McDermott, J. and S. Jajodia, "Orange Locking: Channel-Free
   Database Concurrency Control via Locking", presented at 6th IFIP
   Working Conference on Database Security, August 1992, Vancouver,
   British Columbia, pp. 271-288.
   <a href="1992mcdermott-ifip.ps">
    PostScript</a>

<blockquote>
The concurrency control lock (e.g. file lock, table lock) has long been
used as a canonical example of a covert channel in a database system.
Locking is a fundamental concurrency control technique used in many
kinds of computer systems beside database systems. Locking is generally
considered to be interfering and hence unsuitable for multilevel
systems. In this paper we show how such locks can be used for
concurrency control, without introducing covert channels.
</blockquote>