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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 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
     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 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
     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.  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 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
     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 imported from there
     into FreeBSD 5.0.

BUGS

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