Add support for a configurable output channel to witness(4).

This is useful in environments where system configuration is performed by
automated interaction with the system console, since unexpected witness
output makes such automation difficult. With this change, the new
debug.witness.output_channel sysctl allows one to specify that witness
output is to be printed to the kernel log (using log(9)) rather than the
console.

Reviewed by:	cem, jhb
MFC after:	2 weeks
Relnotes:	yes
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D4183

1 files changed, 18 insertions, 2 deletions

diff --git a/share/man/man4/witness.4 b/share/man/man4/witness.4
index aec7803..02505aa 100644
--- a/share/man/man4/witness.4
+++ b/share/man/man4/witness.4
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 24, 2015
+.Dd November 18, 2015
 .Dt WITNESS 4
 .Os
 .Sh NAME
@@ -47,7 +47,8 @@ Each time a lock is acquired,
 uses these two lists to verify that a lock is not being acquired in the
 wrong order.
 If a lock order violation is detected, then a message is output to the
-kernel console detailing the locks involved and the locations in question.
+kernel console or log detailing the locks involved and the locations in
+question.
 Witness can also be configured to drop into the kernel debugger when an order
 violation occurs.
 .Pp
@@ -125,6 +126,21 @@ The sysctl
 can be set via
 .Xr loader 8 .
 .Pp
+The sysctl
+.Va debug.witness.output_channel
+specifies the output channel used to display warnings emitted by
+.Nm .
+The possible values are
+.Ql console ,
+indicating that warnings are to be printed to the system console,
+.Ql log ,
+indicating that warnings are to be logged via
+.Xr log 9 ,
+and
+.Ql none .
+This sysctl can be set via
+.Xr loader 8 .
+.Pp
 The
 .Nm
 code also provides three extra