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<p class='location'>Module epoch</p><div class="sidebar-elems"><div class="block items"><ul><li><a href="#structs">Structs</a></li><li><a href="#functions">Functions</a></li></ul></div><p class='location'><a href='../index.html'>crossbeam</a></p><script>window.sidebarCurrent = {name: 'epoch', ty: 'mod', relpath: '../'};</script><script defer src="../sidebar-items.js"></script></div>
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<h1 class='fqn'><span class='in-band'>Module <a href='../index.html'>crossbeam</a>::<wbr><a class="mod" href=''>epoch</a></span><span class='out-of-band'><span id='render-detail'>
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<a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">
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[<span class='inner'>−</span>]
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</a>
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</span><a class='srclink' href='../../src/crossbeam/epoch/mod.rs.html#1-265' title='goto source code'>[src]</a></span></h1>
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<div class='docblock'><p>Epoch-based memory management</p>
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<p>This module provides fast, easy to use memory management for lock free data
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structures. It's inspired by <a href="https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-579.pdf">Keir Fraser's <em>epoch-based
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reclamation</em></a>.</p>
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<p>The basic problem this is solving is the fact that when one thread has
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removed a node from a data structure, other threads may still have pointers
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to that node (in the form of snapshots that will be validated through things
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like compare-and-swap), so the memory cannot be immediately freed. Put differently:</p>
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<ol>
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<li>
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<p>There are two sources of reachability at play -- the data structure, and
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the snapshots in threads accessing it. Before we delete a node, we need to know
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that it cannot be reached in either of these ways.</p>
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</li>
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<li>
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<p>Once a node has been unlinked from the data structure, no <em>new</em> snapshots
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reaching it will be created.</p>
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</li>
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</ol>
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<p>Using the epoch scheme is fairly straightforward, and does not require
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understanding any of the implementation details:</p>
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<ul>
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<li>
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<p>When operating on a shared data structure, a thread must "pin the current
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epoch", which is done by calling <code>pin()</code>. This function returns a <code>Guard</code>
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which unpins the epoch when destroyed.</p>
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</li>
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<li>
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<p>When the thread subsequently reads from a lock-free data structure, the
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pointers it extracts act like references with lifetime tied to the
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<code>Guard</code>. This allows threads to safely read from snapshotted data, being
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guaranteed that the data will remain allocated until they exit the epoch.</p>
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</li>
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</ul>
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<p>To put the <code>Guard</code> to use, Crossbeam provides a set of three pointer types meant to work together:</p>
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<ul>
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<li>
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<p><code>Owned<T></code>, akin to <code>Box<T></code>, which points to uniquely-owned data that has
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not yet been published in a concurrent data structure.</p>
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</li>
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<li>
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<p><code>Shared<'a, T></code>, akin to <code>&'a T</code>, which points to shared data that may or may
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not be reachable from a data structure, but it guaranteed not to be freed
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during lifetime <code>'a</code>.</p>
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</li>
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<li>
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<p><code>Atomic<T></code>, akin to <code>std::sync::atomic::AtomicPtr</code>, which provides atomic
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updates to a pointer using the <code>Owned</code> and <code>Shared</code> types, and connects them
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to a <code>Guard</code>.</p>
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</li>
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</ul>
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<p>Each of these types provides further documentation on usage.</p>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">sync</span>::<span class="ident">atomic</span>::<span class="ident">Ordering</span>::{<span class="ident">Acquire</span>, <span class="ident">Release</span>, <span class="ident">Relaxed</span>};
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">ptr</span>;
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<span class="kw">use</span> <span class="ident">crossbeam</span>::<span class="ident">epoch</span>::{<span class="self">self</span>, <span class="ident">Atomic</span>, <span class="ident">Owned</span>};
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<span class="kw">struct</span> <span class="ident">TreiberStack</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="ident">head</span>: <span class="ident">Atomic</span><span class="op"><</span><span class="ident">Node</span><span class="op"><</span><span class="ident">T</span><span class="op">>></span>,
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}
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<span class="kw">struct</span> <span class="ident">Node</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="ident">data</span>: <span class="ident">T</span>,
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<span class="ident">next</span>: <span class="ident">Atomic</span><span class="op"><</span><span class="ident">Node</span><span class="op"><</span><span class="ident">T</span><span class="op">>></span>,
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}
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">TreiberStack</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="kw">fn</span> <span class="ident">new</span>() <span class="op">-></span> <span class="ident">TreiberStack</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="ident">TreiberStack</span> {
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<span class="ident">head</span>: <span class="ident">Atomic</span>::<span class="ident">null</span>()
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}
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}
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<span class="kw">fn</span> <span class="ident">push</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">t</span>: <span class="ident">T</span>) {
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<span class="comment">// allocate the node via Owned</span>
