[dpdk-dev] [PATCH 2/2] lib/ethdev: fix memory ordering for call back functions

[Honnappa Nagarahalli]

Call back functions are registered on the control plane. They
are accessed from the data plane. Hence, correct memory orderings
should be used to avoid race conditions.

Fixes: 4dc294158cac ("ethdev: support optional Rx and Tx callbacks")
Fixes: c8231c63ddcb ("ethdev: insert Rx callback as head of list")
Cc: bruce.richardson at intel.com
Cc: john.mcnamara at intel.com
Cc: reshma.pattan at intel.com
Cc: stable at dpdk.org

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>
Reviewed-by: Ola Liljedahl <ola.liljedahl at arm.com>
---
 lib/librte_ethdev/rte_ethdev.c | 28 +++++++++++++++++++++------
 lib/librte_ethdev/rte_ethdev.h | 35 ++++++++++++++++++++++++++--------
 2 files changed, 49 insertions(+), 14 deletions(-)

diff --git a/lib/librte_ethdev/rte_ethdev.c b/lib/librte_ethdev/rte_ethdev.c
index 59a41c07f..d89fcdc77 100644
--- a/lib/librte_ethdev/rte_ethdev.c
+++ b/lib/librte_ethdev/rte_ethdev.c
@@ -4486,12 +4486,20 @@ rte_eth_add_rx_callback(uint16_t port_id, uint16_t queue_id,
 		rte_eth_devices[port_id].post_rx_burst_cbs[queue_id];
 
 	if (!tail) {
-		rte_eth_devices[port_id].post_rx_burst_cbs[queue_id] = cb;
+		/* Stores to cb->fn and cb->param should complete before
+		 * cb is visible to data plane.
+		 */
+		__atomic_store_n(
+			&rte_eth_devices[port_id].post_rx_burst_cbs[queue_id],
+			cb, __ATOMIC_RELEASE);
 
 	} else {
 		while (tail->next)
 			tail = tail->next;
-		tail->next = cb;
+		/* Stores to cb->fn and cb->param should complete before
+		 * cb is visible to data plane.
+		 */
+		__atomic_store_n(&tail->next, cb, __ATOMIC_RELEASE);
 	}
 	rte_spinlock_unlock(&rte_eth_rx_cb_lock);
 
@@ -4576,12 +4584,20 @@ rte_eth_add_tx_callback(uint16_t port_id, uint16_t queue_id,
 		rte_eth_devices[port_id].pre_tx_burst_cbs[queue_id];
 
 	if (!tail) {
-		rte_eth_devices[port_id].pre_tx_burst_cbs[queue_id] = cb;
+		/* Stores to cb->fn and cb->param should complete before
+		 * cb is visible to data plane.
+		 */
+		__atomic_store_n(
+			&rte_eth_devices[port_id].pre_tx_burst_cbs[queue_id],
+			cb, __ATOMIC_RELEASE);
 
 	} else {
 		while (tail->next)
 			tail = tail->next;
-		tail->next = cb;
+		/* Stores to cb->fn and cb->param should complete before
+		 * cb is visible to data plane.
+		 */
+		__atomic_store_n(&tail->next, cb, __ATOMIC_RELEASE);
 	}
 	rte_spinlock_unlock(&rte_eth_tx_cb_lock);
 
