NAME

     witness — lock validation facility

SYNOPSIS

     options WITNESS
     options WITNESS_COUNT
     options WITNESS_KDB
     options WITNESS_NO_VNODE
     options WITNESS_SKIPSPIN

DESCRIPTION

     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 or log 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, or attempt an upgrade on a shared lock held by another thread.  If any of these checks
     fail, then the kernel will panic.

     The WITNESS_COUNT kernel option controls the maximum number of witness entries that are tracked in the
     kernel.  The maximum number of entries can be queried via the debug.witness.count sysctl.  It can also be
     set from the loader(8) via the debug.witness.count environment variable.

     The WITNESS_NO_VNODE kernel option tells witness to ignore locking issues between vnode(9) objects.

     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 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 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 debug.witness.watch can be set via
     loader(8).

     The sysctl debug.witness.output_channel specifies the output channel used to display warnings emitted by
     witness.  The possible values are ‘console’, indicating that warnings are to be printed to the system
     console, ‘log’, indicating that warnings are to be logged via log(9), and ‘none’.  This 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 console.

     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 imported from there into FreeBSD 5.0.