* half-wait-free/half-blocking queue implementation done by Paul E.
* McKenney.
*
- * Mutual exclusion of __cds_wfcq_* API
- *
- * Unless otherwise stated, the caller must ensure mutual exclusion of
- * queue update operations "dequeue" and "splice" (for source queue).
- * Queue read operations "first" and "next", which are used by
- * "for_each" iterations, need to be protected against concurrent
- * "dequeue" and "splice" (for source queue) by the caller.
- * "enqueue", "splice" (for destination queue), and "empty" are the only
- * operations that can be used without any mutual exclusion.
+ * Mutual exclusion of cds_wfcq_* / __cds_wfcq_* API
+ *
+ * Synchronization table:
+ *
+ * External synchronization techniques described in the API below is
+ * required between pairs marked with "X". No external synchronization
+ * required between pairs marked with "-".
+ *
+ * Legend:
+ * [1] cds_wfcq_enqueue
+ * [2] __cds_wfcq_splice (destination queue)
+ * [3] __cds_wfcq_dequeue
+ * [4] __cds_wfcq_splice (source queue)
+ * [5] __cds_wfcq_first
+ * [6] __cds_wfcq_next
+ *
+ * [1] [2] [3] [4] [5] [6]
+ * [1] - - - - - -
+ * [2] - - - - - -
+ * [3] - - X X X X
+ * [4] - - X - X X
+ * [5] - - X X - -
+ * [6] - - X X - -
+ *
* Mutual exclusion can be ensured by holding cds_wfcq_dequeue_lock().
*
* For convenience, cds_wfcq_dequeue_blocking() and
assert(!ret);
}
+/*
+ * __cds_wfcq_init: initialize wait-free queue.
+ */
+static inline void ___cds_wfcq_init(struct __cds_wfcq_head *head,
+ struct cds_wfcq_tail *tail)
+{
+ /* Set queue head and tail */
+ _cds_wfcq_node_init(&head->node);
+ tail->p = &head->node;
+}
+
/*
* cds_wfcq_empty: return whether wait-free queue is empty.
*
* No memory barrier is issued. No mutual exclusion is required.
+ *
+ * We perform the test on head->node.next to check if the queue is
+ * possibly empty, but we confirm this by checking if the tail pointer
+ * points to the head node because the tail pointer is the linearisation
+ * point of the enqueuers. Just checking the head next pointer could
+ * make a queue appear empty if an enqueuer is preempted for a long time
+ * between xchg() and setting the previous node's next pointer.
*/
-static inline bool _cds_wfcq_empty(struct cds_wfcq_head *head,
+static inline bool _cds_wfcq_empty(cds_wfcq_head_ptr_t u_head,
struct cds_wfcq_tail *tail)
{
+ struct __cds_wfcq_head *head = u_head._h;
/*
* Queue is empty if no node is pointed by head->node.next nor
* tail->p. Even though the tail->p check is sufficient to find
assert(!ret);
}
-static inline void ___cds_wfcq_append(struct cds_wfcq_head *head,
+static inline bool ___cds_wfcq_append(cds_wfcq_head_ptr_t u_head,
struct cds_wfcq_tail *tail,
struct cds_wfcq_node *new_head,
struct cds_wfcq_node *new_tail)
{
+ struct __cds_wfcq_head *head = u_head._h;
struct cds_wfcq_node *old_tail;
/*
* perspective.
*/
CMM_STORE_SHARED(old_tail->next, new_head);
+ /*
+ * Return false if queue was empty prior to adding the node,
+ * else return true.
+ */
+ return old_tail != &head->node;
}
/*
*
* Issues a full memory barrier before enqueue. No mutual exclusion is
* required.
+ *
+ * Returns false if the queue was empty prior to adding the node.
+ * Returns true otherwise.