@@ -4612,7 +4628,7 @@ rte_eth_remove_rx_callback(uint16_t port_id, uint16_t queue_id,
 		cb = *prev_cb;
 		if (cb == user_cb) {
 			/* Remove the user cb from the callback list. */
-			*prev_cb = cb->next;
+			__atomic_store_n(prev_cb, cb->next, __ATOMIC_RELAXED);
 			ret = 0;
 			break;
 		}
@@ -4646,7 +4662,7 @@ rte_eth_remove_tx_callback(uint16_t port_id, uint16_t queue_id,
 		cb = *prev_cb;
 		if (cb == user_cb) {
 			/* Remove the user cb from the callback list. */
-			*prev_cb = cb->next;
+			__atomic_store_n(prev_cb, cb->next, __ATOMIC_RELAXED);
 			ret = 0;
 			break;
 		}
diff --git a/lib/librte_ethdev/rte_ethdev.h b/lib/librte_ethdev/rte_ethdev.h
index 70295d7ab..d810e3e38 100644
--- a/lib/librte_ethdev/rte_ethdev.h
+++ b/lib/librte_ethdev/rte_ethdev.h
@@ -3734,7 +3734,8 @@ struct rte_eth_rxtx_callback;
  *   The callback function
  * @param user_param
  *   A generic pointer parameter which will be passed to each invocation of the
- *   callback function on this port and queue.
+ *   callback function on this port and queue. Inter-thread synchronization
+ *   of any user data changes is the responsibility of the user.
  *
  * @return
  *   NULL on error.
@@ -3763,7 +3764,8 @@ rte_eth_add_rx_callback(uint16_t port_id, uint16_t queue_id,
  *   The callback function
  * @param user_param
  *   A generic pointer parameter which will be passed to each invocation of the
- *   callback function on this port and queue.
+ *   callback function on this port and queue. Inter-thread synchronization
+ *   of any user data changes is the responsibility of the user.
  *
  * @return
  *   NULL on error.
@@ -3791,7 +3793,8 @@ rte_eth_add_first_rx_callback(uint16_t port_id, uint16_t queue_id,
  *   The callback function
  * @param user_param
  *   A generic pointer parameter which will be passed to each invocation of the
- *   callback function on this port and queue.
+ *   callback function on this port and queue. Inter-thread synchronization
+ *   of any user data changes is the responsibility of the user.
  *
  * @return
  *   NULL on error.
@@ -3816,7 +3819,9 @@ rte_eth_add_tx_callback(uint16_t port_id, uint16_t queue_id,
  *   on that queue.
  *
  * - After a short delay - where the delay is sufficient to allow any
- *   in-flight callbacks to complete.
+ *   in-flight callbacks to complete. Alternately, the RCU mechanism can be
+ *   used to detect when data plane threads have ceased referencing the
+ *   callback memory.
  *
  * @param port_id
  *   The port identifier of the Ethernet device.
@@ -3849,7 +3854,9 @@ int rte_eth_remove_rx_callback(uint16_t port_id, uint16_t queue_id,
  *   on that queue.
  *
  * - After a short delay - where the delay is sufficient to allow any
- *   in-flight callbacks to complete.
+ *   in-flight callbacks to complete. Alternately, the RCU mechanism can be
+ *   used to detect when data plane threads have ceased referencing the
+ *   callback memory.
  *
  * @param port_id
  *   The port identifier of the Ethernet device.
@@ -4510,10 +4517,16 @@ rte_eth_rx_burst(uint16_t port_id, uint16_t queue_id,
 				     rx_pkts, nb_pkts);
 
 #ifdef RTE_ETHDEV_RXTX_CALLBACKS
-	if (unlikely(dev->post_rx_burst_cbs[queue_id] != NULL)) {
-		struct rte_eth_rxtx_callback *cb =
-				dev->post_rx_burst_cbs[queue_id];
+	/* __ATOMIC_RELEASE memory order was used when the
+	 * call back was inserted into the list.
+	 * Since there is a clear dependency between loading
+	 * cb and cb->fn/cb->next, __ATOMIC_ACQUIRE memory order is
+	 * not required.
+	 */
+	struct rte_eth_rxtx_callback *cb =
+			dev->post_rx_burst_cbs[queue_id];
 
+	if (unlikely(cb != NULL)) {
 		do {
 			nb_rx = cb->fn.rx(port_id, queue_id, rx_pkts, nb_rx,
 						nb_pkts, cb->param);
@@ -4775,6 +4788,12 @@ rte_eth_tx_burst(uint16_t port_id, uint16_t queue_id,
 #endif
 
 #ifdef RTE_ETHDEV_RXTX_CALLBACKS
+	/* __ATOMIC_RELEASE memory order was used when the
+	 * call back was inserted into the list.
+	 * Since there is a clear dependency between loading
+	 * cb and cb->fn/cb->next, __ATOMIC_ACQUIRE memory order is
+	 * not required.
+	 */
 	struct rte_eth_rxtx_callback *cb = dev->pre_tx_burst_cbs[queue_id];
 
 	if (unlikely(cb != NULL)) {
-- 
2.17.1