Index: sys/dev/vt/vt_core.c
===================================================================
--- sys/dev/vt/vt_core.c
+++ sys/dev/vt/vt_core.c
@@ -2617,10 +2617,17 @@
 		/* Init 25 Hz timer. */
 		callout_init_mtx(&vd->vd_timer, &vd->vd_lock, 0);
 
-		/* Start timer when everything ready. */
+		/*
+		 * Start timer when everything ready.
+		 * Note that the operations here are purposefully ordered.
+		 * We need to ensure vd_timer_armed is non-zero before we set
+		 * the VDF_ASYNC flag. That prevents this function from
+		 * racing with vt_resume_flush_timer() to update the
+		 * callout structure.
+		 */
+		atomic_add_acq_int(&vd->vd_timer_armed, 1);
 		vd->vd_flags |= VDF_ASYNC;
 		callout_reset(&vd->vd_timer, hz / VT_TIMERFREQ, vt_timer, vd);
-		vd->vd_timer_armed = 1;
 		register_handlers = 1;
 	}