net/memif: relax load of ring head for M2S ring

For M2S rings, ring->head is updated by the receiver and eth_memif_rx
function is called in the context of receiving thread. The loads in
the receiver do not need to synchronize with its own stores.

Fixes: a2aafb9aa651 ("net/memif: optimize with one-way barrier")
Cc: stable@dpdk.org

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Jakub Grajciar <jgrajcia@cisco.com>

drivers/net/memif/rte_eth_memif.c

@@ -410,7 +410,11 @@ no_free_bufs:
 
 refill:
 	if (type == MEMIF_RING_M2S) {
-		head = __atomic_load_n(&ring->head, __ATOMIC_ACQUIRE);
+		/* ring->head is updated by the receiver and this function
+		 * is called in the context of receiver thread. The loads in
+		 * the receiver do not need to synchronize with its own stores.
+		 */
+		head = __atomic_load_n(&ring->head, __ATOMIC_RELAXED);
 		n_slots = ring_size - head + mq->last_tail;
 
 		while (n_slots--) {