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</pre><pre class="rust ">
<span class="attribute">#<span class="op">!</span>[<span class="ident">allow</span>(<span class="ident">dead_code</span>)]</span>
<span class="kw">use</span> <span class="kw">super</span>::<span class="ident">Task</span>;
<span class="kw">use</span> <span class="ident">core</span>::<span class="ident">fmt</span>;
<span class="kw">use</span> <span class="ident">core</span>::<span class="ident">cell</span>::<span class="ident">UnsafeCell</span>;
<span class="kw">use</span> <span class="ident">core</span>::<span class="ident">sync</span>::<span class="ident">atomic</span>::<span class="ident">AtomicUsize</span>;
<span class="kw">use</span> <span class="ident">core</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="doccomment">/// A coordinated `Task` handle enabling concurrent operations on a task.</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">AtomicTask</span> {
<span class="ident">state</span>: <span class="ident">AtomicUsize</span>,
<span class="ident">task</span>: <span class="ident">UnsafeCell</span><span class="op"><</span><span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">Task</span><span class="op">>></span>,
}
<span class="doccomment">/// Initial state, the `AtomicTask` is currently not being used.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The value `2` is picked specifically because it between the write lock &</span>
<span class="doccomment">/// read lock values. Since the read lock is represented by an incrementing</span>
<span class="doccomment">/// counter, this enables an atomic fetch_sub operation to be used for releasing</span>
<span class="doccomment">/// a lock.</span>
<span class="kw">const</span> <span class="ident">WAITING</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">2</span>;
<span class="doccomment">/// The `park` function has determined that the task is no longer current. This</span>
<span class="doccomment">/// implies that `AtomicTask::park` is being called from a different task than</span>
<span class="doccomment">/// is represented by the currently stored task. The write lock is obtained to</span>
<span class="doccomment">/// update the task cell.</span>
<span class="kw">const</span> <span class="ident">LOCKED_WRITE</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">0</span>;
<span class="doccomment">/// At least one call to `notify` happened concurrently to `park` updating the</span>
<span class="doccomment">/// task cell. This state is detected when `park` exits the mutation code and</span>
<span class="doccomment">/// signals to `park` that it is responsible for notifying its own task.</span>
<span class="kw">const</span> <span class="ident">LOCKED_WRITE_NOTIFIED</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">1</span>;
<span class="doccomment">/// The `notify` function has locked access to the task cell for notification.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The constant is left here mostly for documentation reasons.</span>
<span class="attribute">#[<span class="ident">allow</span>(<span class="ident">dead_code</span>)]</span>
<span class="kw">const</span> <span class="ident">LOCKED_READ</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">3</span>;
<span class="kw">impl</span> <span class="ident">AtomicTask</span> {
<span class="doccomment">/// Create an `AtomicTask` initialized with the given `Task`</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">new</span>() <span class="op">-></span> <span class="ident">AtomicTask</span> {
<span class="comment">// Make sure that task is Sync</span>
<span class="kw">fn</span> <span class="ident">is_sync</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Sync</span><span class="op">></span>() {}
<span class="ident">is_sync</span>::<span class="op"><</span><span class="ident">Task</span><span class="op">></span>();
<span class="ident">AtomicTask</span> {
<span class="ident">state</span>: <span class="ident">AtomicUsize</span>::<span class="ident">new</span>(<span class="ident">WAITING</span>),
<span class="ident">task</span>: <span class="ident">UnsafeCell</span>::<span class="ident">new</span>(<span class="prelude-val">None</span>),
}
}
<span class="doccomment">/// The caller must ensure mutual exclusion</span>
<span class="kw">pub</span> <span class="kw">unsafe</span> <span class="kw">fn</span> <span class="ident">park</span>(<span class="kw-2">&</span><span class="self">self</span>) {
<span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="kw-2">ref</span> <span class="ident">task</span>) <span class="op">=</span> <span class="kw-2">*</span><span class="self">self</span>.<span class="ident">task</span>.<span class="ident">get</span>() {
<span class="kw">if</span> <span class="ident">task</span>.<span class="ident">will_notify_current</span>() {
<span class="comment">// Nothing more to do</span>
<span class="kw">return</span>
}
}
<span class="comment">// Get a new task handle</span>
<span class="kw">let</span> <span class="ident">task</span> <span class="op">=</span> <span class="kw">super</span>::<span class="ident">current</span>();
<span class="kw">match</span> <span class="self">self</span>.<span class="ident">state</span>.<span class="ident">compare_and_swap</span>(<span class="ident">WAITING</span>, <span class="ident">LOCKED_WRITE</span>, <span class="ident">Acquire</span>) {
<span class="ident">WAITING</span> <span class="op">=></span> {
<span class="comment">// Locked acquired, update the task cell</span>
<span class="kw-2">*</span><span class="self">self</span>.<span class="ident">task</span>.<span class="ident">get</span>() <span class="op">=</span> <span class="prelude-val">Some</span>(<span class="ident">task</span>);
