Index: head/usr.bin/lockf/lockf.1
===================================================================
--- head/usr.bin/lockf/lockf.1
+++ head/usr.bin/lockf/lockf.1
@@ -1,4 +1,4 @@
-.\"
+ .\"
 .\" Copyright (C) 1998 John D. Polstra.  All rights reserved.
 .\"
 .\" Redistribution and use in source and binary forms, with or without
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd June 18, 2020
+.Dd August 26, 2020
 .Dt LOCKF 1
 .Os
 .Sh NAME
@@ -32,7 +32,7 @@
 .Nd execute a command while holding a file lock
 .Sh SYNOPSIS
 .Nm
-.Op Fl kns
+.Op Fl knsw
 .Op Fl t Ar seconds
 .Ar file
 .Ar command
@@ -121,6 +121,14 @@
 is
 .Em not
 executed.
+.It Fl w
+Causes
+.Nm
+to open
+.Ar file
+for writing rather than reading.
+This is necessary on filesystems (including NFSv4) where a file which
+has been opened read-only cannot be exclusively locked.
 .El
 .Pp
 In no event will
Index: head/usr.bin/lockf/lockf.c
===================================================================
--- head/usr.bin/lockf/lockf.c
+++ head/usr.bin/lockf/lockf.c
@@ -62,9 +62,9 @@
 	pid_t child;
 
 	silent = keep = 0;
-	flags = O_CREAT;
+	flags = O_CREAT | O_RDONLY;
 	waitsec = -1;	/* Infinite. */
-	while ((ch = getopt(argc, argv, "sknt:")) != -1) {
+	while ((ch = getopt(argc, argv, "sknt:w")) != -1) {
 		switch (ch) {
 		case 'k':
 			keep = 1;
@@ -84,6 +84,9 @@
 				    "invalid timeout \"%s\"", optarg);
 		}
 			break;
+		case 'w':
+			flags = (flags & ~O_RDONLY) | O_WRONLY;
+			break;
 		default:
 			usage();
 		}
@@ -171,7 +174,7 @@
 {
 	int fd;
 
-	if ((fd = open(name, O_RDONLY|O_EXLOCK|flags, 0666)) == -1) {
+	if ((fd = open(name, O_EXLOCK|flags, 0666)) == -1) {
 		if (errno == EAGAIN || errno == EINTR)
 			return (-1);
 		else if (errno == ENOENT && (flags & O_CREAT) == 0)