rcu lfqueue: make dequeue lockless by helping out other dequeuers

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>

diff --git a/urcu/static/rculfqueue.h b/urcu/static/rculfqueue.h
index 6045c184d22b2b9af8ea0c03bb33a1f026d24be0..f3df7c01f2563bd3d5c7a29314d21ca1333d8a0a 100644 (file)
--- a/urcu/static/rculfqueue.h
+++ b/urcu/static/rculfqueue.h
@@ -59,12 +59,6 @@ struct cds_lfq_node_rcu_dummy {
  * grace period.
  */
 
-static inline
-int is_dummy(struct cds_lfq_queue_rcu *q, struct cds_lfq_node_rcu *node)
-{
-       return node == CMM_LOAD_SHARED(q->dummy);
-}
-
 static inline
 struct cds_lfq_node_rcu *make_dummy(struct cds_lfq_queue_rcu *q,
                                    struct cds_lfq_node_rcu *next)
@@ -79,7 +73,7 @@ struct cds_lfq_node_rcu *make_dummy(struct cds_lfq_queue_rcu *q,
 }
 
 static inline
-void free_dummy(struct rcu_head *head)
+void free_dummy_cb(struct rcu_head *head)
 {
        struct cds_lfq_node_rcu_dummy *dummy =
                caa_container_of(head, struct cds_lfq_node_rcu_dummy, head);
@@ -92,7 +86,16 @@ void rcu_free_dummy(struct cds_lfq_node_rcu *node)
        struct cds_lfq_node_rcu_dummy *dummy;
 
        dummy = caa_container_of(node, struct cds_lfq_node_rcu_dummy, parent);
-       dummy->q->queue_call_rcu(&dummy->head, free_dummy);
+       dummy->q->queue_call_rcu(&dummy->head, free_dummy_cb);
+}
+
+static inline
+void free_dummy(struct cds_lfq_node_rcu *node)
+{
+       struct cds_lfq_node_rcu_dummy *dummy;
+
+       dummy = caa_container_of(node, struct cds_lfq_node_rcu_dummy, parent);
+       free(dummy);
 }
 
 static inline
@@ -123,9 +126,9 @@ int _cds_lfq_destroy_rcu(struct cds_lfq_queue_rcu *q)
        struct cds_lfq_node_rcu *head;
 
        head = rcu_dereference(q->head);
-       if (!(is_dummy(q, head) && head->next == NULL))
+       if (!(head == q->dummy && head->next == NULL))
                return -EPERM;  /* not empty */
-       rcu_free_dummy(head);
+       free_dummy(head);
        return 0;
 }
 
@@ -165,6 +168,21 @@ void _cds_lfq_enqueue_rcu(struct cds_lfq_queue_rcu *q,
        }
 }
 
+static inline
+void reenqueue_dummy(struct cds_lfq_queue_rcu *q)
+{
+       struct cds_lfq_node_rcu *node;
+
+       /* We need to reallocate to protect from ABA. */
+       node = make_dummy(q, NULL);
+       if (uatomic_cmpxchg(&q->dummy, NULL, node) != NULL) {
+               /* other dequeue populated its new dummy */
+               free_dummy(node);
+               return;
+       }
+       _cds_lfq_enqueue_rcu(q, node);
+}
+
 /*
  * Should be called under rcu read lock critical section.
  *
@@ -176,52 +194,39 @@ static inline
 struct cds_lfq_node_rcu *_cds_lfq_dequeue_rcu(struct cds_lfq_queue_rcu *q)
 {
        for (;;) {
-               struct cds_lfq_node_rcu *head, *next;
+               struct cds_lfq_node_rcu *head, *next, *dummy;
 
                head = rcu_dereference(q->head);
                next = rcu_dereference(head->next);
-               if (is_dummy(q, head) && next == NULL)
+               dummy = rcu_dereference(q->dummy);
+               if (head == dummy && next == NULL)
                        return NULL;    /* empty */
                /*
                 * We never, ever allow dequeue to get to a state where
                 * the queue is empty (we need at least one node in the
                 * queue). This is ensured by checking if the head next
-                * is NULL and retry in that case (this means a
-                * concurrent dummy node re-enqueue is in progress).
+                * is NULL. This means a concurrent dummy node
+                * re-enqueue is in progress. We help it reach
+                * completion, and retry.
                 */
-               if (next) {
-                       if (uatomic_cmpxchg(&q->head, head, next) == head) {
-                               if (is_dummy(q, head)) {
-                                       struct cds_lfq_node_rcu *node;
-                                       /*
-                                        * Requeue dummy. We need to
-                                        * reallocate to protect from
-                                        * ABA.
-                                        */
-                                       rcu_free_dummy(head);
-                                       node = make_dummy(q, NULL);
-                                       /*
-                                        * We are the only thread
-                                        * allowed to update dummy (we
-                                        * own the old dummy). Other
-                                        * dequeue threads read it
-                                        * concurrently with RCU
-                                        * read-lock held, which
-                                        * protects from ABA.
-                                        */
-                                       CMM_STORE_SHARED(q->dummy, node);
-                                       _cds_lfq_enqueue_rcu(q, node);
-                                       continue;       /* try again */
-                               }
-                               return head;
-                       } else {
-                               /* Concurrently pushed, retry */
-                               continue;
-                       }
-               } else {
-                       /* Dummy node re-enqueue is in progress, retry. */
-                       continue;
+               if (!next) {
+                       /*
+                        * Dummy node re-enqueue is in progress. Try to
+                        * help.
+                        */
+                       reenqueue_dummy(q);
+                       continue;       /* try again */
+               }
+               if (uatomic_cmpxchg(&q->head, head, next) != head)
+                       continue;       /* Concurrently pushed. */
+               if (head == dummy) {
+                       /* Free old and requeue new dummy. */
+                       rcu_set_pointer(&q->dummy, NULL);
+                       rcu_free_dummy(dummy);
+                       reenqueue_dummy(q);
+                       continue;       /* try again */
                }
+               return head;
        }
 }