virtio-balloon: fix add/get API use

Since ee7cd898 'virtio: expose added
descriptors immediately.', in virtio balloon virtqueue_get_buf might
now run concurrently with virtqueue_kick.  I audited both and this
seems safe in practice but this is not guaranteed by the API.
Additionally, a spurious interrupt might in theory make
virtqueue_get_buf run in parallel with virtqueue_add_buf, which is
racy.

While we might try to protect against spurious callbacks it's
easier to fix the driver: balloon seems to be the only one
(mis)using the API like this, so let's just fix balloon.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (removed unused var)

drivers/virtio/virtio_balloon.c

@@ -47,7 +47,7 @@ struct virtio_balloon
 	struct task_struct *thread;
 
 	/* Waiting for host to ack the pages we released. */
-	struct completion acked;
+	wait_queue_head_t acked;
 
 	/* Number of balloon pages we've told the Host we're not using. */
 	unsigned int num_pages;
@@ -89,29 +89,25 @@ static struct page *balloon_pfn_to_page(u32 pfn)
 
 static void balloon_ack(struct virtqueue *vq)
 {
-	struct virtio_balloon *vb;
-	unsigned int len;
+	struct virtio_balloon *vb = vq->vdev->priv;
 
-	vb = virtqueue_get_buf(vq, &len);
-	if (vb)
-		complete(&vb->acked);
+	wake_up(&vb->acked);
 }
 
 static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
 {
 	struct scatterlist sg;
+	unsigned int len;
 
 	sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
 
-	init_completion(&vb->acked);
-
 	/* We should always be able to add one buffer to an empty queue. */
 	if (virtqueue_add_buf(vq, &sg, 1, 0, vb, GFP_KERNEL) < 0)
 		BUG();
 	virtqueue_kick(vq);
 
 	/* When host has read buffer, this completes via balloon_ack */
-	wait_for_completion(&vb->acked);
+	wait_event(vb->acked, virtqueue_get_buf(vq, &len));
 }
 
 static void set_page_pfns(u32 pfns[], struct page *page)
@@ -231,12 +227,8 @@ static void update_balloon_stats(struct virtio_balloon *vb)
  */
 static void stats_request(struct virtqueue *vq)
 {
-	struct virtio_balloon *vb;
-	unsigned int len;
+	struct virtio_balloon *vb = vq->vdev->priv;
 
-	vb = virtqueue_get_buf(vq, &len);
-	if (!vb)
-		return;
 	vb->need_stats_update = 1;
 	wake_up(&vb->config_change);
 }
@@ -245,11 +237,14 @@ static void stats_handle_request(struct virtio_balloon *vb)
 {
 	struct virtqueue *vq;
 	struct scatterlist sg;
+	unsigned int len;
 
 	vb->need_stats_update = 0;
 	update_balloon_stats(vb);
 
 	vq = vb->stats_vq;
+	if (!virtqueue_get_buf(vq, &len))
+		return;
 	sg_init_one(&sg, vb->stats, sizeof(vb->stats));
 	if (virtqueue_add_buf(vq, &sg, 1, 0, vb, GFP_KERNEL) < 0)
 		BUG();
@@ -358,6 +353,7 @@ static int virtballoon_probe(struct virtio_device *vdev)
 	INIT_LIST_HEAD(&vb->pages);
 	vb->num_pages = 0;
 	init_waitqueue_head(&vb->config_change);
+	init_waitqueue_head(&vb->acked);
 	vb->vdev = vdev;
 	vb->need_stats_update = 0;