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.\" $OpenBSD: witness.4,v 1.4 2018/05/16 14:58:48 visa Exp $
.\"
.\" Copyright (c) 2001 John H. Baldwin <jhb@FreeBSD.org>
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" $FreeBSD: head/share/man/man4/witness.4 301719 2016-06-09 06:55:00Z trasz $
.\"
.Dd $Mdocdate: May 16 2018 $
.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.witnesswatch
.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 .
|
