lfs_node structure.
+ * 2) Using mutual exclusion (e.g. mutexes) to protect cds_lfs_pop
+ * callers.
+ * 3) Ensuring that only ONE thread can call cds_lfs_pop().
+ * (multi-provider/single-consumer scheme).
+ */
+static inline
+struct cds_lfs_node *_cds_lfs_pop(struct cds_lfs_stack *s)
+{
+ for (;;) {
+ struct cds_lfs_node *head;
+
+ head = _CMM_LOAD_SHARED(s->head);
+ if (head) {
+ struct cds_lfs_node *next;
+
+ /*
+ * Read head before head->next. Matches the
+ * implicit memory barrier before
+ * uatomic_cmpxchg() in cds_lfs_push.
+ */
+ cmm_smp_read_barrier_depends();
+ next = _CMM_LOAD_SHARED(head->next);
+ if (uatomic_cmpxchg(&s->head, head, next) == head) {
+ return head;
+ } else {
+ /* Concurrent modification. Retry. */
+ continue;
+ }
+ } else {
+ /* Empty stack */
+ return NULL;
+ }
+ }
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* _URCU_STATIC_LFSTACK_H */
projects / urcu.git / commitdiff
