Index: dev/ata/wd.c
===================================================================
RCS file: /cvsroot/src/sys/dev/ata/wd.c,v
retrieving revision 1.438
diff -u -p -r1.438 wd.c
--- dev/ata/wd.c	7 Jan 2018 11:37:30 -0000	1.438
+++ dev/ata/wd.c	16 Apr 2018 22:34:24 -0000
@@ -765,6 +765,8 @@ wdstart(device_t self)
 	if (!device_is_active(dksc->sc_dev))
 		return;
 
+	mutex_enter(&wd->sc_lock);
+
 	/*
 	 * Do not queue any transfers until flush is finished, so that
 	 * once flush is pending, it will get handled as soon as xfer
@@ -773,9 +775,12 @@ wdstart(device_t self)
 	if (ISSET(wd->sc_flags, WDF_FLUSH_PEND)) {
 		ATADEBUG_PRINT(("wdstart %s flush pend\n",
 		    dksc->sc_xname), DEBUG_XFERS);
+		mutex_exit(&wd->sc_lock);
 		return;
 	}
 
+	mutex_exit(&wd->sc_lock);
+
 	dk_start(dksc, NULL);
 }