--- name: "witness" section: "4" platform: "freebsd" package: "base" version: "16.0-CURRENT-20261027-61c70dc16a79" release: "FreeBSD 16.0-CURRENT (2026-10-27)" platforms: ["linux", "freebsd", "openbsd"] url: "https://new.carta.tech/witness" --- # witness(4) lock validation facility ## NAME **witness** – lock validation facility ## SYNOPSIS **options** **WITNESS** **options** **WITNESS\_COUNT** **options** **WITNESS\_LOCK\_CHILDCOUNT=number** **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.witness\_count* sysctl. It can also be set from the [loader(8)](/loader/8.md) via the *debug.witness.witness\_count* environment variable. The WITNESS\_LOCK\_CHILDCOUNT kernel option specifies the number of entries in the statically allocated pool used by **witness** to record lock-order relationships. The value must be greater than zero. The default is 2048. Increasing this value permits more relationships to be tracked at the cost of additional kernel memory. The WITNESS\_NO\_VNODE kernel option tells **witness** to ignore locking issues between [vnode(9)](/vnode/9.md) 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)](/loader/8.md) 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)](/loader/8.md) 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)](/loader/8.md). The sysctl *debug.witness.trace* controls whether **witness** prints stack traces when it detects lock violations. A value of 0 disables stack printing. A value of 1 specifies that **witness** should print a stack trace when a lock hierarchy violation occurs or non-sleepable locks are held when going to sleep or acquiring a sleepable lock. A value of 2 specifies that **witness** should attempt to display all observed lock chains that contribute to the established lock order, along with stack traces for when those locks were first acquired. The sysctl *debug.witness.trace* can be set via [loader(8)](/loader/8.md). 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)](/log/9.md), and ‘none’. This sysctl can be set via [loader(8)](/loader/8.md). The sysctl *debug.witness.badstacks* displays a listing of detected lock order violations cached in the **witness** module's current view of the lock hierarchy. (This means that it may not display information for locks which have been destroyed.) It displays a similar level of detail to the messages produced by the run-time checks. However, it always tries to show all observed lock chains that contribute to the established lock order. (In other words, it acts like *debug.witness.trace* was set to 2.) It uses '\*\*' to mark lock orders which were attempted but not added to the hierarchy because they violated the hierarchy the **witness** code had previously observed. The **witness** code also provides a few extra [ddb(4)](/ddb/4.md) commands if both **witness** and [ddb(4)](/ddb/4.md) 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** Dumps 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. - **show** **badstacks** Displays a listing of all WITNESS lock order violations. This listing is identical to that produced by the *debug.witness.badstacks* sysctl. ## SEE ALSO [ddb(4)](/ddb/4.md), [loader(8)](/loader/8.md), [sysctl(8)](/sysctl/8.md), [mutex(9)](/mutex/9.md) ## HISTORY The **witness** code first appeared in BSD/OS 5.0 and was imported from there into FreeBSD 5.0.