diff --git a/sys/netproto/smb/smb_dev.c b/sys/netproto/smb/smb_dev.c
--- a/sys/netproto/smb/smb_dev.c
+++ b/sys/netproto/smb/smb_dev.c
@@ -101,6 +101,7 @@
 	cdev_t dev = ap->a_head.a_dev;
 	struct smb_dev *sdp;
 
+	/* si_drv1 setup is an unlocked read-modify-write; serialize with dev_lock */
 	sdp = SMB_GETDEV(dev);
 	if (sdp && (sdp->sd_flags & NSMBFL_OPEN))
 		return EBUSY;