net: calxedaxgmac: fix race with tx queue stop/wake

Since the xgmac transmit start and completion work locklessly, it is
possible for xgmac_xmit to stop the tx queue after the xgmac_tx_complete
has run resulting in the tx queue never being woken up. Fix this by
ensuring that ring buffer index updates are visible and recheck the ring
space after stopping the queue. Also fix an off-by-one bug where we need
to stop the queue when the ring buffer space is equal to MAX_SKB_FRAGS.

The implementation used here was copied from
drivers/net/ethernet/broadcom/tg3.c.
Signed-off-by: NRob Herring <rob.herring@calxeda.com>
Reviewed-by: NBen Hutchings <bhutchings@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/calxeda/xgmac.c

@@ -410,6 +410,9 @@ struct xgmac_priv {
 #define dma_ring_space(h, t, s)	CIRC_SPACE(h, t, s)
 #define dma_ring_cnt(h, t, s)	CIRC_CNT(h, t, s)
 
+#define tx_dma_ring_space(p) \
+	dma_ring_space((p)->tx_head, (p)->tx_tail, DMA_TX_RING_SZ)
+
 /* XGMAC Descriptor Access Helpers */
 static inline void desc_set_buf_len(struct xgmac_dma_desc *p, u32 buf_sz)
 {
@@ -886,8 +889,10 @@ static void xgmac_tx_complete(struct xgmac_priv *priv)
 		priv->tx_tail = dma_ring_incr(entry, DMA_TX_RING_SZ);
 	}
 
-	if (dma_ring_space(priv->tx_head, priv->tx_tail, DMA_TX_RING_SZ) >
-	    MAX_SKB_FRAGS)
+	/* Ensure tx_tail is visible to xgmac_xmit */
+	smp_mb();
+	if (unlikely(netif_queue_stopped(priv->dev) &&
+	    (tx_dma_ring_space(priv) > MAX_SKB_FRAGS)))
 		netif_wake_queue(priv->dev);
 }
 
@@ -1125,10 +1130,15 @@ static netdev_tx_t xgmac_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	priv->tx_head = dma_ring_incr(entry, DMA_TX_RING_SZ);
 
-	if (dma_ring_space(priv->tx_head, priv->tx_tail, DMA_TX_RING_SZ) <
-	    MAX_SKB_FRAGS)
+	/* Ensure tx_head update is visible to tx completion */
+	smp_mb();
+	if (unlikely(tx_dma_ring_space(priv) <= MAX_SKB_FRAGS)) {
 		netif_stop_queue(dev);
-
+		/* Ensure netif_stop_queue is visible to tx completion */
+		smp_mb();
+		if (tx_dma_ring_space(priv) > MAX_SKB_FRAGS)
+			netif_start_queue(dev);
+	}
 	return NETDEV_TX_OK;
 }