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</pre><pre class="rust ">
<span class="doccomment">//! Epoch-based memory management</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! This module provides fast, easy to use memory management for lock free data</span>
<span class="doccomment">//! structures. It's inspired by [Keir Fraser's *epoch-based</span>
<span class="doccomment">//! reclamation*](https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-579.pdf).</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! The basic problem this is solving is the fact that when one thread has</span>
<span class="doccomment">//! removed a node from a data structure, other threads may still have pointers</span>
<span class="doccomment">//! to that node (in the form of snapshots that will be validated through things</span>
<span class="doccomment">//! like compare-and-swap), so the memory cannot be immediately freed. Put differently:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! 1. There are two sources of reachability at play -- the data structure, and</span>
<span class="doccomment">//! the snapshots in threads accessing it. Before we delete a node, we need to know</span>
<span class="doccomment">//! that it cannot be reached in either of these ways.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! 2. Once a node has been unliked from the data structure, no *new* snapshots</span>
<span class="doccomment">//! reaching it will be created.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Using the epoch scheme is fairly straightforward, and does not require</span>
<span class="doccomment">//! understanding any of the implementation details:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! - When operating on a shared data structure, a thread must "pin the current</span>
<span class="doccomment">//! epoch", which is done by calling `pin()`. This function returns a `Guard`</span>
<span class="doccomment">//! which unpins the epoch when destroyed.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! - When the thread subsequently reads from a lock-free data structure, the</span>
<span class="doccomment">//! pointers it extracts act like references with lifetime tied to the</span>
<span class="doccomment">//! `Guard`. This allows threads to safely read from snapshotted data, being</span>
<span class="doccomment">//! guaranteed that the data will remain allocated until they exit the epoch.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! To put the `Guard` to use, Crossbeam provides a set of three pointer types meant to work together:</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! - `Owned<T>`, akin to `Box<T>`, which points to uniquely-owned data that has</span>
<span class="doccomment">//! not yet been published in a concurrent data structure.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! - `Shared<'a, T>`, akin to `&'a T`, which points to shared data that may or may</span>
<span class="doccomment">//! not be reachable from a data structure, but it guaranteed not to be freed</span>
<span class="doccomment">//! during lifetime `'a`.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! - `Atomic<T>`, akin to `std::sync::atomic::AtomicPtr`, which provides atomic</span>
<span class="doccomment">//! updates to a pointer using the `Owned` and `Shared` types, and connects them</span>
<span class="doccomment">//! to a `Guard`.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! Each of these types provides further documentation on usage.</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! # Example</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! ```</span>
<span class="doccomment">//! use std::sync::atomic::Ordering::{Acquire, Release, Relaxed};</span>
<span class="doccomment">//! use std::ptr;</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! use crossbeam::mem::epoch::{self, Atomic, Owned};</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! struct TreiberStack<T> {</span>
<span class="doccomment">//! head: Atomic<Node<T>>,</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! struct Node<T> {</span>
<span class="doccomment">//! data: T,</span>
<span class="doccomment">//! next: Atomic<Node<T>>,</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! impl<T> TreiberStack<T> {</span>
<span class="doccomment">//! fn new() -> TreiberStack<T> {</span>
<span class="doccomment">//! TreiberStack {</span>
<span class="doccomment">//! head: Atomic::null()</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! fn push(&self, t: T) {</span>
<span class="doccomment">//! // allocate the node via Owned</span>
<span class="doccomment">//! let mut n = Owned::new(Node {</span>
<span class="doccomment">//! data: t,</span>
<span class="doccomment">//! next: Atomic::null(),</span>
<span class="doccomment">//! });</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! // become active</span>
<span class="doccomment">//! let guard = epoch::pin();</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! loop {</span>
<span class="doccomment">//! // snapshot current head</span>
<span class="doccomment">//! let head = self.head.load(Relaxed, &guard);</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! // update `next` pointer with snapshot</span>
<span class="doccomment">//! n.next.store_shared(head, Relaxed);</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! // if snapshot is still good, link in the new node</span>
<span class="doccomment">//! match self.head.cas_and_ref(head, n, Release, &guard) {</span>
<span class="doccomment">//! Ok(_) => return,</span>
<span class="doccomment">//! Err(owned) => n = owned,</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! fn pop(&self) -> Option<T> {</span>
<span class="doccomment">//! // become active</span>
<span class="doccomment">//! let guard = epoch::pin();</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! loop {</span>
<span class="doccomment">//! // take a snapshot</span>
<span class="doccomment">//! match self.head.load(Acquire, &guard) {</span>
<span class="doccomment">//! // the stack is non-empty</span>
<span class="doccomment">//! Some(head) => {</span>
<span class="doccomment">//! // read through the snapshot, *safely*!</span>
<span class="doccomment">//! let next = head.next.load(Relaxed, &guard);</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! // if snapshot is still good, update from `head` to `next`</span>
