X-Git-Url: http://git.liburcu.org/?p=urcu.git;a=blobdiff_plain;f=urcu%2Fstatic%2Flfstack.h;fp=urcu%2Fstatic%2Flfstack.h;h=7acbf54c1f2f3e4e25ea240d06da7b3a1d9b35e8;hp=0000000000000000000000000000000000000000;hb=d3bfcb24240b01619686641ddd92ae8222b3ad32;hpb=69328034b804a0fc180c08f7ca6be1a4446bb105

diff --git a/urcu/static/lfstack.h b/urcu/static/lfstack.h
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+++ b/urcu/static/lfstack.h
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+#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 */