When witness(4) detects lock order reversals (LORs), it prints
information about the stack trace which caused the LOR. If available,
it can also print information about the first stack trace which
established the other lock ordering. However, it only does this for
"simple" LORs where the two locks in question were directly locked
in the opposite order. When the lock order was established through
a more complex pattern of intermediate locks, WITNESS only prints
the stack trace where it detected the LOR.
This commit provides new functionality to provide more verbose
information about the lock chain(s) which established the lock
ordering. The new functionality can be disabled by setting the
debug.witness.trace sysctl/tunable to 1. The new functionality
is also available through the debug.witness.badstacks sysctl,
which has been modified to always show the more verbose
information.
Reviewed by: markj, glebius (previous version), kib (previous version)
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D54785
MFC after: 1 month
Jonathan T. Looney
fb4b0c9119