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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.	 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 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.  This sysctl can be written to
     in	order to disable witness, however it may not be	used to	enable wit-
     ness.  The	sysctl can be set via loader(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	con-
     sole 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	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.

     The witness code currently	does not handle	recursion of shared sx(9)
     locks properly.

BSD			       February	18, 2001			   BSD


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