[PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi

[Koichiro Den <den@klaipeden.com>]

Facing the possible unbounded delay relying on freeing on xmit path,
we also better to invoke and clear the upper layer zerocopy callback
beforehand to keep them from waiting for unbounded duration in vain.
For instance, this removes the possible deadlock in the case that the
upper layer is a zerocopy-enabled vhost-net.
This does not apply if napi_tx is enabled since it will be called in
reasonale time.

Signed-off-by: Koichiro Den <den@klaipeden.com>
---
 drivers/net/virtio_net.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 4302f313d9a7..f7deaa5b7b50 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1290,6 +1290,14 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
 
 	/* Don't wait up for transmitted skbs to be freed. */
 	if (!use_napi) {
+		if (skb_shinfo(skb)->tx_flags & SKBTX_DEV_ZEROCOPY) {
+			struct ubuf_info *uarg;
+			uarg = skb_shinfo(skb)->destructor_arg;
+			if (uarg->callback)
+			    uarg->callback(uarg, true);
+			skb_shinfo(skb)->destructor_arg = NULL;
+			skb_shinfo(skb)->tx_flags &= ~SKBTX_DEV_ZEROCOPY;
+		}
 		skb_orphan(skb);
 		nf_reset(skb);
 	}
-- 
2.9.4