ck_ring: Various improvements to readability.

No regressions in unit tests, so I'm dropping volatile
semantics for unnecessary volatile load/store on producer/consumer
side.

include/ck_ring.h

@@ -53,6 +53,7 @@
 			    struct type *buffer,				\
 			    unsigned int size)					\
 	{									\
+										\
 		ck_pr_store_uint(&ring->size, size);				\
 		ck_pr_store_uint(&ring->mask, size - 1);			\
 		ck_pr_store_uint(&ring->p_tail, 0);				\
@@ -82,13 +83,13 @@
 		unsigned int consumer, producer;				\
 										\
 		consumer = ck_pr_load_uint(&ring->c_head);			\
-		producer = ck_pr_load_uint(&ring->p_tail);			\
+		producer = ring->p_tail;					\
 										\
 		if (((producer + 1) & ring->mask) == consumer) 			\
 			return (false);						\
 										\
 		ring->ring[producer] = *entry;					\
-		ck_pr_fence_store();						\
+		ck_pr_fence_memory();						\
 		ck_pr_store_uint(&ring->p_tail,	(producer + 1) & ring->mask);	\
 										\
 		return (true);							\
@@ -99,7 +100,7 @@
 	{									\
 		unsigned int consumer, producer;				\
 										\
-		consumer = ck_pr_load_uint(&ring->c_head);			\
+		consumer = ring->c_head;					\
 		producer = ck_pr_load_uint(&ring->p_tail);			\
 										\
 		if (consumer == producer)					\
@@ -154,7 +155,7 @@ CK_CC_INLINE static unsigned int
 ck_ring_capacity(struct ck_ring *ring)
 {
 
-	return ck_pr_load_uint(&ring->size);
+	return ring->size;
 }
 
 CK_CC_INLINE static bool
@@ -163,15 +164,19 @@ ck_ring_enqueue_spsc(struct ck_ring *ring, void *entry)
 	unsigned int consumer, producer;
 
 	consumer = ck_pr_load_uint(&ring->c_head);
-	producer = ck_pr_load_uint(&ring->p_tail);
+	producer = ring->p_tail;
 
 	if (((producer + 1) & ring->mask) == consumer)
 		return (false);
 
-	ck_pr_store_ptr(&ring->ring[producer], entry);
+	ring->ring[producer] = entry;
+
+	/*
+	 * Make sure to update slot value before indicating
+	 * that the slot is available for consumption.
+	 */
 	ck_pr_fence_store();
 	ck_pr_store_uint(&ring->p_tail, (producer + 1) & ring->mask);
-
 	return (true);
 }
 
@@ -182,19 +187,33 @@ CK_CC_INLINE static bool
 ck_ring_dequeue_spsc(struct ck_ring *ring, void *data)
 {
 	unsigned int consumer, producer;
-	void *value;
 
-	consumer = ck_pr_load_uint(&ring->c_head);
+	consumer = ring->c_head;
 	producer = ck_pr_load_uint(&ring->p_tail);
 
 	if (consumer == producer)
 		return (false);
 
+	/*
+	 * Make sure to serialize with respect to our snapshot
+	 * of the producer counter.
+	 */
 	ck_pr_fence_load();
-	value = ck_pr_load_ptr(ring->ring + consumer);
-	ck_pr_store_ptr(data, value);
-	ck_pr_store_uint(&ring->c_head, (consumer + 1) & ring->mask);
 
+	/*
+	 * This is used to work-around aliasing issues (C
+	 * lacks a generic pointer to pointer despite it
+	 * being a reality on POSIX). This interface is
+	 * troublesome on platforms where sizeof(void *)
+	 * is not guaranteed to be sizeof(T *).
+	 *
+	 * It is guaranteed that the value of
+	 * ring->ring[consumer] is either in *data or aliased
+	 * to some location whose value is to be stored in data
+	 * before this operation completes.
+	 */
+	ck_pr_store_ptr(data, ring->ring[consumer]);
+	ck_pr_store_uint(&ring->c_head, (consumer + 1) & ring->mask);
 	return (true);
 }