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     witness — lock validation facility


     options WITNESS
     options WITNESS_COUNT
     options WITNESS_KDB
     options WITNESS_NO_VNODE


     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)

     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 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 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.


     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.