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WITNESS(4) Device Drivers Manual WITNESS(4)

witness
lock validation facility

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.witnesswatch 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

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

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.
May 16, 2018 OpenBSD-current