WITNESS(4) | Device Drivers Manual | WITNESS(4)

# NAME

`witness` — lock validation facility

# 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 printed to the kernel console detailing the locks involved and the locations in question.

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 behaviour of `witness` is controlled by the kern.witness.watch sysctl(8) variable. Valid values for it are:

-1
  disable `witness` completely
0
  disable error checking
1
  print a message if an error is detected
2
  print a message and a stack trace if possible
3
  like 2 but also drop into the kernel debugger

# SEE ALSO

ddb(4), options(4), sysctl(8), mutex(9), rwlock(9)

# HISTORY

The `witness` code first appeared in BSD/OS 5.0 and was imported from there into FreeBSD 5.0. The OpenBSD port was derived from FreeBSD and first appeared in OpenBSD 6.2.

January 29, 2019 | Debian