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">n</span> <span class="op">=</span> <span class="ident">Owned</span>::<span class="ident">new</span>(<span class="ident">Node</span> {
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<span class="ident">data</span>: <span class="ident">t</span>,
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<span class="ident">next</span>: <span class="ident">Atomic</span>::<span class="ident">null</span>(),
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});
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<span class="comment">// become active</span>
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<span class="kw">let</span> <span class="ident">guard</span> <span class="op">=</span> <span class="ident">epoch</span>::<span class="ident">pin</span>();
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<span class="kw">loop</span> {
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<span class="comment">// snapshot current head</span>
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<span class="kw">let</span> <span class="ident">head</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">head</span>.<span class="ident">load</span>(<span class="ident">Relaxed</span>, <span class="kw-2">&</span><span class="ident">guard</span>);
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<span class="comment">// update `next` pointer with snapshot</span>
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<span class="ident">n</span>.<span class="ident">next</span>.<span class="ident">store_shared</span>(<span class="ident">head</span>, <span class="ident">Relaxed</span>);
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<span class="comment">// if snapshot is still good, link in the new node</span>
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<span class="kw">match</span> <span class="self">self</span>.<span class="ident">head</span>.<span class="ident">cas_and_ref</span>(<span class="ident">head</span>, <span class="ident">n</span>, <span class="ident">Release</span>, <span class="kw-2">&</span><span class="ident">guard</span>) {
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<span class="prelude-val">Ok</span>(_) <span class="op">=></span> <span class="kw">return</span>,
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<span class="prelude-val">Err</span>(<span class="ident">owned</span>) <span class="op">=></span> <span class="ident">n</span> <span class="op">=</span> <span class="ident">owned</span>,
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}
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}
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}
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<span class="kw">fn</span> <span class="ident">pop</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="comment">// become active</span>
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<span class="kw">let</span> <span class="ident">guard</span> <span class="op">=</span> <span class="ident">epoch</span>::<span class="ident">pin</span>();
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<span class="kw">loop</span> {
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<span class="comment">// take a snapshot</span>
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<span class="kw">match</span> <span class="self">self</span>.<span class="ident">head</span>.<span class="ident">load</span>(<span class="ident">Acquire</span>, <span class="kw-2">&</span><span class="ident">guard</span>) {
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<span class="comment">// the stack is non-empty</span>
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<span class="prelude-val">Some</span>(<span class="ident">head</span>) <span class="op">=></span> {
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<span class="comment">// read through the snapshot, *safely*!</span>
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<span class="kw">let</span> <span class="ident">next</span> <span class="op">=</span> <span class="ident">head</span>.<span class="ident">next</span>.<span class="ident">load</span>(<span class="ident">Relaxed</span>, <span class="kw-2">&</span><span class="ident">guard</span>);
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<span class="comment">// if snapshot is still good, update from `head` to `next`</span>
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<span class="kw">if</span> <span class="self">self</span>.<span class="ident">head</span>.<span class="ident">cas_shared</span>(<span class="prelude-val">Some</span>(<span class="ident">head</span>), <span class="ident">next</span>, <span class="ident">Release</span>) {
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<span class="kw">unsafe</span> {
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<span class="comment">// mark the node as unlinked</span>
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<span class="ident">guard</span>.<span class="ident">unlinked</span>(<span class="ident">head</span>);
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<span class="comment">// extract out the data from the now-unlinked node</span>
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<span class="kw">return</span> <span class="prelude-val">Some</span>(<span class="ident">ptr</span>::<span class="ident">read</span>(<span class="kw-2">&</span>(<span class="kw-2">*</span><span class="ident">head</span>).<span class="ident">data</span>))
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}
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}
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}
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<span class="comment">// we observed the stack empty</span>
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<span class="prelude-val">None</span> <span class="op">=></span> <span class="kw">return</span> <span class="prelude-val">None</span>
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}
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}
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}
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}</pre>
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</div><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
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<table>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.Atomic.html"
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title='struct crossbeam::epoch::Atomic'>Atomic</a></td>
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<td class='docblock-short'>
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<p>Like <code>std::sync::atomic::AtomicPtr</code>.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.Guard.html"
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title='struct crossbeam::epoch::Guard'>Guard</a></td>
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<td class='docblock-short'>
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<p>An RAII-style guard for pinning the current epoch.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.Owned.html"
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title='struct crossbeam::epoch::Owned'>Owned</a></td>
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<td class='docblock-short'>
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<p>Like <code>Box<T></code>: an owned, heap-allocated data value of type <code>T</code>.</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.Shared.html"
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title='struct crossbeam::epoch::Shared'>Shared</a></td>
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<td class='docblock-short'>
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<p>Like <code>&'a T</code>: a shared reference valid for lifetime <code>'a</code>.</p>
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</td>
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</tr></table><h2 id='functions' class='section-header'><a href="#functions">Functions</a></h2>
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<table>
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<tr class=' module-item'>
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<td><a class="fn" href="fn.pin.html"
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title='fn crossbeam::epoch::pin'>pin</a></td>
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<td class='docblock-short'>
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<p>Pin the current epoch.</p>
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</td>
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