[PATCH v6 1/6] ring: add a pointer-width headtail structure

[Gage Eads <gage.eads@intel.com>]

For 64-bit systems, at current CPU speeds, 64-bit head and tail indexes
will not wrap-around within the author's lifetime. This is important to
avoiding the ABA problem -- in which a thread mistakes reading the same
tail index in two accesses to mean that the ring was not modified in the
intervening time -- in the upcoming lock-free ring implementation. Using a
64-bit index makes the possibility of this occurring effectively zero. This
commit uses pointer-width indexes so the lock-free ring can support 32-bit
systems as well.

This commit places the new producer and consumer structures in the same
location in struct rte_ring as their 32-bit counterparts. Since the 32-bit
versions are padded out to a cache line, there is space for the new
structure without affecting the layout of struct rte_ring. Thus, the ABI is
preserved.

Signed-off-by: Gage Eads <gage.eads@intel.com>
---
 lib/librte_ring/rte_ring.h         |  21 +++++-
 lib/librte_ring/rte_ring_c11_mem.h | 143 +++++++++++++++++++++++++++++++++++++
 lib/librte_ring/rte_ring_generic.h | 130 +++++++++++++++++++++++++++++++++
 3 files changed, 291 insertions(+), 3 deletions(-)

diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h
index af5444a9f..c78db6916 100644
--- a/lib/librte_ring/rte_ring.h
+++ b/lib/librte_ring/rte_ring.h
@@ -1,6 +1,6 @@
 /* SPDX-License-Identifier: BSD-3-Clause
  *
- * Copyright (c) 2010-2017 Intel Corporation
+ * Copyright (c) 2010-2019 Intel Corporation
  * Copyright (c) 2007-2009 Kip Macy kmacy@freebsd.org
  * All rights reserved.
  * Derived from FreeBSD's bufring.h
@@ -70,6 +70,13 @@ struct rte_ring_headtail {
 	uint32_t single;         /**< True if single prod/cons */
 };
 
+/* Structure to hold a pair of pointer-sized head/tail values and metadata */
+struct rte_ring_headtail_ptr {
+	volatile uintptr_t head; /**< Prod/consumer head. */
+	volatile uintptr_t tail; /**< Prod/consumer tail. */
+	uint32_t single;         /**< True if single prod/cons */
+};
+
 /**
  * An RTE ring structure.
  *
@@ -97,11 +104,19 @@ struct rte_ring {
 	char pad0 __rte_cache_aligned; /**< empty cache line */
 
 	/** Ring producer status. */
-	struct rte_ring_headtail prod __rte_cache_aligned;
+	RTE_STD_C11
+	union {
+		struct rte_ring_headtail prod __rte_cache_aligned;
+		struct rte_ring_headtail_ptr prod_ptr __rte_cache_aligned;
+	};
 	char pad1 __rte_cache_aligned; /**< empty cache line */
 
 	/** Ring consumer status. */
-	struct rte_ring_headtail cons __rte_cache_aligned;
+	RTE_STD_C11
+	union {
+		struct rte_ring_headtail cons __rte_cache_aligned;
+		struct rte_ring_headtail_ptr cons_ptr __rte_cache_aligned;
+	};
 	char pad2 __rte_cache_aligned; /**< empty cache line */
 };
 
diff --git a/lib/librte_ring/rte_ring_c11_mem.h b/lib/librte_ring/rte_ring_c11_mem.h
index 0fb73a337..545caf257 100644
--- a/lib/librte_ring/rte_ring_c11_mem.h
+++ b/lib/librte_ring/rte_ring_c11_mem.h
@@ -178,4 +178,147 @@ __rte_ring_move_cons_head(struct rte_ring *r, int is_sc,
 	return n;
 }
 
