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<section id="file-locking-release-notes">
<h1>File Locking Release Notes<a class="headerlink" href="#file-locking-release-notes" title="Link to this heading">¶</a></h1>
<blockquote>
<div><p>Andy Walker <<a class="reference external" href="mailto:andy%40lysaker.kvaerner.no">andy<span>@</span>lysaker<span>.</span>kvaerner<span>.</span>no</a>></p>
<blockquote>
<div><p>12 May 1997</p>
</div></blockquote>
</div></blockquote>
<section id="what-s-new">
<h2>1. What’s New?<a class="headerlink" href="#what-s-new" title="Link to this heading">¶</a></h2>
<section id="broken-flock-emulation">
<h3>1.1 Broken Flock Emulation<a class="headerlink" href="#broken-flock-emulation" title="Link to this heading">¶</a></h3>
<p>The old flock(2) emulation in the kernel was swapped for proper BSD
compatible flock(2) support in the 1.3.x series of kernels. With the
release of the 2.1.x kernel series, support for the old emulation has
been totally removed, so that we don’t need to carry this baggage
forever.</p>
<p>This should not cause problems for anybody, since everybody using a
2.1.x kernel should have updated their C library to a suitable version
anyway (see the file “<a class="reference internal" href="../process/changes.html"><span class="doc">Minimal requirements to compile the Kernel</span></a>”.)</p>
</section>
<section id="allow-mixed-locks-again">
<h3>1.2 Allow Mixed Locks Again<a class="headerlink" href="#allow-mixed-locks-again" title="Link to this heading">¶</a></h3>
<section id="typical-problems-sendmail">
<h4>1.2.1 Typical Problems - Sendmail<a class="headerlink" href="#typical-problems-sendmail" title="Link to this heading">¶</a></h4>
<p>Because sendmail was unable to use the old <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">flock()</span></code> emulation, many sendmail
installations use fcntl() instead of <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">flock()</span></code>. This is true of Slackware 3.0
for example. This gave rise to some other subtle problems if sendmail was
configured to rebuild the alias file. Sendmail tried to lock the aliases.dir
file with fcntl() at the same time as the GDBM routines tried to lock this
file with <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">flock()</span></code>. With pre 1.3.96 kernels this could result in deadlocks that,
over time, or under a very heavy mail load, would eventually cause the kernel
to lock solid with deadlocked processes.</p>
</section>
<section id="the-solution">
<h4>1.2.2 The Solution<a class="headerlink" href="#the-solution" title="Link to this heading">¶</a></h4>
<p>The solution I have chosen, after much experimentation and discussion,
is to make <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">flock()</span></code> and fcntl() locks oblivious to each other. Both can
exists, and neither will have any effect on the other.</p>
<p>I wanted the two lock styles to be cooperative, but there were so many
race and deadlock conditions that the current solution was the only
practical one. It puts us in the same position as, for example, SunOS
4.1.x and several other commercial Unices. The only OS’s that support
cooperative <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">flock()</span></code>/fcntl() are those that emulate <code class="xref c c-func broken_xref docutils literal notranslate"><span class="pre">flock()</span></code> using
fcntl(), with all the problems that implies.</p>
</section>
</section>
<section id="mandatory-locking-as-a-mount-option">
<h3>1.3 Mandatory Locking As A Mount Option<a class="headerlink" href="#mandatory-locking-as-a-mount-option" title="Link to this heading">¶</a></h3>
<p>Mandatory locking was prior to this release a general configuration option
that was valid for all mounted filesystems. This had a number of inherent
dangers, not the least of which was the ability to freeze an NFS server by
asking it to read a file for which a mandatory lock existed.</p>
<p>Such option was dropped in Kernel v5.14.</p>
</section>
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