Re: The ubufs->refcount maybe be subtracted twice when tun_get_user failed

On 2016年11月29日 21:27, wangyunjian wrote:

Sorry, I didn't describe it clearly. In fact, the second subtraction happens in the function handle_tx,
when tun_get_user fails and zcopy_used is ture. Fllowing the steps:

I get your meaning. Thanks for the reporting. Will post patches (since 
macvtap has similar issue).

static ssize_t tun_get_user(struct tun_struct *tun, struct tun_file *tfile,
                                   void *msg_control, struct iov_iter *from,
                                   int noblock) {
            ...

            if (zerocopy)
                      err = zerocopy_sg_from_iter(skb, from);
            else {
                      err = skb_copy_datagram_from_iter(skb, 0, from, len);
                      if (!err && msg_control) {
                               struct ubuf_info *uarg = msg_control;
                               uarg->callback(uarg, false);                       --> step 1, the ubufs->refcount is subtracted frist.
                      }
            }

            if (err) {
                      this_cpu_inc(tun->pcpu_stats->rx_dropped);
                      kfree_skb(skb);
                      return -EFAULT;
            }

            err = virtio_net_hdr_to_skb(skb, &gso, tun_is_little_endian(tun));
            if (err) {
                      this_cpu_inc(tun->pcpu_stats->rx_frame_errors);
                      kfree_skb(skb);
                      return -EINVAL;                                         -->step 2, return err.
            }
}

static void handle_tx(struct vhost_net *net)
{
	...
	/* TODO: Check specific error and bomb out unless ENOBUFS? */
	err = sock->ops->sendmsg(sock, &msg, len);
	if (unlikely(err < 0)) {
		if (zcopy_used) {
			vhost_net_ubuf_put(ubufs);                                        --> step 3, the ubufs->refcount will be subtracted twice, when sendmsg execution err.
			nvq->upend_idx = ((unsigned)nvq->upend_idx - 1)
				% UIO_MAXIOV;
		}
		vhost_discard_vq_desc(vq, 1);
		break;
	}
	...
}

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