virtio-balloon: fix add/get API use

Since ee7cd8981e15bcb365fc762afe3fc47b8242f630 '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)

1 files changed, 10 insertions, 14 deletions

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index bfbc15ca38dd..0908e6044333 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/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;