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WITNESS(4)		 BSD Kernel Interfaces Manual		    WITNESS(4)

     witness --	lock validation	facility

     options WITNESS
     options WITNESS_KDB

     The witness 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,	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 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 debug-
     ger 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,	or attempt an upgrade
     on	a shared lock held by another thread.  If any of these checks fail,
     then the kernel 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 option is speci-
     fied, 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	mu-
     texes.  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 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 can	be set via loader(8).

     The witness code also provides three extra	ddb(4) commands	if both
     witness and ddb(4)	are compiled into the kernel:

     show locks	[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 thread argument may be either a TID, PID, or
     pointer to	a thread structure.  If	thread is not specified, then the
     locks held	by the current thread are displayed.

     show all locks
     Outputs the list of locks held by all threads in the system to the	kernel

     show witness
     Dump the current order list to the	kernel console.	 The code first	dis-
     plays 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.

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

     The witness code first appeared in	BSD/OS 5.0 and was imported from there
     into FreeBSD 5.0.

BSD				 May 30, 2012				   BSD


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