| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-1.28.2/src/sync/task/atomic_waker.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>atomic_waker.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../../../normalize.css"><link rel="stylesheet" href="../../../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../../../ayu.css" disabled><link rel="stylesheet" href="../../../../dark.css" disabled><link rel="stylesheet" href="../../../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../../../storage.js"></script><script defer src="../../../../source-script.js"></script><script defer src="../../../../source-files.js"></script><script defer src="../../../../main.js"></script><noscript><link rel="stylesheet" href="../../../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../../../tokio/index.html"><div class="logo-container"><img class="rust-logo" src="../../../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../../../tokio/index.html"><img class="rust-logo" src="../../../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span> |
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| </pre><pre class="rust"><code><span class="attribute">#![cfg_attr(any(loom, not(feature = <span class="string">"sync"</span>)), allow(dead_code, unreachable_pub))] |
| |
| </span><span class="kw">use </span><span class="kw">crate</span>::loom::cell::UnsafeCell; |
| <span class="kw">use </span><span class="kw">crate</span>::loom::hint; |
| <span class="kw">use </span><span class="kw">crate</span>::loom::sync::atomic::AtomicUsize; |
| |
| <span class="kw">use </span>std::fmt; |
| <span class="kw">use </span>std::panic::{resume_unwind, AssertUnwindSafe, RefUnwindSafe, UnwindSafe}; |
| <span class="kw">use </span>std::sync::atomic::Ordering::{AcqRel, Acquire, Release}; |
| <span class="kw">use </span>std::task::Waker; |
| |
| <span class="doccomment">/// A synchronization primitive for task waking. |
| /// |
| /// `AtomicWaker` will coordinate concurrent wakes with the consumer |
| /// potentially "waking" the underlying task. This is useful in scenarios |
| /// where a computation completes in another thread and wants to wake the |
| /// consumer, but the consumer is in the process of being migrated to a new |
| /// logical task. |
| /// |
| /// Consumers should call `register` before checking the result of a computation |
| /// and producers should call `wake` after producing the computation (this |
| /// differs from the usual `thread::park` pattern). It is also permitted for |
| /// `wake` to be called **before** `register`. This results in a no-op. |
| /// |
| /// A single `AtomicWaker` may be reused for any number of calls to `register` or |
| /// `wake`. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>AtomicWaker { |
| state: AtomicUsize, |
| waker: UnsafeCell<<span class="prelude-ty">Option</span><Waker>>, |
| } |
| |
| <span class="kw">impl </span>RefUnwindSafe <span class="kw">for </span>AtomicWaker {} |
| <span class="kw">impl </span>UnwindSafe <span class="kw">for </span>AtomicWaker {} |
| |
| <span class="comment">// `AtomicWaker` is a multi-consumer, single-producer transfer cell. The cell |
| // stores a `Waker` value produced by calls to `register` and many threads can |
| // race to take the waker by calling `wake`. |
| // |
| // If a new `Waker` instance is produced by calling `register` before an existing |
| // one is consumed, then the existing one is overwritten. |
| // |
| // While `AtomicWaker` is single-producer, the implementation ensures memory |
| // safety. In the event of concurrent calls to `register`, there will be a |
| // single winner whose waker will get stored in the cell. The losers will not |
| // have their tasks woken. As such, callers should ensure to add synchronization |
| // to calls to `register`. |
| // |
| // The implementation uses a single `AtomicUsize` value to coordinate access to |
| // the `Waker` cell. There are two bits that are operated on independently. These |
| // are represented by `REGISTERING` and `WAKING`. |
| // |
| // The `REGISTERING` bit is set when a producer enters the critical section. The |
| // `WAKING` bit is set when a consumer enters the critical section. Neither |
| // bit being set is represented by `WAITING`. |
| // |
| // A thread obtains an exclusive lock on the waker cell by transitioning the |
| // state from `WAITING` to `REGISTERING` or `WAKING`, depending on the |
| // operation the thread wishes to perform. When this transition is made, it is |
| // guaranteed that no other thread will access the waker cell. |
