From: Brian Norris <banorris@uci.edu>
Date: Fri, 8 Mar 2013 22:47:27 +0000 (-0800)
Subject: ms-queue: revert Brian D. changes
X-Git-Tag: oopsla2013-final~18
X-Git-Url: http://demsky.eecs.uci.edu/git/?p=model-checker-benchmarks.git;a=commitdiff_plain;h=7fd7e04652df0491d9ebf56da6a179e9b9ee2f05

ms-queue: revert Brian D. changes

On second (or third or fourth) thought, these changes were ill-advised.
We will make a different set of tweaks shortly.

This reverts commits 647601d3f341bb2a2a79534b3d044c039695839d and
fad3891f3158a4285984bb60415c1e4ebc2b767b.
---

diff --git a/ms-queue/my_queue.c b/ms-queue/my_queue.c
index dfdf819..d658407 100644
--- a/ms-queue/my_queue.c
+++ b/ms-queue/my_queue.c
@@ -61,7 +61,6 @@ void enqueue(queue_t *q, unsigned int val)
 	pointer next;
 	pointer tmp;
 
-	/* Get a fresh node, set its value field, then set it's next reference to null */
 	node = new_node();
 	store_32(&q->nodes[node].value, val);
 	tmp = atomic_load_explicit(&q->nodes[node].next, relaxed);
@@ -69,30 +68,29 @@ void enqueue(queue_t *q, unsigned int val)
 	atomic_store_explicit(&q->nodes[node].next, tmp, relaxed);
 
 	while (!success) {
-		//tail isn't actually synchronization point
-		tail = atomic_load_explicit(&q->tail, relaxed);
-		//acquire reference...
-		next = atomic_load_explicit(&q->nodes[get_ptr(tail)].next, memory_order_seq_cst);
-		if (tail == atomic_load_explicit(&q->tail, memory_order_seq_cst)) {
+		tail = atomic_load_explicit(&q->tail, acquire);
+		next = atomic_load_explicit(&q->nodes[get_ptr(tail)].next, acquire);
+		if (tail == atomic_load_explicit(&q->tail, relaxed)) {
 			if (get_ptr(next) == 0) { // == NULL
 				pointer value = MAKE_POINTER(node, get_count(next) + 1);
 				success = atomic_compare_exchange_strong_explicit(&q->nodes[get_ptr(tail)].next,
-						&next, value, memory_order_seq_cst, memory_order_seq_cst);
+						&next, value, memory_order_acq_rel, memory_order_acq_rel);
 			}
 			if (!success) {
-				unsigned int ptr = get_ptr(atomic_load_explicit(&q->nodes[get_ptr(tail)].next, memory_order_relaxed));
+				unsigned int ptr = get_ptr(atomic_load_explicit(&q->nodes[get_ptr(tail)].next, memory_order_seq_cst));
 				pointer value = MAKE_POINTER(ptr,
 						get_count(tail) + 1);
 				atomic_compare_exchange_strong_explicit(&q->tail,
 						&tail, value,
-						memory_order_seq_cst, memory_order_seq_cst);
+						memory_order_acq_rel, memory_order_acq_rel);
+				thrd_yield();
 			}
 		}
 	}
 	atomic_compare_exchange_strong_explicit(&q->tail,
 			&tail,
 			MAKE_POINTER(node, get_count(tail) + 1),
-			memory_order_seq_cst, memory_order_seq_cst);
+			memory_order_acq_rel, memory_order_acq_rel);
 }
 
 unsigned int dequeue(queue_t *q)
@@ -104,10 +102,10 @@ unsigned int dequeue(queue_t *q)
 	pointer next;
 
 	while (!success) {
-		head = atomic_load_explicit(&q->head, memory_order_seq_cst);
-		tail = atomic_load_explicit(&q->tail, memory_order_seq_cst);
-		next = atomic_load_explicit(&q->nodes[get_ptr(head)].next, memory_order_seq_cst);
-		if (atomic_load_explicit(&q->head, memory_order_seq_cst) == head) {
+		head = atomic_load_explicit(&q->head, acquire);
+		tail = atomic_load_explicit(&q->tail, acquire);
+		next = atomic_load_explicit(&q->nodes[get_ptr(head)].next, acquire);
+		if (atomic_load_explicit(&q->head, relaxed) == head) {
 			if (get_ptr(head) == get_ptr(tail)) {
 				if (get_ptr(next) == 0) { // NULL
 					return 0; // NULL
@@ -115,13 +113,16 @@ unsigned int dequeue(queue_t *q)
 				atomic_compare_exchange_strong_explicit(&q->tail,
 						&tail,
 						MAKE_POINTER(get_ptr(next), get_count(tail) + 1),
-						memory_order_seq_cst, memory_order_seq_cst);
+						memory_order_acq_rel, memory_order_acq_rel);
+				thrd_yield();
 			} else {
 				value = load_32(&q->nodes[get_ptr(next)].value);
 				success = atomic_compare_exchange_strong_explicit(&q->head,
 						&head,
 						MAKE_POINTER(get_ptr(next), get_count(head) + 1),
-						memory_order_seq_cst, memory_order_seq_cst);
+						memory_order_acq_rel, memory_order_acq_rel);
+				if (!success)
+					thrd_yield();
 			}
 		}
 	}