ck_fifo_spsc: Fix implementation.

Do not be overly pedantic. Fix misplaced store
barrier and make sure to atomically load stub
pointer on reader side.

include/ck_fifo.h

@@ -105,11 +105,9 @@ ck_fifo_spsc_init(struct ck_fifo_spsc *fifo, struct ck_fifo_spsc_entry *stub)
 
 	ck_spinlock_init(&fifo->m_head);
 	ck_spinlock_init(&fifo->m_tail);
-	ck_pr_store_ptr(&stub->next, NULL);
-	ck_pr_store_ptr(&fifo->head, stub);
-	ck_pr_store_ptr(&fifo->tail, stub);
-	ck_pr_store_ptr(&fifo->head_snapshot, stub);
-	ck_pr_store_ptr(&fifo->garbage, stub);
+
+	stub->next = NULL;
+	fifo->head = fifo->tail = fifo->head_snapshot = fifo->garbage = stub;
 	return;
 }
 
@@ -119,11 +117,13 @@ ck_fifo_spsc_enqueue(struct ck_fifo_spsc *fifo,
 		     void *value)
 {
 
-	ck_pr_store_ptr(&entry->value, value);
-	ck_pr_store_ptr(&entry->next, NULL);
-	ck_pr_store_ptr(&fifo->tail->next, entry);
+	entry->value = value;
+	entry->next = NULL;
+
+	/* If stub->next is visible, guarantee that entry is consistent. */
 	ck_pr_fence_store();
-	ck_pr_store_ptr(&fifo->tail, entry);
+	ck_pr_store_ptr(&fifo->tail->next, entry);
+	fifo->tail = entry;
 	return;
 }
 
@@ -138,15 +138,14 @@ ck_fifo_spsc_dequeue(struct ck_fifo_spsc *fifo, void *value)
 	 * If the stub entry does not point to an entry, then the queue is
 	 * empty.
 	 */
-	stub = ck_pr_load_ptr(&fifo->head);
-	ck_pr_fence_load();
-	entry = stub->next;
+	entry = ck_pr_load_ptr(&fifo->head->next);
 	if (entry == NULL)
 		return (false);
 
-	store = ck_pr_load_ptr(&entry->value);
-	ck_pr_store_ptr(value, store);
-	ck_pr_store_ptr(&fifo->head, entry);
+	/* If entry is visible, guarantee store to value is visible. */
+	ck_pr_fence_load();
+	ck_pr_store_ptr(value, entry->value);
+	fifo->head = entry;
 	return (true);
 }
 
@@ -157,21 +156,17 @@ ck_fifo_spsc_dequeue(struct ck_fifo_spsc *fifo, void *value)
 CK_CC_INLINE static struct ck_fifo_spsc_entry *
 ck_fifo_spsc_recycle(struct ck_fifo_spsc *fifo)
 {
-	struct ck_fifo_spsc_entry *p, *garbage;
-
-	garbage = ck_pr_load_ptr(&fifo->garbage);
-	p = ck_pr_load_ptr(&fifo->head_snapshot);
+	struct ck_fifo_spsc_entry *garbage;
 
-	if (garbage == p) {
-		p = ck_pr_load_ptr(&fifo->head);
-		ck_pr_store_ptr(&fifo->head_snapshot, p);
-		if (garbage == p)
-			return (NULL);
+	if (fifo->head_snapshot == fifo->garbage) {
+		fifo->head_snapshot = ck_pr_load_ptr(&fifo->head);
+		if (fifo->head_snapshot == fifo->garbage)
+			return NULL;
 	}
 
-	p = garbage;
-	ck_pr_store_ptr(&fifo->garbage, garbage->next);
-	return (p);
+	garbage = fifo->garbage;
+	fifo->garbage = garbage->next;
+	return (garbage);
 }
 
 CK_CC_INLINE static bool