<span class="comment">// Release the lock. If the state transitioned to</span>
<span class="comment">// `LOCKED_NOTIFIED`, this means that an notify has been</span>
<span class="comment">// signaled, so notify the task.</span>
<span class="kw">if</span> <span class="ident">LOCKED_WRITE_NOTIFIED</span> <span class="op">==</span> <span class="self">self</span>.<span class="ident">state</span>.<span class="ident">swap</span>(<span class="ident">WAITING</span>, <span class="ident">Release</span>) {
(<span class="kw-2">*</span><span class="self">self</span>.<span class="ident">task</span>.<span class="ident">get</span>()).<span class="ident">as_ref</span>().<span class="ident">unwrap</span>().<span class="ident">notify</span>();
}
}
<span class="ident">state</span> <span class="op">=></span> {
<span class="macro">debug_assert</span><span class="macro">!</span>(<span class="ident">state</span> <span class="op">!=</span> <span class="ident">LOCKED_WRITE</span>, <span class="string">"unexpected state LOCKED_WRITE"</span>);
<span class="macro">debug_assert</span><span class="macro">!</span>(<span class="ident">state</span> <span class="op">!=</span> <span class="ident">LOCKED_WRITE_NOTIFIED</span>, <span class="string">"unexpected state LOCKED_WRITE_NOTIFIED"</span>);
<span class="comment">// Currently in a read locked state, this implies that `notify`</span>
<span class="comment">// is currently being called on the old task handle. So, we call</span>
<span class="comment">// notify on the new task handle</span>
<span class="ident">task</span>.<span class="ident">notify</span>();
}
}
}
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">notify</span>(<span class="kw-2">&</span><span class="self">self</span>) {
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">curr</span> <span class="op">=</span> <span class="ident">WAITING</span>;
<span class="kw">loop</span> {
<span class="kw">if</span> <span class="ident">curr</span> <span class="op">==</span> <span class="ident">LOCKED_WRITE</span> {
<span class="comment">// Transition the state to LOCKED_NOTIFIED</span>
<span class="kw">let</span> <span class="ident">actual</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">state</span>.<span class="ident">compare_and_swap</span>(<span class="ident">LOCKED_WRITE</span>, <span class="ident">LOCKED_WRITE_NOTIFIED</span>, <span class="ident">Release</span>);
<span class="kw">if</span> <span class="ident">curr</span> <span class="op">==</span> <span class="ident">actual</span> {
<span class="comment">// Success, return</span>
<span class="kw">return</span>;
}
<span class="comment">// update current state variable and try again</span>
<span class="ident">curr</span> <span class="op">=</span> <span class="ident">actual</span>;
} <span class="kw">else</span> <span class="kw">if</span> <span class="ident">curr</span> <span class="op">==</span> <span class="ident">LOCKED_WRITE_NOTIFIED</span> {
<span class="comment">// Currently in `LOCKED_WRITE_NOTIFIED` state, nothing else to do.</span>
<span class="kw">return</span>;
} <span class="kw">else</span> {
<span class="comment">// Currently in a LOCKED_READ state, so attempt to increment the</span>
<span class="comment">// lock count.</span>
<span class="kw">let</span> <span class="ident">actual</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">state</span>.<span class="ident">compare_and_swap</span>(<span class="ident">curr</span>, <span class="ident">curr</span> <span class="op">+</span> <span class="number">1</span>, <span class="ident">Acquire</span>);
<span class="comment">// Locked acquired</span>
<span class="kw">if</span> <span class="ident">actual</span> <span class="op">==</span> <span class="ident">curr</span> {
<span class="comment">// Notify the task</span>
<span class="kw">unsafe</span> {
<span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="kw-2">ref</span> <span class="ident">task</span>) <span class="op">=</span> <span class="kw-2">*</span><span class="self">self</span>.<span class="ident">task</span>.<span class="ident">get</span>() {
<span class="ident">task</span>.<span class="ident">notify</span>();
}
}
<span class="comment">// Release the lock</span>
<span class="self">self</span>.<span class="ident">state</span>.<span class="ident">fetch_sub</span>(<span class="number">1</span>, <span class="ident">Release</span>);
<span class="comment">// Done</span>
<span class="kw">return</span>;
}
<span class="comment">// update current state variable and try again</span>
<span class="ident">curr</span> <span class="op">=</span> <span class="ident">actual</span>;
}
}
}
}
<span class="kw">impl</span> <span class="ident">fmt</span>::<span class="ident">Debug</span> <span class="kw">for</span> <span class="ident">AtomicTask</span> {
<span class="kw">fn</span> <span class="ident">fmt</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">fmt</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">fmt</span>::<span class="ident">Formatter</span>) <span class="op">-></span> <span class="ident">fmt</span>::<span class="prelude-ty">Result</span> {
<span class="macro">write</span><span class="macro">!</span>(<span class="ident">fmt</span>, <span class="string">"AtomicTask"</span>)
}
}
<span class="kw">unsafe</span> <span class="kw">impl</span> <span class="ident">Send</span> <span class="kw">for</span> <span class="ident">AtomicTask</span> {}
<span class="kw">unsafe</span> <span class="kw">impl</span> <span class="ident">Sync</span> <span class="kw">for</span> <span class="ident">AtomicTask</span> {}
</pre>
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