hwrng: virtio - don't wait on cleanup

When virtio-rng device was dropped by the hwrng core we were forced
to wait the buffer to come back from the device to not have
remaining ongoing operation that could spoil the buffer.

But now, as the buffer is internal to the virtio-rng we can release
the waiting loop immediately, the buffer will be retrieve and use
when the virtio-rng driver will be selected again.

This avoids to hang on an rng_current write command if the virtio-rng
device is blocked by a lack of entropy. This allows to select
another entropy source if the current one is empty.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Link: https://lore.kernel.org/r/20211028101111.128049-3-lvivier@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/char/hw_random/virtio-rng.c

@@ -82,6 +82,11 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)
 		ret = wait_for_completion_killable(&vi->have_data);
 		if (ret < 0)
 			return ret;
+		/* if vi->data_avail is 0, we have been interrupted
+		 * by a cleanup, but buffer stays in the queue
+		 */
+		if (vi->data_avail == 0)
+			return read;
 
 		chunk = min_t(unsigned int, size, vi->data_avail);
 		memcpy(buf + read, vi->data, chunk);
@@ -105,7 +110,7 @@ static void virtio_cleanup(struct hwrng *rng)
 	struct virtrng_info *vi = (struct virtrng_info *)rng->priv;
 
 	if (vi->busy)
-		wait_for_completion(&vi->have_data);
+		complete(&vi->have_data);
 }
 
 static int probe_common(struct virtio_device *vdev)