Implement enqueue-to-head. It can run concurrently with enqueue and
splice to queue, but requires a mutex against all other queue
operations.
Useful for special-cases where a queue needs to have nodes enqueued into
its head.
This patch is only compile-tested.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoy...@efficios.com>
---
include/linux/wfcqueue.h | 54 ++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 47 insertions(+), 7 deletions(-)
Index: linux/include/linux/wfcqueue.h
===================================================================
--- linux.orig/include/linux/wfcqueue.h
+++ linux/include/linux/wfcqueue.h
@@ -55,14 +55,16 @@
* [4] __wfcq_splice (source queue)
* [5] __wfcq_first
* [6] __wfcq_next
+ * [7] __wfcq_enqueue_head
*
- * [1] [2] [3] [4] [5] [6]
- * [1] - - - - - -
- * [2] - - - - - -
- * [3] - - X X X X
- * [4] - - X - X X
- * [5] - - X X - -
- * [6] - - X X - -
+ * [1] [2] [3] [4] [5] [6] [7]
+ * [1] - - - - - - -
+ * [2] - - - - - - X
+ * [3] - - X X X X X
+ * [4] - - X - X X X
+ * [5] - - X X - - X
+ * [6] - - X X - - X
+ * [7] - X X X X X X
*
* Besides locking, mutual exclusion of dequeue, splice and iteration
* can be ensured by performing all of those operations from a single
@@ -230,6 +232,44 @@ ___wfcq_node_sync_next(struct wfcq_node
}
/*
+ * __wfcq_enqueue_head: prepend a node into a queue.
+ *
+ * No memory barriers are issued. Mutual exclusion is the responsibility
+ * of the caller.
+ *
+ * Returns false if the queue was empty prior to adding the node.
+ * Returns true otherwise.
+ */
+static inline bool __wfcq_enqueue_head(struct wfcq_head *head,
+ struct wfcq_tail *tail,
+ struct wfcq_node *node)
+{
+ bool not_empty = 0;
+
+ /*
+ * Move tail if queue was empty. Tail pointer is the
+ * linearization point of enqueuers.
+ */
+ if (cmpxchg(&tail->p, &head->node, node) != &head->node) {
+ not_empty = 1;
+ /*
+ * Queue was non-empty. We need to wait for
+ * head->node.next to become non-NULL, because a
+ * concurrent wfcq_append may be updating it.
+ */
+ node->next = ___wfcq_node_sync_next(&head->node);
+ }
+ /*
+ * From this point, we know that wfcq_append cannot touch
+ * head->node.next, either because we successfully moved tail->p
+ * to node, or because we waited for head->node.next to become
+ * non-NULL. It is therefore safe to update it.
+ */
+ head->node.next = node;
+ return not_empty;
+}
+
+/*
* __wfcq_first: get first node of a queue, without dequeuing.
*
* Content written into the node before enqueue is guaranteed to be
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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