*/
-static inline void _cds_wfcq_enqueue(struct cds_wfcq_head *head,
+static inline bool _cds_wfcq_enqueue(cds_wfcq_head_ptr_t head,
struct cds_wfcq_tail *tail,
struct cds_wfcq_node *new_tail)
{
- ___cds_wfcq_append(head, tail, new_tail, new_tail);
+ return ___cds_wfcq_append(head, tail, new_tail, new_tail);
+}
+
+/*
+ * ___cds_wfcq_busy_wait: adaptative busy-wait.
+ *
+ * Returns 1 if nonblocking and needs to block, 0 otherwise.
+ */
+static inline bool
+___cds_wfcq_busy_wait(int *attempt, int blocking)
+{
+ if (!blocking)
+ return 1;
+ if (++(*attempt) >= WFCQ_ADAPT_ATTEMPTS) {
+ (void) poll(NULL, 0, WFCQ_WAIT); /* Wait for 10ms */
+ *attempt = 0;
+ } else {
+ caa_cpu_relax();
+ }
+ return 0;
}
/*
* Adaptative busy-looping waiting for enqueuer to complete enqueue.
*/
while ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
- if (!blocking)
+ if (___cds_wfcq_busy_wait(&attempt, blocking))
return CDS_WFCQ_WOULDBLOCK;
- if (++attempt >= WFCQ_ADAPT_ATTEMPTS) {
- poll(NULL, 0, WFCQ_WAIT); /* Wait for 10ms */
- attempt = 0;
- } else {
- caa_cpu_relax();
- }
}
return next;
}
static inline struct cds_wfcq_node *
-___cds_wfcq_first(struct cds_wfcq_head *head,
+___cds_wfcq_first(cds_wfcq_head_ptr_t u_head,
struct cds_wfcq_tail *tail,
int blocking)
{
+ struct __cds_wfcq_head *head = u_head._h;
struct cds_wfcq_node *node;
if (_cds_wfcq_empty(head, tail))
* Used by for-like iteration macros in urcu/wfqueue.h:
* __cds_wfcq_for_each_blocking()
* __cds_wfcq_for_each_blocking_safe()
+ *
+ * Returns NULL if queue is empty, first node otherwise.
*/
static inline struct cds_wfcq_node *
-___cds_wfcq_first_blocking(struct cds_wfcq_head *head,
+___cds_wfcq_first_blocking(cds_wfcq_head_ptr_t head,
struct cds_wfcq_tail *tail)
{
return ___cds_wfcq_first(head, tail, 1);
* it needs to block.
*/
static inline struct cds_wfcq_node *
-___cds_wfcq_first_nonblocking(struct cds_wfcq_head *head,
+___cds_wfcq_first_nonblocking(cds_wfcq_head_ptr_t head,
struct cds_wfcq_tail *tail)
{
return ___cds_wfcq_first(head, tail, 0);
}
static inline struct cds_wfcq_node *
-___cds_wfcq_next(struct cds_wfcq_head *head,
+___cds_wfcq_next(cds_wfcq_head_ptr_t head,
struct cds_wfcq_tail *tail,
struct cds_wfcq_node *node,
int blocking)
* Used by for-like iteration macros in urcu/wfqueue.h:
* __cds_wfcq_for_each_blocking()
* __cds_wfcq_for_each_blocking_safe()
+ *
+ * Returns NULL if reached end of queue, non-NULL next queue node
+ * otherwise.
*/
static inline struct cds_wfcq_node *
-___cds_wfcq_next_blocking(struct cds_wfcq_head *head,
+___cds_wfcq_next_blocking(cds_wfcq_head_ptr_t head,
struct cds_wfcq_tail *tail,
struct cds_wfcq_node *node)
{
* it needs to block.
