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
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.
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.
code first appeared in
and was imported from there into
. The OpenBSD
was derived from FreeBSD
and first appeared in