.\" Copyright (c) 2001 John H. Baldwin <jhb@FreeBSD.org>
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.Dd May 30, 2012
.Dt WITNESS 4
.Os
.Sh NAME
.Nm witness
.Nd lock validation facility
.Sh SYNOPSIS
.Cd options WITNESS
.Cd options WITNESS_KDB
.Cd options WITNESS_SKIPSPIN
.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 output to the
kernel console 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
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 flag that controls whether or not the kernel debugger is entered when a
lock order violation is detected can be set in a variety of ways.
By default, the flag is off, but if the
.Dv WITNESS_KDB
kernel option is
specified, then the flag will default to on.
It can also be set from the
.Xr loader 8
via the
.Va debug.witness.kdb
environment variable or after the kernel has booted via the
.Va debug.witness.kdb
sysctl.
If the flag is set to zero, then the debugger will not be entered.
If the flag is non-zero, then the debugger will be entered.
.Pp
The
.Nm
code can also be configured to skip all checks on spin mutexes.
By default, this flag defaults to off, but it can be turned on by
specifying the
.Dv WITNESS_SKIPSPIN
kernel option.
The flag can also be set via the
.Xr loader 8
environment variable
.Va debug.witness.skipspin .
If the variable is set to a non-zero value, then spin mutexes are skipped.
Once the kernel has booted, the status of this flag can be examined but not
set via the read-only sysctl
.Va debug.witness.skipspin .
.Pp
The sysctl
.Va debug.witness.watch
specifies the level of witness involvement in the system.
A value of 1 specifies that witness is enabled.
A value of 0 specifies that witness is disabled, but that can be enabled
again.  This will maintain a small amount of overhead in the system.
A value of -1 specifies that witness is disabled permanently and
cannot be enabled again.
The sysctl
.Va debug.witness.watch
can be set via
.Xr loader 8 .
.Pp
The
.Nm
code also provides two extra
.Xr ddb 4
commands if both
.Nm
and
.Xr ddb 4
are compiled into the kernel:
.Bl -ohang
.It Ic show locks Op thread
Outputs the list of locks held by a thread to the kernel console
along with the filename and line number at which each lock was last acquired
by the thread.
The optional
.Ar thread
argument may be either a TID,
PID,
or pointer to a thread structure.
If
.Ar thread
is not specified,
then the locks held by the current thread are displayed.
.It Ic show all locks
Outputs the list of locks held by all threads in the system to the
kernel console.
.It Ic show witness
Dump the current order list to the kernel console.
The code first displays the lock order tree for all of the sleep locks.
Then it displays the lock order tree for all of the spin locks.
Finally, it displays a list of locks that have not yet been acquired.
.El
.Sh SEE ALSO
.Xr ddb 4 ,
.Xr loader 8 ,
.Xr sysctl 8 ,
.Xr mutex 9
.Sh HISTORY
The
.Nm
code first appeared in
.Bsx 5.0
and was imported from there into
.Fx 5.0 .