+/**
+ * @internal This function updates the producer head for enqueue using
+ *	     pointer-sized head/tail values.
+ *
+ * @param r
+ *   A pointer to the ring structure
+ * @param is_sp
+ *   Indicates whether multi-producer path is needed or not
+ * @param n
+ *   The number of elements we will want to enqueue, i.e. how far should the
+ *   head be moved
+ * @param behavior
+ *   RTE_RING_QUEUE_FIXED:    Enqueue a fixed number of items from a ring
+ *   RTE_RING_QUEUE_VARIABLE: Enqueue as many items as possible from ring
+ * @param old_head
+ *   Returns head value as it was before the move, i.e. where enqueue starts
+ * @param new_head
+ *   Returns the current/new head value i.e. where enqueue finishes
+ * @param free_entries
+ *   Returns the amount of free space in the ring BEFORE head was moved
+ * @return
+ *   Actual number of objects enqueued.
+ *   If behavior == RTE_RING_QUEUE_FIXED, this will be 0 or n only.
+ */
+static __rte_always_inline unsigned int
+__rte_ring_move_prod_head_ptr(struct rte_ring *r, unsigned int is_sp,
+		unsigned int n, enum rte_ring_queue_behavior behavior,
+		uintptr_t *old_head, uintptr_t *new_head,
+		uint32_t *free_entries)
+{
+	const uint32_t capacity = r->capacity;
+	uintptr_t cons_tail;
+	unsigned int max = n;
+	int success;
+
+	*old_head = __atomic_load_n(&r->prod_ptr.head, __ATOMIC_RELAXED);
+	do {
+		/* Reset n to the initial burst count */
+		n = max;
+
+		/* Ensure the head is read before tail */
+		__atomic_thread_fence(__ATOMIC_ACQUIRE);
+
+		/* load-acquire synchronize with store-release of ht->tail
+		 * in update_tail.
+		 */
+		cons_tail = __atomic_load_n(&r->cons_ptr.tail,
+					__ATOMIC_ACQUIRE);
+
+		*free_entries = (capacity + cons_tail - *old_head);
+
+		/* check that we have enough room in ring */
+		if (unlikely(n > *free_entries))
+			n = (behavior == RTE_RING_QUEUE_FIXED) ?
+					0 : *free_entries;
+
+		if (n == 0)
+			return 0;
+
+		*new_head = *old_head + n;
+		if (is_sp)
+			r->prod_ptr.head = *new_head, success = 1;
+		else
+			/* on failure, *old_head is updated */
+			success = __atomic_compare_exchange_n(&r->prod_ptr.head,
+					old_head, *new_head,
+					0, __ATOMIC_RELAXED,
+					__ATOMIC_RELAXED);
+	} while (unlikely(success == 0));
+	return n;
+}
+
+/**
+ * @internal This function updates the consumer head for dequeue using
+ *	     pointer-sized head/tail values.
+ *
+ * @param r
+ *   A pointer to the ring structure
+ * @param is_sc
+ *   Indicates whether multi-consumer path is needed or not
+ * @param n
+ *   The number of elements we will want to enqueue, i.e. how far should the
+ *   head be moved
+ * @param behavior
+ *   RTE_RING_QUEUE_FIXED:    Dequeue a fixed number of items from a ring
+ *   RTE_RING_QUEUE_VARIABLE: Dequeue as many items as possible from ring
+ * @param old_head
+ *   Returns head value as it was before the move, i.e. where dequeue starts
+ * @param new_head
+ *   Returns the current/new head value i.e. where dequeue finishes
+ * @param entries
+ *   Returns the number of entries in the ring BEFORE head was moved
+ * @return
+ *   - Actual number of objects dequeued.
+ *     If behavior == RTE_RING_QUEUE_FIXED, this will be 0 or n only.
+ */
+static __rte_always_inline unsigned int
+__rte_ring_move_cons_head_ptr(struct rte_ring *r, unsigned int is_sc,
+		unsigned int n, enum rte_ring_queue_behavior behavior,
+		uintptr_t *old_head, uintptr_t *new_head,
+		uint32_t *entries)
+{
+	unsigned int max = n;
+	uintptr_t prod_tail;
+	int success;
+
+	/* move cons.head atomically */
+	*old_head = __atomic_load_n(&r->cons_ptr.head, __ATOMIC_RELAXED);
+	do {
+		/* Restore n as it may change every loop */
+		n = max;
+
+		/* Ensure the head is read before tail */
+		__atomic_thread_fence(__ATOMIC_ACQUIRE);
+
+		/* this load-acquire synchronize with store-release of ht->tail
+		 * in update_tail.
+		 */
+		prod_tail = __atomic_load_n(&r->prod_ptr.tail,
+					__ATOMIC_ACQUIRE);
+
+		*entries = (prod_tail - *old_head);
+
+		/* Set the actual entries for dequeue */
+		if (n > *entries)
+			n = (behavior == RTE_RING_QUEUE_FIXED) ? 0 : *entries;
+
+		if (unlikely(n == 0))
+			return 0;
+
+		*new_head = *old_head + n;
+		if (is_sc)
+			r->cons_ptr.head = *new_head, success = 1;
+		else
+			/* on failure, *old_head will be updated */
+			success = __atomic_compare_exchange_n(&r->cons_ptr.head,
+							old_head, *new_head,
+							0, __ATOMIC_RELAXED,
+							__ATOMIC_RELAXED);
+	} while (unlikely(success == 0));
+	return n;
+}
+
 #endif /* _RTE_RING_C11_MEM_H_ */
diff --git a/lib/librte_ring/rte_ring_generic.h b/lib/librte_ring/rte_ring_generic.h
index ea7dbe5b9..6a0e1bbfb 100644
--- a/lib/librte_ring/rte_ring_generic.h
+++ b/lib/librte_ring/rte_ring_generic.h
@@ -167,4 +167,134 @@ __rte_ring_move_cons_head(struct rte_ring *r, unsigned int is_sc,
 	return n;
 }
 