*/
static inline struct cds_wfcq_node *
-___cds_wfcq_next_nonblocking(struct cds_wfcq_head *head,
+___cds_wfcq_next_nonblocking(cds_wfcq_head_ptr_t head,
struct cds_wfcq_tail *tail,
struct cds_wfcq_node *node)
{
}
static inline struct cds_wfcq_node *
-___cds_wfcq_dequeue(struct cds_wfcq_head *head,
+___cds_wfcq_dequeue_with_state(cds_wfcq_head_ptr_t u_head,
struct cds_wfcq_tail *tail,
+ int *state,
int blocking)
{
+ struct __cds_wfcq_head *head = u_head._h;
struct cds_wfcq_node *node, *next;
- if (_cds_wfcq_empty(head, tail))
+ if (state)
+ *state = 0;
+
+ if (_cds_wfcq_empty(head, tail)) {
return NULL;
+ }
node = ___cds_wfcq_node_sync_next(&head->node, blocking);
- if (!blocking && node == CDS_WFCQ_WOULDBLOCK)
+ if (!blocking && node == CDS_WFCQ_WOULDBLOCK) {
return CDS_WFCQ_WOULDBLOCK;
+ }
if ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
/*
* content.
*/
_cds_wfcq_node_init(&head->node);
- if (uatomic_cmpxchg(&tail->p, node, &head->node) == node)
+ if (uatomic_cmpxchg(&tail->p, node, &head->node) == node) {
+ if (state)
+ *state |= CDS_WFCQ_STATE_LAST;
return node;
+ }
next = ___cds_wfcq_node_sync_next(node, blocking);
/*
* In nonblocking mode, if we would need to block to
}
/*
- * __cds_wfcq_dequeue_blocking: dequeue a node from the queue.
+ * __cds_wfcq_dequeue_with_state_blocking: dequeue node from queue, with state.
*
* Content written into the node before enqueue is guaranteed to be
* consistent, but no other memory ordering is ensured.
* caller.
*/
static inline struct cds_wfcq_node *
-___cds_wfcq_dequeue_blocking(struct cds_wfcq_head *head,
+___cds_wfcq_dequeue_with_state_blocking(cds_wfcq_head_ptr_t head,
+ struct cds_wfcq_tail *tail, int *state)
+{
+ return ___cds_wfcq_dequeue_with_state(head, tail, state, 1);
+}
+
+/*
+ * ___cds_wfcq_dequeue_blocking: dequeue node from queue.
+ *
+ * Same as __cds_wfcq_dequeue_with_state_blocking, but without saving
+ * state.
+ */
+static inline struct cds_wfcq_node *
+___cds_wfcq_dequeue_blocking(cds_wfcq_head_ptr_t head,
struct cds_wfcq_tail *tail)
{
- return ___cds_wfcq_dequeue(head, tail, 1);
+ return ___cds_wfcq_dequeue_with_state_blocking(head, tail, NULL);
}
/*
- * __cds_wfcq_dequeue_nonblocking: dequeue a node from a wait-free queue.
+ * __cds_wfcq_dequeue_with_state_nonblocking: dequeue node, with state.
*
* Same as __cds_wfcq_dequeue_blocking, but returns CDS_WFCQ_WOULDBLOCK
* if it needs to block.
*/
static inline struct cds_wfcq_node *
-___cds_wfcq_dequeue_nonblocking(struct cds_wfcq_head *head,
+___cds_wfcq_dequeue_with_state_nonblocking(cds_wfcq_head_ptr_t head,
+ struct cds_wfcq_tail *tail, int *state)
+{
+ return ___cds_wfcq_dequeue_with_state(head, tail, state, 0);
+}
+
+/*
+ * ___cds_wfcq_dequeue_nonblocking: dequeue node from queue.
+ *
+ * Same as __cds_wfcq_dequeue_with_state_nonblocking, but without saving
+ * state.
+ */
+static inline struct cds_wfcq_node *
+___cds_wfcq_dequeue_nonblocking(cds_wfcq_head_ptr_t head,
struct cds_wfcq_tail *tail)
{
- return ___cds_wfcq_dequeue(head, tail, 0);
+ return ___cds_wfcq_dequeue_with_state_nonblocking(head, tail, NULL);
}
-static inline int
+/*
+ * __cds_wfcq_splice: enqueue all src_q nodes at the end of dest_q.
