On Tuesday 27 May 2008 21:06:26 Mark McLoughlin wrote:
> On Mon, 2008-05-26 at 17:42 +1000, Rusty Russell wrote:
> > If we fail to transmit a packet, we assume the queue is full and put
> > the skb into last_xmit_skb. However, if more space frees up before we
> > xmit it, we loop, and the result can be transmitting the same skb twice.
> >
> > Fix is simple: set skb to NULL if we've used it in some way, and check
> > before sending.
Great! It's a corner case, but it's no more complicated to do it your way.
Minor mod, I find it clearer to have the vi->last_xmit_skb = NULL; under
the branch:
Subject: [PATCH] virtio_net: Delay dropping tx skbs
Date: Tue, 27 May 2008 12:06:26 +0100
From: Mark McLoughlin <markmc@xxxxxxxxxx>
Currently we drop the skb in start_xmit() if we have a
queued buffer and fail to transmit it.
However, if we delay dropping it until we've stopped the
queue and enabled the tx notification callback, then there
is a chance space might become available for it.
Signed-off-by: Mark McLoughlin <markmc@xxxxxxxxxx>
Signed-off-by: Rusty Russell <rusty@xxxxxxxxxxxxxxx> (move last_xmit_skb = NULL)
---
drivers/net/virtio_net.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -308,13 +308,8 @@ again:
/* If we has a buffer left over from last time, send it now. */
if (unlikely(vi->last_xmit_skb)) {
- if (xmit_skb(vi, vi->last_xmit_skb) != 0) {
- /* Drop this skb: we only queue one. */
- vi->dev->stats.tx_dropped++;
- kfree_skb(skb);
- skb = NULL;
+ if (xmit_skb(vi, vi->last_xmit_skb) != 0)
goto stop_queue;
- }
vi->last_xmit_skb = NULL;
}
@@ -341,6 +336,11 @@ stop_queue:
vi->svq->vq_ops->disable_cb(vi->svq);
netif_start_queue(dev);
goto again;
+ }
+ if (skb) {
+ /* Drop this skb: we only queue one. */
+ vi->dev->stats.tx_dropped++;
+ kfree_skb(skb);
}
goto done;
}
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