[dpdk-dev] [PATCH v2 4/8] net/memif: relax the load of ring head pointer for S2M ring

[Honnappa Nagarahalli]

For S2M rings, ring->head is updated by the sender and eth_memif_tx
function is called in the context of sending thread. The loads in
the sender do not need to synchronize with its own stores.

Fixes: a2aafb9aa651 ("net/memif: optimize with one-way barrier")
Cc: phil.yang at arm.com
Cc: stable at dpdk.org

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli at arm.com>
Reviewed-by: Phil Yang <phil.yang at arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang at arm.com>
---
 drivers/net/memif/rte_eth_memif.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/drivers/net/memif/rte_eth_memif.c b/drivers/net/memif/rte_eth_memif.c
index 159b45180..96db76121 100644
--- a/drivers/net/memif/rte_eth_memif.c
+++ b/drivers/net/memif/rte_eth_memif.c
@@ -585,7 +585,13 @@ eth_memif_tx(void *queue, struct rte_mbuf **bufs, uint16_t nb_pkts)
 	mask = ring_size - 1;
 
 	if (type == MEMIF_RING_S2M) {
-		slot = __atomic_load_n(&ring->head, __ATOMIC_ACQUIRE);
+		/* For S2M queues ring->head is updated by the sender and
+		 * this function is called in the context of sending thread.
+		 * The loads in the sender do not need to synchronize with
+		 * its own stores. Hence, the following load can be a
+		 * relaxed load.
+		 */
+		slot = __atomic_load_n(&ring->head, __ATOMIC_RELAXED);
 		n_free = ring_size - slot +
 				__atomic_load_n(&ring->tail, __ATOMIC_ACQUIRE);
 	} else {
-- 
2.17.1