Index: sys/dev/dksubr.c
===================================================================
RCS file: /cvsroot/src/sys/dev/dksubr.c,v
retrieving revision 1.93
diff -u -p -r1.93 dksubr.c
--- sys/dev/dksubr.c	8 Dec 2016 12:22:56 -0000	1.93
+++ sys/dev/dksubr.c	22 Dec 2016 09:39:03 -0000
@@ -447,6 +447,23 @@ dk_done1(struct dk_softc *dksc, struct b
 	rnd_add_uint32(&dksc->sc_rnd_source, bp->b_rawblkno);
 
 	biodone(bp);
+
+	if (lock)
+		mutex_enter(&dksc->sc_iolock);
+	if (dksc->sc_deferred) {
+		struct buf *tmp = dksc->sc_deferred;
+		const struct dkdriver *dkd = dksc->sc_dkdev.dk_driver;
+		dksc->sc_deferred = NULL;
+		disk_busy(&dksc->sc_dkdev);
+		mutex_exit(&dksc->sc_iolock);
+		int error = dkd->d_diskstart(dksc->sc_dev, tmp);
+		mutex_enter(&dksc->sc_iolock);
+		if (error) {
+			dksc->sc_deferred = tmp;
+		}
+	}
+	if (lock)
+		mutex_exit(&dksc->sc_iolock);
 }
 
 void