Make lf stack push return if the stack was empty

[urcu.git] / urcu / static / rculfstack.h

diff --git a/urcu/static/rculfstack.h b/urcu/static/rculfstack.h
index ba26231e87206371965c653189596137abb818cd..1df121b461389fbb0b621e6eeefb37c4b4e131eb 100644 (file)
--- a/urcu/static/rculfstack.h
+++ b/urcu/static/rculfstack.h
@@ -43,8 +43,34 @@ void _cds_lfs_init_rcu(struct cds_lfs_stack_rcu *s)
        s->head = NULL;
 }
 
+/*
+ * 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
-void _cds_lfs_push_rcu(struct cds_lfs_stack_rcu *s, struct cds_lfs_node_rcu *node)
+int _cds_lfs_push_rcu(struct cds_lfs_stack_rcu *s,
+                         struct cds_lfs_node_rcu *node)
 {
        struct cds_lfs_node_rcu *head = NULL;
 
@@ -60,10 +86,11 @@ void _cds_lfs_push_rcu(struct cds_lfs_stack_rcu *s, struct cds_lfs_node_rcu *nod
                if (old_head == head)
                        break;
        }
+       return (int) !!((unsigned long) head);
 }
 
 /*
- * Should be called under rcu read-side lock.
+ * Acts as a RCU reader.
  *
  * The caller must wait for a grace period to pass before freeing the returned
  * node or modifying the cds_lfs_node_rcu structure.
@@ -76,18 +103,22 @@ _cds_lfs_pop_rcu(struct cds_lfs_stack_rcu *s)
        for (;;) {
                struct cds_lfs_node_rcu *head;
 
+               rcu_read_lock();
                head = rcu_dereference(s->head);
                if (head) {
                        struct cds_lfs_node_rcu *next = rcu_dereference(head->next);
 
                        if (uatomic_cmpxchg(&s->head, head, next) == head) {
+                               rcu_read_unlock();
                                return head;
                        } else {
                                /* Concurrent modification. Retry. */
+                               rcu_read_unlock();
                                continue;
                        }
                } else {
                        /* Empty stack */
+                       rcu_read_unlock();
                        return NULL;
                }
        }