Commit 94d250fa authored May 20, 2019 by Bernd Eckstein Committed by David S. Miller May 21, 2019

usbnet: ipheth: fix racing condition

Fix a racing condition in ipheth.c that can lead to slow performance.

Bug: In ipheth_tx(), netif_wake_queue() may be called on the callback
ipheth_sndbulk_callback(), _before_ netif_stop_queue() is called.
When this happens, the queue is stopped longer than it needs to be,
thus reducing network performance.

Fix: Move netif_stop_queue() in front of usb_submit_urb(). Now the order
is always correct. In case, usb_submit_urb() fails, the queue is woken up
again as callback will not fire.

Testing: This racing condition is usually not noticeable, as it has to
occur very frequently to slowdown the network. The callback from the USB
is usually triggered slow enough, so the situation does not appear.
However, on a Ubuntu Linux on VMWare Workstation, running on Windows 10,
the we loose the race quite often and the following speedup can be noticed:

Without this patch: Download: 4.10 Mbit/s, Upload: 4.01 Mbit/s
With this patch: Download: 36.23 Mbit/s, Upload: 17.61 Mbit/s

Signed-off-by: Oliver Zweigle <Oliver.Zweigle@faro.com>
Signed-off-by: Bernd Eckstein <3ernd.Eckstein@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent af8f3fb7

drivers/net/usb/ipheth.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -383,17 +383,18 @@ static int ipheth_tx(struct sk_buff skb, struct net_device net)
		dev);
		dev->tx_urb->transfer_flags \|= URB_NO_TRANSFER_DMA_MAP;

		netif_stop_queue(net);
		retval = usb_submit_urb(dev->tx_urb, GFP_ATOMIC);
		if (retval) {
		dev_err(&dev->intf->dev, "%s: usb_submit_urb: %d\n",
		__func__, retval);
		dev->net->stats.tx_errors++;
		dev_kfree_skb_any(skb);
		netif_wake_queue(net);
		} else {
		dev->net->stats.tx_packets++;
		dev->net->stats.tx_bytes += skb->len;
		dev_consume_skb_any(skb);
		netif_stop_queue(net);
		}

		return NETDEV_TX_OK;

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