| // |
| // # Registering |
| // |
| // On a call to `register`, an attempt to transition the state from WAITING to |
| // REGISTERING is made. On success, the caller obtains a lock on the waker cell. |
| // |
| // If the lock is obtained, then the thread sets the waker cell to the waker |
| // provided as an argument. Then it attempts to transition the state back from |
| // `REGISTERING` -> `WAITING`. |
| // |
| // If this transition is successful, then the registering process is complete |
| // and the next call to `wake` will observe the waker. |
| // |
| // If the transition fails, then there was a concurrent call to `wake` that |
| // was unable to access the waker cell (due to the registering thread holding the |
| // lock). To handle this, the registering thread removes the waker it just set |
| // from the cell and calls `wake` on it. This call to wake represents the |
| // attempt to wake by the other thread (that set the `WAKING` bit). The |
| // state is then transitioned from `REGISTERING | WAKING` back to `WAITING`. |
| // This transition must succeed because, at this point, the state cannot be |
| // transitioned by another thread. |
| // |
| // # Waking |
| // |
| // On a call to `wake`, an attempt to transition the state from `WAITING` to |
| // `WAKING` is made. On success, the caller obtains a lock on the waker cell. |
| // |
| // If the lock is obtained, then the thread takes ownership of the current value |
| // in the waker cell, and calls `wake` on it. The state is then transitioned |
| // back to `WAITING`. This transition must succeed as, at this point, the state |
| // cannot be transitioned by another thread. |
| // |
| // If the thread is unable to obtain the lock, the `WAKING` bit is still set. |
| // This is because it has either been set by the current thread but the previous |
| // value included the `REGISTERING` bit **or** a concurrent thread is in the |
| // `WAKING` critical section. Either way, no action must be taken. |
| // |
| // If the current thread is the only concurrent call to `wake` and another |
| // thread is in the `register` critical section, when the other thread **exits** |
| // the `register` critical section, it will observe the `WAKING` bit and |
| // handle the waker itself. |
| // |
| // If another thread is in the `waker` critical section, then it will handle |
| // waking the caller task. |
| // |
| // # A potential race (is safely handled). |
| // |
| // Imagine the following situation: |
| // |
| // * Thread A obtains the `wake` lock and wakes a task. |
| // |
| // * Before thread A releases the `wake` lock, the woken task is scheduled. |
| // |
| // * Thread B attempts to wake the task. In theory this should result in the |
| // task being woken, but it cannot because thread A still holds the wake |
| // lock. |
| // |
| // This case is handled by requiring users of `AtomicWaker` to call `register` |
| // **before** attempting to observe the application state change that resulted |
| // in the task being woken. The wakers also change the application state |
| // before calling wake. |
| // |
| // Because of this, the task will do one of two things. |
| // |
| // 1) Observe the application state change that Thread B is waking on. In |
| // this case, it is OK for Thread B's wake to be lost. |
| // |
| // 2) Call register before attempting to observe the application state. Since |
| // Thread A still holds the `wake` lock, the call to `register` will result |
| // in the task waking itself and get scheduled again. |
| |
| </span><span class="doccomment">/// Idle state. |
| </span><span class="kw">const </span>WAITING: usize = <span class="number">0</span>; |
| |
| <span class="doccomment">/// A new waker value is being registered with the `AtomicWaker` cell. |
| </span><span class="kw">const </span>REGISTERING: usize = <span class="number">0b01</span>; |
| |
| <span class="doccomment">/// The task currently registered with the `AtomicWaker` cell is being woken. |
| </span><span class="kw">const </span>WAKING: usize = <span class="number">0b10</span>; |
| |
| <span class="kw">impl </span>AtomicWaker { |
| <span class="doccomment">/// Create an `AtomicWaker` |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>new() -> AtomicWaker { |
| AtomicWaker { |
| state: AtomicUsize::new(WAITING), |
| waker: UnsafeCell::new(<span class="prelude-val">None</span>), |
| } |
| } |
| |
| <span class="comment">/* |
| /// Registers the current waker to be notified on calls to `wake`. |
