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Add a dedicated sysctl(2) node for witness(4).

The new node contains the subsystem's main control variable,
kern.witness.watch. It is aliased by the old name, kern.witnesswatch.
The alias will be removed in the future.

OK anton@ mpi@

.\" $OpenBSD: witness.4,v 1.5 2019/01/29 14:07:15 visa Exp $
.\"
.\" Copyright (c) 2001 John H. Baldwin <jhb@FreeBSD.org>
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.\" $FreeBSD: head/share/man/man4/witness.4 301719 2016-06-09 06:55:00Z trasz $
.\"
.Dd $Mdocdate: January 29 2019 $
.Dt WITNESS 4
.Os
.Sh NAME
.Nm witness
.Nd lock validation facility
.Sh DESCRIPTION
The
.Nm
module keeps track of the locks acquired and released by each thread.
It also keeps track of the order in which locks are acquired with respect
to each other.
Each time a lock is acquired,
.Nm
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 printed to the
kernel console detailing the locks involved and the locations in question.
.Pp
The
.Nm
code also checks various other conditions such as verifying that one
does not recurse on a non-recursive lock,
or attempt an upgrade on a shared lock held by another thread.
If any of these checks fail, then the kernel will panic.
.Pp
The behaviour of
.Nm
is controlled by the
.Va kern.witness.watch
.Xr sysctl 8
variable.
Valid values for it are:
.Pp
.Bl -tag -width 1nr -compact -offset indent
.It \-1
disable
.Nm witness
completely
.It 0
disable error checking
.It 1
print a message if an error is detected
.It 2
print a message and a stack trace if possible
.It 3
like 2 but also drop into the kernel debugger
.El
.Sh SEE ALSO
.Xr ddb 4 ,
.Xr options 4 ,
.Xr sysctl 8 ,
.Xr mutex 9 ,
.Xr rwlock 9
.Sh HISTORY
The
.Nm
code first appeared in
.Bsx 5.0
and was imported from there into
.Fx 5.0 .
The
.Ox
port was derived from
.Fx
and first appeared in
.Ox 6.2 .