Add flag to disable inital reboot(8) userland sync

Add -N flag to reboot(8) which bypasses the userland sync(2) during
reboot(8) while still allow the kernel sync during the reboot(2) syscall
to occur.

An example use of this is when rebooting with disconnected iSCSI sessions
which would otherwise cause the reboot to hang on BIOs that will never
complete.

Reviewed by:	bjk
MFC after:	2 weeks
Sponsored by:	Multiplay
Differential Revision:	https://reviews.freebsd.org/D4449

1 files changed, 15 insertions, 5 deletions

diff --git a/sbin/reboot/reboot.8 b/sbin/reboot/reboot.8
index ac3bd47..c95333c 100644
--- a/sbin/reboot/reboot.8
+++ b/sbin/reboot/reboot.8
@@ -28,7 +28,7 @@
 .\"	@(#)reboot.8	8.1 (Berkeley) 6/9/93
 .\" $FreeBSD$
 .\"
-.Dd May 22, 2015
+.Dd Dec 12, 2015
 .Dt REBOOT 8
 .Os
 .Sh NAME
@@ -39,16 +39,16 @@
 .Nd stopping and restarting the system
 .Sh SYNOPSIS
 .Nm halt
-.Op Fl lnpq
+.Op Fl lNnpq
 .Op Fl k Ar kernel
 .Nm
-.Op Fl dlnpqr
+.Op Fl dlNnpqr
 .Op Fl k Ar kernel
 .Nm fasthalt
-.Op Fl lnpq
+.Op Fl lNnpq
 .Op Fl k Ar kernel
 .Nm fastboot
-.Op Fl dlnpq
+.Op Fl dlNnpq
 .Op Fl k Ar kernel
 .Sh DESCRIPTION
 The
@@ -94,6 +94,16 @@ that call
 or
 .Nm halt
 and log this themselves.
+.It Fl N
+The file system cache is not flushed during the initial process clean-up,
+however the kernel level
+.Xr reboot 2
+is still processed with a sync.
+This option can be useful for performing a
+.Dq best-effort
+reboot when devices might be unavailable.
+This can happen when devices have been disconnected, such as with
+.Xr iscsi 4 .
 .It Fl n
 The file system cache is not flushed.
 This option should probably not be used.