[PATCH AUTOSEL 4.4 23/53] net: axienet: Fix race condition causing TX hang

[Sasha Levin]

From: Robert Hancock <hancock@xxxxxxxxxxxxx>

[ Upstream commit 7de44285c1f69ccfbe8be1d6a16fcd956681fee6 ]

It is possible that the interrupt handler fires and frees up space in
the TX ring in between checking for sufficient TX ring space and
stopping the TX queue in axienet_start_xmit. If this happens, the
queue wake from the interrupt handler will occur before the queue is
stopped, causing a lost wakeup and the adapter's transmit hanging.

To avoid this, after stopping the queue, check again whether there is
sufficient space in the TX ring. If so, wake up the queue again.

Signed-off-by: Robert Hancock <hancock@xxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 .../net/ethernet/xilinx/xilinx_axienet_main.c | 20 ++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
index 58ba579793f8..f1e969128a4e 100644
--- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
+++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
@@ -613,6 +613,10 @@ static void axienet_start_xmit_done(struct net_device *ndev)
 
 	ndev->stats.tx_packets += packets;
 	ndev->stats.tx_bytes += size;
+
+	/* Matches barrier in axienet_start_xmit */
+	smp_mb();
+
 	netif_wake_queue(ndev);
 }
 
@@ -667,9 +671,19 @@ static int axienet_start_xmit(struct sk_buff *skb, struct net_device *ndev)
 	cur_p = &lp->tx_bd_v[lp->tx_bd_tail];
 
 	if (axienet_check_tx_bd_space(lp, num_frag)) {
-		if (!netif_queue_stopped(ndev))
-			netif_stop_queue(ndev);
-		return NETDEV_TX_BUSY;
+		if (netif_queue_stopped(ndev))
+			return NETDEV_TX_BUSY;
+
+		netif_stop_queue(ndev);
+
+		/* Matches barrier in axienet_start_xmit_done */
+		smp_mb();
+
+		/* Space might have just been freed - check again */
+		if (axienet_check_tx_bd_space(lp, num_frag))
+			return NETDEV_TX_BUSY;
+
+		netif_wake_queue(ndev);
 	}
 
 	if (skb->ip_summed == CHECKSUM_PARTIAL) {
-- 
2.20.1