<span class="doccomment">//! if self.head.cas_shared(Some(head), next, Release) {</span>
<span class="doccomment">//! unsafe {</span>
<span class="doccomment">//! // mark the node as unlinked</span>
<span class="doccomment">//! guard.unlinked(head);</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! // extract out the data from the now-unlinked node</span>
<span class="doccomment">//! return Some(ptr::read(&(*head).data))</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! // we observed the stack empty</span>
<span class="doccomment">//! None => return None</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! }</span>
<span class="doccomment">//! ```</span>
<span class="comment">// FIXME: document implementation details</span>
<span class="kw">mod</span> <span class="ident">atomic</span>;
<span class="kw">mod</span> <span class="ident">garbage</span>;
<span class="kw">mod</span> <span class="ident">global</span>;
<span class="kw">mod</span> <span class="ident">guard</span>;
<span class="kw">mod</span> <span class="ident">local</span>;
<span class="kw">mod</span> <span class="ident">participant</span>;
<span class="kw">mod</span> <span class="ident">participants</span>;
<span class="kw">pub</span> <span class="kw">use</span> <span class="self">self</span>::<span class="ident">atomic</span>::<span class="ident">Atomic</span>;
<span class="kw">pub</span> <span class="kw">use</span> <span class="self">self</span>::<span class="ident">guard</span>::{<span class="ident">pin</span>, <span class="ident">Guard</span>};
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">ops</span>::{<span class="ident">Deref</span>, <span class="ident">DerefMut</span>};
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">ptr</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">mem</span>;
<span class="doccomment">/// Like `Box<T>`: an owned, heap-allocated data value of type `T`.</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>)]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Owned</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="ident">data</span>: <span class="ident">Box</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
}
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Owned</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="doccomment">/// Move `t` to a new heap allocation.</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">new</span>(<span class="ident">t</span>: <span class="ident">T</span>) <span class="op">-></span> <span class="ident">Owned</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="ident">Owned</span> { <span class="ident">data</span>: <span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">t</span>) }
}
<span class="kw">fn</span> <span class="ident">as_raw</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">T</span> {
<span class="self">self</span>.<span class="ident">deref</span>() <span class="kw">as</span> <span class="kw-2">*</span><span class="kw">const</span> _ <span class="kw">as</span> <span class="kw-2">*</span><span class="kw-2">mut</span> _
}
<span class="doccomment">/// Move data out of the owned box, deallocating the box.</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">into_inner</span>(<span class="self">self</span>) <span class="op">-></span> <span class="ident">T</span> {
<span class="kw-2">*</span><span class="self">self</span>.<span class="ident">data</span>
}
}
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Deref</span> <span class="kw">for</span> <span class="ident">Owned</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">type</span> <span class="ident">Target</span> <span class="op">=</span> <span class="ident">T</span>;
<span class="kw">fn</span> <span class="ident">deref</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="ident">T</span> {
<span class="kw-2">&</span><span class="self">self</span>.<span class="ident">data</span>
}
}
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">DerefMut</span> <span class="kw">for</span> <span class="ident">Owned</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">deref_mut</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">T</span> {
<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>.<span class="ident">data</span>
}
}
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">PartialEq</span>, <span class="ident">Eq</span>)]</span>
<span class="doccomment">/// Like `&'a T`: a shared reference valid for lifetime `'a`.</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>)]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Shared</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="lifetime">'a</span><span class="op">></span> {
<span class="ident">data</span>: <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">T</span>,
}
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> <span class="ident">Copy</span> <span class="kw">for</span> <span class="ident">Shared</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> {}
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> <span class="ident">Clone</span> <span class="kw">for</span> <span class="ident">Shared</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">clone</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="ident">Shared</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> {
<span class="ident">Shared</span> { <span class="ident">data</span>: <span class="self">self</span>.<span class="ident">data</span> }
}
}
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> <span class="ident">Deref</span> <span class="kw">for</span> <span class="ident">Shared</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> {
<span class="kw">type</span> <span class="ident">Target</span> <span class="op">=</span> <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">T</span>;
<span class="kw">fn</span> <span class="ident">deref</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="op">&&</span><span class="lifetime">'a</span> <span class="ident">T</span> {
<span class="kw-2">&</span><span class="self">self</span>.<span class="ident">data</span>
}
}
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> <span class="ident">Shared</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> {
<span class="kw">unsafe</span> <span class="kw">fn</span> <span class="ident">from_raw</span>(<span class="ident">raw</span>: <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">T</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">Shared</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">>></span> {
