#include <urcu/uatomic.h>
#include <assert.h>
#include <errno.h>
-/* A urcu implementation header should be already included. */
#ifdef __cplusplus
extern "C" {
* Lock-free RCU queue. Enqueue and dequeue operations hold a RCU read
* lock to deal with cmpxchg ABA problem. This queue is *not* circular:
* head points to the oldest node, tail points to the newest node.
- * Dummy nodes are kept to ensure enqueue and dequeue can always proceed
+ * A dummy node is kept to ensure enqueue and dequeue can always proceed
* concurrently. Keeping a separate head and tail helps with large
* queues: enqueue and dequeue can proceed concurrently without
* wrestling for exclusive access to the same variables.
*
- * We keep two dummy nodes in the queue to distinguish between empty queue
- * state and intermediate state while a dummy node dequeue/requeue is
- * being performed. Dequeue retry if it detects that it would be
- * dequeueing the last node (it means a dummy node dequeue-requeue is in
- * progress). This ensures that there is always at least one node in
- * the queue. In a situation where the two dummy nodes are being
- * requeued (they therefore don't appear in the queue at a given
- * moment), we are certain that there is at least one non-dummy node in
- * the queue (ensured by the test for NULL next node upon dequeue).
+ * Dequeue retry if it detects that it would be dequeueing the last node
+ * (it means a dummy node dequeue-requeue is in progress). This ensures
+ * that there is always at least one node in the queue.
*
- * In the dequeue operation, we internally reallocate the dummy nodes
- * upon dequeue/requeue and use call_rcu to free them after a grace
- * period.
+ * In the dequeue operation, we internally reallocate the dummy node
+ * upon dequeue/requeue and use call_rcu to free the old one after a
+ * grace period.
*/
static inline
-int is_dummy(struct cds_lfq_node_rcu *node)
+int is_dummy(struct cds_lfq_queue_rcu *q, struct cds_lfq_node_rcu *node)
{
- return ((unsigned long) node) & 0x1UL;
+ return node == q->dummy;
}
static inline
struct cds_lfq_node_rcu_dummy *dummy;
dummy = malloc(sizeof(struct cds_lfq_node_rcu_dummy));
+ assert(dummy);
dummy->parent.next = next;
dummy->q = q;
- return (struct cds_lfq_node_rcu *) (((unsigned long) &dummy->parent) | 0x1UL);
-}
-
-static inline
-struct cds_lfq_node_rcu *get_node(struct cds_lfq_node_rcu *node)
-{
- return (struct cds_lfq_node_rcu *) (((unsigned long )node) & ~0x1UL);
+ return &dummy->parent;
}
static inline
{
struct cds_lfq_node_rcu_dummy *dummy;
- dummy = caa_container_of(get_node(node), struct cds_lfq_node_rcu_dummy,
- parent);
+ dummy = caa_container_of(node, struct cds_lfq_node_rcu_dummy, parent);
dummy->q->queue_call_rcu(&dummy->head, free_dummy);
}
void (*func)(struct rcu_head *head)))
{
q->tail = make_dummy(q, NULL);
- q->head = make_dummy(q, q->tail);
+ q->dummy = q->tail;
+ q->head = q->tail;
q->queue_call_rcu = queue_call_rcu;
}
static inline
int _cds_lfq_destroy_rcu(struct cds_lfq_queue_rcu *q)
{
- struct cds_lfq_node_rcu *head, *next;
+ struct cds_lfq_node_rcu *head;
head = rcu_dereference(q->head);
- next = rcu_dereference(get_node(head)->next);
- if (!(is_dummy(head) && is_dummy(next) && get_node(next)->next == NULL))
+ if (!(is_dummy(q, head) && head->next == NULL))
return -EPERM; /* not empty */
rcu_free_dummy(head);
- rcu_free_dummy(next);
return 0;
}
struct cds_lfq_node_rcu *tail, *next;
tail = rcu_dereference(q->tail);
- next = uatomic_cmpxchg(&get_node(tail)->next, NULL, node);
+ next = uatomic_cmpxchg(&tail->next, NULL, node);
if (next == NULL) {
/*
* Tail was at the end of queue, we successfully
struct cds_lfq_node_rcu *head, *next;
head = rcu_dereference(q->head);
- next = rcu_dereference(get_node(head)->next);
- if (is_dummy(head) && is_dummy(next) && get_node(next)->next == NULL)
+ next = rcu_dereference(head->next);
+ if (is_dummy(q, head) && next == NULL)
return NULL; /* empty */
/*
* We never, ever allow dequeue to get to a state where
*/
if (next) {
if (uatomic_cmpxchg(&q->head, head, next) == head) {
- if (is_dummy(head)) {
+ if (is_dummy(q, head)) {
struct cds_lfq_node_rcu *node;
/*
* Requeue dummy. We need to
*/
rcu_free_dummy(head);
node = make_dummy(q, NULL);
+ /*
+ * We are the only thread
+ * allowed to update dummy (we
+ * own the old dummy).
+ */
+ q->dummy = node;
_cds_lfq_enqueue_rcu(q, node);
continue; /* try again */
}