| pub(crate) fn register(&self, waker: Waker) { |
| self.do_register(waker); |
| } |
| */ |
| |
| </span><span class="doccomment">/// Registers the provided waker to be notified on calls to `wake`. |
| /// |
| /// The new waker will take place of any previous wakers that were registered |
| /// by previous calls to `register`. Any calls to `wake` that happen after |
| /// a call to `register` (as defined by the memory ordering rules), will |
| /// wake the `register` caller's task. |
| /// |
| /// It is safe to call `register` with multiple other threads concurrently |
| /// calling `wake`. This will result in the `register` caller's current |
| /// task being woken once. |
| /// |
| /// This function is safe to call concurrently, but this is generally a bad |
| /// idea. Concurrent calls to `register` will attempt to register different |
| /// tasks to be woken. One of the callers will win and have its task set, |
| /// but there is no guarantee as to which caller will succeed. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>register_by_ref(<span class="kw-2">&</span><span class="self">self</span>, waker: <span class="kw-2">&</span>Waker) { |
| <span class="self">self</span>.do_register(waker); |
| } |
| |
| <span class="kw">fn </span>do_register<W>(<span class="kw-2">&</span><span class="self">self</span>, waker: W) |
| <span class="kw">where |
| </span>W: WakerRef, |
| { |
| <span class="kw">fn </span>catch_unwind<F: FnOnce() -> R, R>(f: F) -> std::thread::Result<R> { |
| std::panic::catch_unwind(AssertUnwindSafe(f)) |
| } |
| |
| <span class="kw">match </span><span class="self">self |
| </span>.state |
| .compare_exchange(WAITING, REGISTERING, Acquire, Acquire) |
| .unwrap_or_else(|x| x) |
| { |
| WAITING => { |
| <span class="kw">unsafe </span>{ |
| <span class="comment">// If `into_waker` panics (because it's code outside of |
| // AtomicWaker) we need to prime a guard that is called on |
| // unwind to restore the waker to a WAITING state. Otherwise |
| // any future calls to register will incorrectly be stuck |
| // believing it's being updated by someone else. |
| </span><span class="kw">let </span>new_waker_or_panic = catch_unwind(<span class="kw">move </span>|| waker.into_waker()); |
| |
| <span class="comment">// Set the field to contain the new waker, or if |
| // `into_waker` panicked, leave the old value. |
| </span><span class="kw">let </span><span class="kw-2">mut </span>maybe_panic = <span class="prelude-val">None</span>; |
| <span class="kw">let </span><span class="kw-2">mut </span>old_waker = <span class="prelude-val">None</span>; |
| <span class="kw">match </span>new_waker_or_panic { |
| <span class="prelude-val">Ok</span>(new_waker) => { |
| old_waker = <span class="self">self</span>.waker.with_mut(|t| (<span class="kw-2">*</span>t).take()); |
| <span class="self">self</span>.waker.with_mut(|t| <span class="kw-2">*</span>t = <span class="prelude-val">Some</span>(new_waker)); |
| } |
| <span class="prelude-val">Err</span>(panic) => maybe_panic = <span class="prelude-val">Some</span>(panic), |
| } |
| |
| <span class="comment">// Release the lock. If the state transitioned to include |
| // the `WAKING` bit, this means that a wake has been |
| // called concurrently, so we have to remove the waker and |
| // wake it.` |
| // |
| // Start by assuming that the state is `REGISTERING` as this |
| // is what we jut set it to. |
| </span><span class="kw">let </span>res = <span class="self">self |
| </span>.state |
| .compare_exchange(REGISTERING, WAITING, AcqRel, Acquire); |
| |
| <span class="kw">match </span>res { |
| <span class="prelude-val">Ok</span>(<span class="kw">_</span>) => { |
| <span class="comment">// We don't want to give the caller the panic if it |
| // was someone else who put in that waker. |
| </span><span class="kw">let _ </span>= catch_unwind(<span class="kw">move </span>|| { |
| drop(old_waker); |
| }); |
| } |
| <span class="prelude-val">Err</span>(actual) => { |
| <span class="comment">// This branch can only be reached if a |
| // concurrent thread called `wake`. In this |
| // case, `actual` **must** be `REGISTERING | |
| // WAKING`. |
| </span><span class="macro">debug_assert_eq!</span>(actual, REGISTERING | WAKING); |
| |
| <span class="comment">// Take the waker to wake once the atomic operation has |
| // completed. |
| </span><span class="kw">let </span><span class="kw-2">mut </span>waker = <span class="self">self</span>.waker.with_mut(|t| (<span class="kw-2">*</span>t).take()); |
| |
| <span class="comment">// Just swap, because no one could change state |
