vhost: handle wrap around in # of bufs math

The meth for calculating the # of outstanding buffers gives
incorrect results when vq->upend_idx wraps around zero.
Fix that.

Signed-off-by: Shirley Ma <xma@us.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 248b250..882a51f 100644 (file)
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -182,15 +182,21 @@ static void handle_tx(struct vhost_net *net)
                        break;
                /* Nothing new?  Wait for eventfd to tell us they refilled. */
                if (head == vq->num) {
+                       int num_pends;
+
                        wmem = atomic_read(&sock->sk->sk_wmem_alloc);
                        if (wmem >= sock->sk->sk_sndbuf * 3 / 4) {
                                tx_poll_start(net, sock);
                                set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
                                break;
                        }
-                       /* If more outstanding DMAs, queue the work */
-                       if (unlikely(vq->upend_idx - vq->done_idx >
-                                    VHOST_MAX_PEND)) {
+                       /* If more outstanding DMAs, queue the work.
+                        * Handle upend_idx wrap around
+                        */
+                       num_pends = likely(vq->upend_idx >= vq->done_idx) ?
+                                   (vq->upend_idx - vq->done_idx) :
+                                   (vq->upend_idx + UIO_MAXIOV - vq->done_idx);
+                       if (unlikely(num_pends > VHOST_MAX_PEND)) {
                                tx_poll_start(net, sock);
                                set_bit(SOCK_ASYNC_NOSPACE, &sock->flags);
                                break;