<span class="kw">if</span> <span class="ident">raw</span> <span class="op">==</span> <span class="ident">ptr</span>::<span class="ident">null_mut</span>() { <span class="prelude-val">None</span> }
<span class="kw">else</span> {
<span class="prelude-val">Some</span>(<span class="ident">Shared</span> {
<span class="ident">data</span>: <span class="ident">mem</span>::<span class="ident">transmute</span>::<span class="op"><</span><span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">T</span>, <span class="kw-2">&</span><span class="ident">T</span><span class="op">></span>(<span class="ident">raw</span>)
})
}
}
<span class="kw">unsafe</span> <span class="kw">fn</span> <span class="ident">from_ref</span>(<span class="ident">r</span>: <span class="kw-2">&</span><span class="ident">T</span>) <span class="op">-></span> <span class="ident">Shared</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> {
<span class="ident">Shared</span> { <span class="ident">data</span>: <span class="ident">mem</span>::<span class="ident">transmute</span>(<span class="ident">r</span>) }
}
<span class="kw">unsafe</span> <span class="kw">fn</span> <span class="ident">from_owned</span>(<span class="ident">owned</span>: <span class="ident">Owned</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>) <span class="op">-></span> <span class="ident">Shared</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> {
<span class="kw">let</span> <span class="ident">ret</span> <span class="op">=</span> <span class="ident">Shared</span>::<span class="ident">from_ref</span>(<span class="ident">owned</span>.<span class="ident">deref</span>());
<span class="ident">mem</span>::<span class="ident">forget</span>(<span class="ident">owned</span>);
<span class="ident">ret</span>
}
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">as_raw</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">T</span> {
<span class="self">self</span>.<span class="ident">data</span> <span class="kw">as</span> <span class="kw-2">*</span><span class="kw">const</span> _ <span class="kw">as</span> <span class="kw-2">*</span><span class="kw-2">mut</span> _
}
}
<span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">test</span>)]</span>
<span class="kw">mod</span> <span class="ident">test</span> {
<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="kw">use</span> <span class="kw">super</span>::<span class="kw-2">*</span>;
<span class="kw">use</span> <span class="ident">mem</span>::<span class="ident">epoch</span>;
<span class="attribute">#[<span class="ident">test</span>]</span>
<span class="kw">fn</span> <span class="ident">test_no_drop</span>() {
<span class="kw">static</span> <span class="kw-2">mut</span> <span class="ident">DROPS</span>: <span class="ident">i32</span> <span class="op">=</span> <span class="number">0</span>;
<span class="kw">struct</span> <span class="ident">Test</span>;
<span class="kw">impl</span> <span class="ident">Drop</span> <span class="kw">for</span> <span class="ident">Test</span> {
<span class="kw">fn</span> <span class="ident">drop</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) {
<span class="kw">unsafe</span> {
<span class="ident">DROPS</span> <span class="op">+=</span> <span class="number">1</span>;
}
}
}
<span class="kw">let</span> <span class="ident">g</span> <span class="op">=</span> <span class="ident">pin</span>();
<span class="kw">let</span> <span class="ident">x</span> <span class="op">=</span> <span class="ident">Atomic</span>::<span class="ident">null</span>();
<span class="ident">x</span>.<span class="ident">store</span>(<span class="prelude-val">Some</span>(<span class="ident">Owned</span>::<span class="ident">new</span>(<span class="ident">Test</span>)), <span class="ident">Ordering</span>::<span class="ident">Relaxed</span>);
<span class="ident">x</span>.<span class="ident">store_and_ref</span>(<span class="ident">Owned</span>::<span class="ident">new</span>(<span class="ident">Test</span>), <span class="ident">Ordering</span>::<span class="ident">Relaxed</span>, <span class="kw-2">&</span><span class="ident">g</span>);
<span class="kw">let</span> <span class="ident">y</span> <span class="op">=</span> <span class="ident">x</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Relaxed</span>, <span class="kw-2">&</span><span class="ident">g</span>);
<span class="kw">let</span> <span class="ident">z</span> <span class="op">=</span> <span class="ident">x</span>.<span class="ident">cas_and_ref</span>(<span class="ident">y</span>, <span class="ident">Owned</span>::<span class="ident">new</span>(<span class="ident">Test</span>), <span class="ident">Ordering</span>::<span class="ident">Relaxed</span>, <span class="kw-2">&</span><span class="ident">g</span>).<span class="ident">ok</span>();
<span class="kw">let</span> _ <span class="op">=</span> <span class="ident">x</span>.<span class="ident">cas</span>(<span class="ident">z</span>, <span class="prelude-val">Some</span>(<span class="ident">Owned</span>::<span class="ident">new</span>(<span class="ident">Test</span>)), <span class="ident">Ordering</span>::<span class="ident">Relaxed</span>);
<span class="ident">x</span>.<span class="ident">swap</span>(<span class="prelude-val">Some</span>(<span class="ident">Owned</span>::<span class="ident">new</span>(<span class="ident">Test</span>)), <span class="ident">Ordering</span>::<span class="ident">Relaxed</span>, <span class="kw-2">&</span><span class="ident">g</span>);
<span class="kw">unsafe</span> {
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">DROPS</span>, <span class="number">0</span>);
}
}
<span class="attribute">#[<span class="ident">test</span>]</span>
<span class="kw">fn</span> <span class="ident">test_new</span>() {
<span class="kw">let</span> <span class="ident">guard</span> <span class="op">=</span> <span class="ident">epoch</span>::<span class="ident">pin</span>();
<span class="kw">let</span> <span class="ident">my_atomic</span> <span class="op">=</span> <span class="ident">Atomic</span>::<span class="ident">new</span>(<span class="number">42</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="kw-2">*</span><span class="kw-2">*</span><span class="ident">my_atomic</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">Relaxed</span>, <span class="kw-2">&</span><span class="ident">guard</span>).<span class="ident">unwrap</span>(), <span class="number">42</span>);
}
}
</pre>
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