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
witnesscompletely - 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
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.