From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Tue, 21 Oct 2014 12:32:43 +0000 (-0400)
Subject: workqueue: preserve FIFO work order
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=e10c65b3279d1868a7ef83588bb91dd46cfe428a;p=userspace-rcu.git

workqueue: preserve FIFO work order

Never merge work from a sibling into a non-empty worker queue, because
doing so does not keep FIFO ordering of the work.

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

diff --git a/urcu/workqueue-fifo.h b/urcu/workqueue-fifo.h
index 1d7528d..bc19967 100644
--- a/urcu/workqueue-fifo.h
+++ b/urcu/workqueue-fifo.h
@@ -29,6 +29,7 @@
 #include <urcu/wfcqueue.h>
 #include <urcu/rculist.h>
 #include <pthread.h>
+#include <assert.h>
 
 /*
  * We use RCU to steal work from siblings. Therefore, one of RCU flavors
@@ -170,31 +171,37 @@ void urcu_worker_unregister(struct urcu_workqueue *queue,
  * Try stealing work from siblings when we have nothing to do.
  */
 static inline
-void ___urcu_steal_work(struct urcu_worker *worker,
+bool ___urcu_steal_work(struct urcu_worker *worker,
 		struct urcu_worker *sibling)
 {
+	enum cds_wfcq_ret splice_ret;
+
 	/*
 	 * Don't bother grabbing the sibling queue lock if it is empty.
 	 */
 	if (cds_wfcq_empty(&sibling->head, &sibling->tail))
-		return;
+		return false;
 	cds_wfcq_dequeue_lock(&sibling->head, &sibling->tail);
-	(void) __cds_wfcq_splice_blocking(&worker->head,
+	splice_ret = __cds_wfcq_splice_blocking(&worker->head,
 			&worker->tail,
 			&sibling->head,
 			&sibling->tail);
 	cds_wfcq_dequeue_unlock(&sibling->head, &sibling->tail);
+	/* Ensure that we preserve FIFO work order. */
+	assert(splice_ret != CDS_WFCQ_RET_DEST_NON_EMPTY);
+	return splice_ret != CDS_WFCQ_RET_SRC_EMPTY;
 }
 
 static inline
-int __urcu_steal_work(struct urcu_workqueue *queue,
+bool __urcu_steal_work(struct urcu_workqueue *queue,
 		struct urcu_worker *worker)
 {
 	struct urcu_worker *sibling_prev, *sibling_next;
 	struct cds_list_head *sibling_node;
+	bool steal_performed = 0;
 
 	if (!(worker->flags & URCU_WORKER_STEAL))
-		return 0;
+		return false;
 
 	rcu_read_lock();
 
@@ -204,7 +211,9 @@ int __urcu_steal_work(struct urcu_workqueue *queue,
 	sibling_next = caa_container_of(sibling_node, struct urcu_worker,
 			sibling_node);
 	if (sibling_next != worker)
-		___urcu_steal_work(worker, sibling_next);
+		steal_performed = ___urcu_steal_work(worker, sibling_next);
+	if (steal_performed)
+		goto end;
 
 	sibling_node = rcu_dereference(worker->sibling_node.prev);
 	if (sibling_node == &queue->sibling_head)
@@ -212,11 +221,11 @@ int __urcu_steal_work(struct urcu_workqueue *queue,
 	sibling_prev = caa_container_of(sibling_node, struct urcu_worker,
 			sibling_node);
 	if (sibling_prev != worker && sibling_prev != sibling_next)
-		___urcu_steal_work(worker, sibling_prev);
-
+		steal_performed =  ___urcu_steal_work(worker, sibling_prev);
+end:
 	rcu_read_unlock();
 
-	return !cds_wfcq_empty(&worker->head, &worker->tail);
+	return steal_performed;
 }
 
 static inline