From 74938cbb7f2c538a9185f8af87e38a90167e7372 Mon Sep 17 00:00:00 2001
From: Ian Lepore <ian@FreeBSD.org>
Date: Wed, 13 Feb 2013 15:09:16 +0000
Subject: [PATCH] Make the F_READAHEAD option to fcntl(2) work as documented: 
 a value of zero now disables read-ahead.  It used to effectively restore the
 system default readahead hueristic if it had been changed; a negative value
 now restores the default.

Reviewed by:	kib
---
 lib/libc/sys/fcntl.2    | 4 ++--
 sys/kern/kern_descrip.c | 2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/lib/libc/sys/fcntl.2 b/lib/libc/sys/fcntl.2
index c1745636747c..8e0ca73c399e 100644
--- a/lib/libc/sys/fcntl.2
+++ b/lib/libc/sys/fcntl.2
@@ -28,7 +28,7 @@
 .\"     @(#)fcntl.2	8.2 (Berkeley) 1/12/94
 .\" $FreeBSD$
 .\"
-.Dd July 27, 2012
+.Dd February 8, 2013
 .Dt FCNTL 2
 .Os
 .Sh NAME
@@ -171,7 +171,7 @@ argument,
 which is rounded up to the nearest block size.
 A zero value in
 .Fa arg
-turns off read ahead.
+turns off read ahead, a negative value restores the system default.
 .It Dv F_RDAHEAD
 Equivalent to Darwin counterpart which sets read ahead amount of 128KB
 when the third argument,
diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
index 87d75ea9ca82..fc74bdbd8d47 100644
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -776,7 +776,7 @@ kern_fcntl(struct thread *td, int fd, int cmd, intptr_t arg)
 		}
 		fhold(fp);
 		FILEDESC_SUNLOCK(fdp);
-		if (arg != 0) {
+		if (arg >= 0) {
 			vp = fp->f_vnode;
 			error = vn_lock(vp, LK_SHARED);
 			if (error != 0) {