3 files changed, 49 insertions, 15 deletions
diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 0098810df69..1f572c00a1b 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -192,7 +192,8 @@ static void leak_balloon(struct virtio_balloon *vb, size_t num)
 	 * virtio_has_feature(vdev, VIRTIO_BALLOON_F_MUST_TELL_HOST);
 	 * is true, we *have* to do it in this order
 	 */
-	tell_host(vb, vb->deflate_vq);
+	if (vb->num_pfns != 0)
+		tell_host(vb, vb->deflate_vq);
 	mutex_unlock(&vb->balloon_lock);
 	release_pages_by_pfn(vb->pfns, vb->num_pfns);
 }
diff --git a/drivers/virtio/virtio_pci.c b/drivers/virtio/virtio_pci.c
index a7ce73029f5..1aba255b587 100644
--- a/drivers/virtio/virtio_pci.c
+++ b/drivers/virtio/virtio_pci.c
@@ -289,9 +289,9 @@ static void vp_free_vectors(struct virtio_device *vdev)
 
 		pci_disable_msix(vp_dev->pci_dev);
 		vp_dev->msix_enabled = 0;
-		vp_dev->msix_vectors = 0;
 	}
 
+	vp_dev->msix_vectors = 0;
 	vp_dev->msix_used_vectors = 0;
 	kfree(vp_dev->msix_names);
 	vp_dev->msix_names = NULL;
@@ -309,6 +309,8 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
 	unsigned i, v;
 	int err = -ENOMEM;
 
+	vp_dev->msix_vectors = nvectors;
+
 	vp_dev->msix_entries = kmalloc(nvectors * sizeof *vp_dev->msix_entries,
 				       GFP_KERNEL);
 	if (!vp_dev->msix_entries)
@@ -336,7 +338,6 @@ static int vp_request_msix_vectors(struct virtio_device *vdev, int nvectors,
 		err = -ENOSPC;
 	if (err)
 		goto error;
-	vp_dev->msix_vectors = nvectors;
 	vp_dev->msix_enabled = 1;
 
 	/* Set the vector used for configuration */
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index c70207478e5..6b4a4db4404 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -576,19 +576,21 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
 EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
 
 /**
- * virtqueue_enable_cb - restart callbacks after disable_cb.
+ * virtqueue_enable_cb_prepare - restart callbacks after disable_cb
  * @vq: the struct virtqueue we're talking about.
  *
- * This re-enables callbacks; it returns "false" if there are pending
- * buffers in the queue, to detect a possible race between the driver
- * checking for more work, and enabling callbacks.
+ * This re-enables callbacks; it returns current queue state
+ * in an opaque unsigned value. This value should be later tested by
+ * virtqueue_poll, to detect a possible race between the driver checking for
+ * more work, and enabling callbacks.
  *
  * Caller must ensure we don't call this with other virtqueue
  * operations at the same time (except where noted).
  */
-bool virtqueue_enable_cb(struct virtqueue *_vq)
+unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
+	u16 last_used_idx;
 
 	START_USE(vq);
 
@@ -598,15 +600,45 @@ bool virtqueue_enable_cb(struct virtqueue *_vq)
 	 * either clear the flags bit or point the event index at the next
 	 * entry. Always do both to keep code simple. */
 	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
-	vring_used_event(&vq->vring) = vq->last_used_idx;
+	vring_used_event(&vq->vring) = last_used_idx = vq->last_used_idx;
+	END_USE(vq);
+	return last_used_idx;
+}
+EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
+
+/**
+ * virtqueue_poll - query pending used buffers
+ * @vq: the struct virtqueue we're talking about.
+ * @last_used_idx: virtqueue state (from call to virtqueue_enable_cb_prepare).
+ *
+ * Returns "true" if there are pending used buffers in the queue.
+ *
+ * This does not need to be serialized.
+ */
+bool virtqueue_poll(struct virtqueue *_vq, unsigned last_used_idx)
+{
+	struct vring_virtqueue *vq = to_vvq(_vq);
+
 	virtio_mb(vq->weak_barriers);
-	if (unlikely(more_used(vq))) {
-		END_USE(vq);
-		return false;
-	}
+	return (u16)last_used_idx != vq->vring.used->idx;
+}
+EXPORT_SYMBOL_GPL(virtqueue_poll);
 
-	END_USE(vq);
-	return true;
+/**
+ * virtqueue_enable_cb - restart callbacks after disable_cb.
+ * @vq: the struct virtqueue we're talking about.
+ *
+ * This re-enables callbacks; it returns "false" if there are pending
+ * buffers in the queue, to detect a possible race between the driver
+ * checking for more work, and enabling callbacks.
+ *
+ * Caller must ensure we don't call this with other virtqueue
+ * operations at the same time (except where noted).
+ */
+bool virtqueue_enable_cb(struct virtqueue *_vq)
+{
+	unsigned last_used_idx = virtqueue_enable_cb_prepare(_vq);
+	return !virtqueue_poll(_vq, last_used_idx);
 }
 EXPORT_SYMBOL_GPL(virtqueue_enable_cb);