<qandaentry>

        <question id="lock-order-reversal">

          <para>What is a <errorname>lock order reversal</errorname>?</para>

        </question>

        <answer>

          <para>&a.rwatson; answered this question very succinctly on

            the freebsd-current list in a thread entitled <quote><ulink

            url="http://docs.freebsd.org/cgi/getmsg.cgi?fetch=65165+0+/usr/local/www/db/text/2003/freebsd-current/20031221.freebsd-current">lock

            order reversals - what do they mean?</ulink></quote></para>

          <blockquote>

            <attribution>&a.rwatson; on freebsd-current, December 14,

              2003</attribution>

            <para>These warnings are generated by Witness, a run-time lock

              diagnostic system found in FreeBSD 5-CURRENT kernels (but

              removed in releases).  You can read more about Witness in the

              &man.witness.4; man page, which talks about its capabilities.  Among

              other things, Witness performs run-time lock order verification

              using a combination of hard coded lock orders, and run-time

              detected lock orders, and generates console warnings when lock

              orders are violated.  The intent of this is to detect the

              potential for deadlocks due to lock order violations; it's worth

              observing that Witness is actually slightly conservative, and so

              it's possible to get false positives.  In the event that Witness

              is accurately reporting a lock order problem, it's basically

              saying "If you were unlucky, a deadlock would have happened

              here".  There are a couple of "well known" false positives,

              which we need to do a better job of documenting to prevent

              spurious reports.  The non-well-known ones typically correspond

              to bugs in newly added locking, as lock order reversals usually

              get fixed pretty quickly because Witness is busy generating

              warnings :-).</para>

          </blockquote>

          <note>

            <para>See <ulink

              url="http://sources.zabbadoz.net/freebsd/lor.html">Bjoern

              Zeeb's lock order reversal page</ulink> for the status of

              known lock order reversals.</para>

          </note>

        </answer>

      </qandaentry>