virtio: Improve vq->broken access to avoid any compiler optimization

Currently vq->broken field is read by virtqueue_is_broken() in busy
loop in one context by virtnet_send_command().

vq->broken is set to true in other process context by
virtio_break_device(). Reader and writer are accessing it without any
synchronization. This may lead to a compiler optimization which may
result to optimize reading vq->broken only once.

Hence, force reading vq->broken on each invocation of
virtqueue_is_broken() and also force writing it so that such
update is visible to the readers.

It is a theoretical fix that isn't yet encountered in the field.
Signed-off-by: NParav Pandit <parav@nvidia.com>
Link: https://lore.kernel.org/r/20210721142648.1525924-2-parav@nvidia.comSigned-off-by: NMichael S. Tsirkin <mst@redhat.com>

drivers/virtio/virtio_ring.c

@@ -2373,7 +2373,7 @@ bool virtqueue_is_broken(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 
-	return vq->broken;
+	return READ_ONCE(vq->broken);
 }
 EXPORT_SYMBOL_GPL(virtqueue_is_broken);
 
@@ -2387,7 +2387,9 @@ void virtio_break_device(struct virtio_device *dev)
 
 	list_for_each_entry(_vq, &dev->vqs, list) {
 		struct vring_virtqueue *vq = to_vvq(_vq);
-		vq->broken = true;
+
+		/* Pairs with READ_ONCE() in virtqueue_is_broken(). */
+		WRITE_ONCE(vq->broken, true);
 	}
 }
 EXPORT_SYMBOL_GPL(virtio_break_device);