+/**
+ * @internal This function updates the producer head for enqueue using
+ *	     pointer-sized head/tail values.
+ *
+ * @param r
+ *   A pointer to the ring structure
+ * @param is_sp
+ *   Indicates whether multi-producer path is needed or not
+ * @param n
+ *   The number of elements we will want to enqueue, i.e. how far should the
+ *   head be moved
+ * @param behavior
+ *   RTE_RING_QUEUE_FIXED:    Enqueue a fixed number of items from a ring
+ *   RTE_RING_QUEUE_VARIABLE: Enqueue as many items as possible from ring
+ * @param old_head
+ *   Returns head value as it was before the move, i.e. where enqueue starts
+ * @param new_head
+ *   Returns the current/new head value i.e. where enqueue finishes
+ * @param free_entries
+ *   Returns the amount of free space in the ring BEFORE head was moved
+ * @return
+ *   Actual number of objects enqueued.
+ *   If behavior == RTE_RING_QUEUE_FIXED, this will be 0 or n only.
+ */
+static __rte_always_inline unsigned int
+__rte_ring_move_prod_head_ptr(struct rte_ring *r, unsigned int is_sp,
+		unsigned int n, enum rte_ring_queue_behavior behavior,
+		uintptr_t *old_head, uintptr_t *new_head,
+		uint32_t *free_entries)
+{
+	const uint32_t capacity = r->capacity;
+	unsigned int max = n;
+	int success;
+
+	do {
+		/* Reset n to the initial burst count */
+		n = max;
+
+		*old_head = r->prod_ptr.head;
+
+		/* add rmb barrier to avoid load/load reorder in weak
+		 * memory model. It is noop on x86
+		 */
+		rte_smp_rmb();
+
+		*free_entries = (capacity + r->cons_ptr.tail - *old_head);
+
+		/* check that we have enough room in ring */
+		if (unlikely(n > *free_entries))
+			n = (behavior == RTE_RING_QUEUE_FIXED) ?
+					0 : *free_entries;
+
+		if (n == 0)
+			return 0;
+
+		*new_head = *old_head + n;
+		if (is_sp)
+			r->prod_ptr.head = *new_head, success = 1;
+		else
+			success = __sync_bool_compare_and_swap(
+					&r->prod_ptr.head,
+					*old_head, *new_head);
+	} while (unlikely(success == 0));
+	return n;
+}
+
+/**
+ * @internal This function updates the consumer head for dequeue using
+ *	     pointer-sized head/tail values.
+ *
+ * @param r
+ *   A pointer to the ring structure
+ * @param is_sc
+ *   Indicates whether multi-consumer path is needed or not
+ * @param n
+ *   The number of elements we will want to enqueue, i.e. how far should the
+ *   head be moved
+ * @param behavior
+ *   RTE_RING_QUEUE_FIXED:    Dequeue a fixed number of items from a ring
+ *   RTE_RING_QUEUE_VARIABLE: Dequeue as many items as possible from ring
+ * @param old_head
+ *   Returns head value as it was before the move, i.e. where dequeue starts
+ * @param new_head
+ *   Returns the current/new head value i.e. where dequeue finishes
+ * @param entries
+ *   Returns the number of entries in the ring BEFORE head was moved
+ * @return
+ *   - Actual number of objects dequeued.
+ *     If behavior == RTE_RING_QUEUE_FIXED, this will be 0 or n only.
+ */
+static __rte_always_inline unsigned int
+__rte_ring_move_cons_head_ptr(struct rte_ring *r, unsigned int is_sc,
+		unsigned int n, enum rte_ring_queue_behavior behavior,
+		uintptr_t *old_head, uintptr_t *new_head,
+		uint32_t *entries)
+{
+	unsigned int max = n;
+	int success;
+
+	do {
+		/* Restore n as it may change every loop */
+		n = max;
+
+		*old_head = r->cons_ptr.head;
+
+		/* add rmb barrier to avoid load/load reorder in weak
+		 * memory model. It is noop on x86
+		 */
+		rte_smp_rmb();
+
+		*entries = (r->prod_ptr.tail - *old_head);
+
+		/* Set the actual entries for dequeue */
+		if (n > *entries)
+			n = (behavior == RTE_RING_QUEUE_FIXED) ? 0 : *entries;
+
+		if (unlikely(n == 0))
+			return 0;
+
+		*new_head = *old_head + n;
+		if (is_sc)
+			r->cons_ptr.head = *new_head, success = 1;
+		else
+			success = __sync_bool_compare_and_swap(
+					&r->cons_ptr.head,
+					*old_head, *new_head);
+	} while (unlikely(success == 0));
+	return n;
+}
+
 #endif /* _RTE_RING_GENERIC_H_ */
-- 
2.13.6