virtio-rng: skip reading when we start to remove the device

commit f4981956 upstream.

Before we really unregister the hwrng device, reading will get stuck if
the virtio device is reset. We should return error for reading when we
start to remove the device.
Signed-off-by: Amos Kong <akong@redhat.com>
Reviewed-by: Amit Shah <amit.shah@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/char/hw_random/virtio-rng.c

@@ -37,6 +37,7 @@ struct virtrng_info {
 	char name[25];
 	int index;
 	bool hwrng_register_done;
+	bool hwrng_removed;
 };
 
 
@@ -69,6 +70,9 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
 	int ret;
 	struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
 
+	if (vi->hwrng_removed)
+		return -ENODEV;
+
 	if (!vi->busy) {
 		vi->busy = true;
 		init_completion(&vi->have_data);
@@ -137,6 +141,7 @@ static void remove_common(struct virtio_device *vdev)
 {
 	struct virtrng_info *vi = vdev->priv;
 
+	vi->hwrng_removed = true;
 	vi->data_avail = 0;
 	complete(&vi->have_data);
 	vdev->config->reset(vdev);