+ *
+ * Dequeue all nodes from src_q.
+ * dest_q must be already initialized.
+ * Mutual exclusion for src_q should be ensured by the caller as
+ * specified in the "Synchronisation table".
+ * Returns enum cds_wfcq_ret which indicates the state of the src or
+ * dest queue.
+ */
+static inline enum cds_wfcq_ret
___cds_wfcq_splice(
- struct cds_wfcq_head *dest_q_head,
+ cds_wfcq_head_ptr_t u_dest_q_head,
struct cds_wfcq_tail *dest_q_tail,
- struct cds_wfcq_head *src_q_head,
+ cds_wfcq_head_ptr_t u_src_q_head,
struct cds_wfcq_tail *src_q_tail,
int blocking)
{
+ struct __cds_wfcq_head *dest_q_head = u_dest_q_head._h;
+ struct __cds_wfcq_head *src_q_head = u_src_q_head._h;
struct cds_wfcq_node *head, *tail;
+ int attempt = 0;
+ /*
+ * Initial emptiness check to speed up cases where queue is
+ * empty: only require loads to check if queue is empty.
+ */
if (_cds_wfcq_empty(src_q_head, src_q_tail))
- return 0;
+ return CDS_WFCQ_RET_SRC_EMPTY;
- head = ___cds_wfcq_node_sync_next(&src_q_head->node, blocking);
- if (head == CDS_WFCQ_WOULDBLOCK)
- return -1;
- _cds_wfcq_node_init(&src_q_head->node);
+ for (;;) {
+ /*
+ * Open-coded _cds_wfcq_empty() by testing result of
+ * uatomic_xchg, as well as tail pointer vs head node
+ * address.
+ */
+ head = uatomic_xchg(&src_q_head->node.next, NULL);
+ if (head)
+ break; /* non-empty */
+ if (CMM_LOAD_SHARED(src_q_tail->p) == &src_q_head->node)
+ return CDS_WFCQ_RET_SRC_EMPTY;
+ if (___cds_wfcq_busy_wait(&attempt, blocking))
+ return CDS_WFCQ_RET_WOULDBLOCK;
+ }
/*
* Memory barrier implied before uatomic_xchg() orders store to
* Append the spliced content of src_q into dest_q. Does not
* require mutual exclusion on dest_q (wait-free).
*/
- ___cds_wfcq_append(dest_q_head, dest_q_tail, head, tail);
- return 0;
+ if (___cds_wfcq_append(dest_q_head, dest_q_tail, head, tail))
+ return CDS_WFCQ_RET_DEST_NON_EMPTY;
+ else
+ return CDS_WFCQ_RET_DEST_EMPTY;
}
-
/*
* __cds_wfcq_splice_blocking: enqueue all src_q nodes at the end of dest_q.
*
* Dequeue all nodes from src_q.
* dest_q must be already initialized.
- * Dequeue/splice/iteration mutual exclusion for src_q should be ensured
- * by the caller.
+ * Mutual exclusion for src_q should be ensured by the caller as
+ * specified in the "Synchronisation table".
+ * Returns enum cds_wfcq_ret which indicates the state of the src or
+ * dest queue. Never returns CDS_WFCQ_RET_WOULDBLOCK.
*/
-static inline void
+static inline enum cds_wfcq_ret
___cds_wfcq_splice_blocking(
- struct cds_wfcq_head *dest_q_head,
+ cds_wfcq_head_ptr_t dest_q_head,
struct cds_wfcq_tail *dest_q_tail,
- struct cds_wfcq_head *src_q_head,
+ cds_wfcq_head_ptr_t src_q_head,
struct cds_wfcq_tail *src_q_tail)
{
- (void) ___cds_wfcq_splice(dest_q_head, dest_q_tail,
+ return ___cds_wfcq_splice(dest_q_head, dest_q_tail,
src_q_head, src_q_tail, 1);
}
/*
* __cds_wfcq_splice_nonblocking: enqueue all src_q nodes at the end of dest_q.
