macvtap: handle ubuf refcount correctly when meet errors

We trigger uarg->callback() immediately after we decide do datacopy
even if caller want to do zerocopy. This will cause the callback
(vhost_net_zerocopy_callback) decrease the refcount. But when we meet
an error afterwards, the error handling in vhost handle_tx() will try
to decrease it again. This is wrong and fix this by delay the
uarg->callback() until we're sure there's no errors.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/macvtap.c

@@ -742,13 +742,8 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
 
 	if (zerocopy)
 		err = zerocopy_sg_from_iter(skb, from);
-	else {
+	else
 		err = skb_copy_datagram_from_iter(skb, 0, from, len);
-		if (!err && m && m->msg_control) {
-			struct ubuf_info *uarg = m->msg_control;
-			uarg->callback(uarg, false);
-		}
-	}
 
 	if (err)
 		goto err_kfree;
@@ -779,7 +774,11 @@ static ssize_t macvtap_get_user(struct macvtap_queue *q, struct msghdr *m,
 		skb_shinfo(skb)->destructor_arg = m->msg_control;
 		skb_shinfo(skb)->tx_flags |= SKBTX_DEV_ZEROCOPY;
 		skb_shinfo(skb)->tx_flags |= SKBTX_SHARED_FRAG;
+	} else if (m && m->msg_control) {
+		struct ubuf_info *uarg = m->msg_control;
+		uarg->callback(uarg, false);
 	}
+
 	if (vlan) {
 		skb->dev = vlan->dev;
 		dev_queue_xmit(skb);