net/mlx4_en: mlx4_en_xmit() reads ring->cons once, and ahead of time to avoid stalls

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/drivers/net/ethernet/mellanox/mlx4/en_tx.c b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
index d9aaeb2..99875c8 100644 (file)
--- a/drivers/net/ethernet/mellanox/mlx4/en_tx.c
+++ b/drivers/net/ethernet/mellanox/mlx4/en_tx.c
@@ -691,10 +691,17 @@ netdev_tx_t mlx4_en_xmit(struct sk_buff *skb, struct net_device *dev)
        void *fragptr;
        bool bounce = false;
        bool send_doorbell;
+       u32 ring_cons;
 
        if (!priv->port_up)
                goto tx_drop;
 
+       tx_ind = skb_get_queue_mapping(skb);
+       ring = priv->tx_ring[tx_ind];
+
+       /* fetch ring->cons far ahead before needing it to avoid stall */
+       ring_cons = ACCESS_ONCE(ring->cons);
+
        real_size = get_real_size(skb, dev, &lso_header_size);
        if (unlikely(!real_size))
                goto tx_drop;
@@ -708,13 +715,11 @@ netdev_tx_t mlx4_en_xmit(struct sk_buff *skb, struct net_device *dev)
                goto tx_drop;
        }
 
-       tx_ind = skb->queue_mapping;
-       ring = priv->tx_ring[tx_ind];
        if (vlan_tx_tag_present(skb))
                vlan_tag = vlan_tx_tag_get(skb);
 
        /* Check available TXBBs And 2K spare for prefetch */
-       if (unlikely(((int)(ring->prod - ring->cons)) >
+       if (unlikely(((int)(ring->prod - ring_cons)) >
                     ring->size - HEADROOM - MAX_DESC_TXBBS)) {
                /* every full Tx ring stops queue */
                netif_tx_stop_queue(ring->tx_queue);
@@ -728,7 +733,8 @@ netdev_tx_t mlx4_en_xmit(struct sk_buff *skb, struct net_device *dev)
                 */
                wmb();
 
-               if (unlikely(((int)(ring->prod - ring->cons)) <=
+               ring_cons = ACCESS_ONCE(ring->cons);
+               if (unlikely(((int)(ring->prod - ring_cons)) <=
                             ring->size - HEADROOM - MAX_DESC_TXBBS)) {
                        netif_tx_wake_queue(ring->tx_queue);
                        ring->wake_queue++;
@@ -741,7 +747,7 @@ netdev_tx_t mlx4_en_xmit(struct sk_buff *skb, struct net_device *dev)
 
        /* Track current inflight packets for performance analysis */
        AVG_PERF_COUNTER(priv->pstats.inflight_avg,
-                        (u32) (ring->prod - ring->cons - 1));
+                        (u32)(ring->prod - ring_cons - 1));
 
        /* Packet is good - grab an index and transmit it */
        index = ring->prod & ring->size_mask;