*
- * Same as __cds_wfcq_splice_blocking, but returns nonzero if it needs to
- * block.
+ * Same as __cds_wfcq_splice_blocking, but returns
+ * CDS_WFCQ_RET_WOULDBLOCK if it needs to block.
*/
-static inline int
+static inline enum cds_wfcq_ret
___cds_wfcq_splice_nonblocking(
- struct cds_wfcq_head *dest_q_head,
+ cds_wfcq_head_ptr_t dest_q_head,
struct cds_wfcq_tail *dest_q_tail,
- struct cds_wfcq_head *src_q_head,
+ cds_wfcq_head_ptr_t src_q_head,
struct cds_wfcq_tail *src_q_tail)
{
return ___cds_wfcq_splice(dest_q_head, dest_q_tail,
}
/*
- * cds_wfcq_dequeue_blocking: dequeue a node from a wait-free queue.
+ * cds_wfcq_dequeue_with_state_blocking: dequeue a node from a wait-free queue.
*
* Content written into the node before enqueue is guaranteed to be
* consistent, but no other memory ordering is ensured.
- * Mutual exlusion with cds_wfcq_splice_blocking and dequeue lock is
+ * Mutual exclusion with cds_wfcq_splice_blocking and dequeue lock is
* ensured.
* It is valid to reuse and free a dequeued node immediately.
*/
static inline struct cds_wfcq_node *
-_cds_wfcq_dequeue_blocking(struct cds_wfcq_head *head,
- struct cds_wfcq_tail *tail)
+_cds_wfcq_dequeue_with_state_blocking(struct cds_wfcq_head *head,
+ struct cds_wfcq_tail *tail, int *state)
{
struct cds_wfcq_node *retval;
_cds_wfcq_dequeue_lock(head, tail);
- retval = ___cds_wfcq_dequeue_blocking(head, tail);
+ retval = ___cds_wfcq_dequeue_with_state_blocking(head, tail, state);
_cds_wfcq_dequeue_unlock(head, tail);
return retval;
}
+/*
+ * cds_wfcq_dequeue_blocking: dequeue node from queue.
+ *
+ * Same as cds_wfcq_dequeue_blocking, but without saving state.
+ */
+static inline struct cds_wfcq_node *
+_cds_wfcq_dequeue_blocking(struct cds_wfcq_head *head,
+ struct cds_wfcq_tail *tail)
+{
+ return _cds_wfcq_dequeue_with_state_blocking(head, tail, NULL);
+}
+
/*
* cds_wfcq_splice_blocking: enqueue all src_q nodes at the end of dest_q.
*
* dest_q must be already initialized.
* Content written into the node before enqueue is guaranteed to be
* consistent, but no other memory ordering is ensured.
- * Mutual exlusion with cds_wfcq_dequeue_blocking and dequeue lock is
+ * Mutual exclusion with cds_wfcq_dequeue_blocking and dequeue lock is
* ensured.
+ * Returns enum cds_wfcq_ret which indicates the state of the src or
+ * dest queue. Never returns CDS_WFCQ_RET_WOULDBLOCK.
*/
-static inline void
+static inline enum cds_wfcq_ret
_cds_wfcq_splice_blocking(
struct cds_wfcq_head *dest_q_head,
struct cds_wfcq_tail *dest_q_tail,
struct cds_wfcq_head *src_q_head,
struct cds_wfcq_tail *src_q_tail)
{
+ enum cds_wfcq_ret ret;
+
_cds_wfcq_dequeue_lock(src_q_head, src_q_tail);
- ___cds_wfcq_splice_blocking(dest_q_head, dest_q_tail,
+ ret = ___cds_wfcq_splice_blocking(dest_q_head, dest_q_tail,
src_q_head, src_q_tail);
_cds_wfcq_dequeue_unlock(src_q_head, src_q_tail);
+ return ret;
}
#ifdef __cplusplus
projects / urcu.git / blobdiff