witness(4)

NAME

witness - lock validation facility

SYNOPSIS

options WITNESS
options WITNESS_KDB
options WITNESS_SKIPSPIN

DESCRIPTION

The witness module keeps track of the locks acquired and re

leased 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,

witness 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 mes

sage 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.

The witness code also checks various other conditions such

as verifying
that one does not recurse on a non-recursive lock. For

sleep locks,
witness verifies that a new process would not be switched to

when a lock
is released or a lock is blocked on during an acquire while

any spin
locks are held. If any of these checks fail, then the ker

nel will panic.

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 WITNESS_KDB kernel op

tion is specified, then the flag will default to on. It can also be set

from the
loader(8) via the debug.witness.kdb environment variable or

after the
kernel has booted via the 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.

The witness 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 WITNESS_SKIPSPIN kernel option. The flag

can also be
set via the loader(8) environment variable

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 debug.witness.skipspin.

The sysctl debug.witness.watch specifies the level of wit

ness involvement
in the system. A value of 1 specifies that witness is en

abled. A value
of 0 specifies that witness is disabled. This sysctl can be

written to
in order to disable witness, however it may not be used to

enable witness. The sysctl debug.witness.watch can be set via load

er(8).

The witness code also provides two extra ddb(4) commands if

both witness
and ddb(4) are compiled into the kernel:

show locks
Outputs the list of locks held by the current thread to the

kernel console along with the filename and line number at which each

lock was last
acquired by this thread.

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.

SEE ALSO

ddb(4), loader(8), sysctl(8), mutex(9)

HISTORY

The witness code first appeared in BSD/OS 5.0 and was im

ported from there
into FreeBSD 5.0.

BUGS

The witness code currently does not handle recursion of

shared sx(9)
locks properly.

BSD February 18, 2001

BSD