| // while state == `Registering | `Waking` |
| </span><span class="self">self</span>.state.swap(WAITING, AcqRel); |
| |
| <span class="comment">// If `into_waker` panicked, then the waker in the |
| // waker slot is actually the old waker. |
| </span><span class="kw">if </span>maybe_panic.is_some() { |
| old_waker = waker.take(); |
| } |
| |
| <span class="comment">// We don't want to give the caller the panic if it |
| // was someone else who put in that waker. |
| </span><span class="kw">if let </span><span class="prelude-val">Some</span>(old_waker) = old_waker { |
| <span class="kw">let _ </span>= catch_unwind(<span class="kw">move </span>|| { |
| old_waker.wake(); |
| }); |
| } |
| |
| <span class="comment">// The atomic swap was complete, now wake the waker |
| // and return. |
| // |
| // If this panics, we end up in a consumed state and |
| // return the panic to the caller. |
| </span><span class="kw">if let </span><span class="prelude-val">Some</span>(waker) = waker { |
| <span class="macro">debug_assert!</span>(maybe_panic.is_none()); |
| waker.wake(); |
| } |
| } |
| } |
| |
| <span class="kw">if let </span><span class="prelude-val">Some</span>(panic) = maybe_panic { |
| <span class="comment">// If `into_waker` panicked, return the panic to the caller. |
| </span>resume_unwind(panic); |
| } |
| } |
| } |
| WAKING => { |
| <span class="comment">// Currently in the process of waking the task, i.e., |
| // `wake` is currently being called on the old waker. |
| // So, we call wake on the new waker. |
| // |
| // If this panics, someone else is responsible for restoring the |
| // state of the waker. |
| </span>waker.wake(); |
| |
| <span class="comment">// This is equivalent to a spin lock, so use a spin hint. |
| </span>hint::spin_loop(); |
| } |
| state => { |
| <span class="comment">// In this case, a concurrent thread is holding the |
| // "registering" lock. This probably indicates a bug in the |
| // caller's code as racing to call `register` doesn't make much |
| // sense. |
| // |
| // We just want to maintain memory safety. It is ok to drop the |
| // call to `register`. |
| </span><span class="macro">debug_assert!</span>(state == REGISTERING || state == REGISTERING | WAKING); |
| } |
| } |
| } |
| |
| <span class="doccomment">/// Wakes the task that last called `register`. |
| /// |
| /// If `register` has not been called yet, then this does nothing. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>wake(<span class="kw-2">&</span><span class="self">self</span>) { |
| <span class="kw">if let </span><span class="prelude-val">Some</span>(waker) = <span class="self">self</span>.take_waker() { |
| <span class="comment">// If wake panics, we've consumed the waker which is a legitimate |
| // outcome. |
| </span>waker.wake(); |
| } |
| } |
| |
| <span class="doccomment">/// Attempts to take the `Waker` value out of the `AtomicWaker` with the |
| /// intention that the caller will wake the task later. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>take_waker(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="prelude-ty">Option</span><Waker> { |
| <span class="comment">// AcqRel ordering is used in order to acquire the value of the `waker` |
| // cell as well as to establish a `release` ordering with whatever |
| // memory the `AtomicWaker` is associated with. |
| </span><span class="kw">match </span><span class="self">self</span>.state.fetch_or(WAKING, AcqRel) { |
| WAITING => { |
| <span class="comment">// The waking lock has been acquired. |
| </span><span class="kw">let </span>waker = <span class="kw">unsafe </span>{ <span class="self">self</span>.waker.with_mut(|t| (<span class="kw-2">*</span>t).take()) }; |
| |
| <span class="comment">// Release the lock |
| </span><span class="self">self</span>.state.fetch_and(!WAKING, Release); |
| |
| waker |
| } |
| state => { |
| <span class="comment">// There is a concurrent thread currently updating the |
| // associated waker. |
| // |
| // Nothing more to do as the `WAKING` bit has been set. It |
| // doesn't matter if there are concurrent registering threads or |
| // not. |
| // |
| </span><span class="macro">debug_assert!</span>( |
| state == REGISTERING || state == REGISTERING | WAKING || state == WAKING |
| ); |
| <span class="prelude-val">None |
| </span>} |
| } |
| } |
| } |
| |
| <span class="kw">impl </span>Default <span class="kw">for </span>AtomicWaker { |
| <span class="kw">fn </span>default() -> <span class="self">Self </span>{ |
| AtomicWaker::new() |
| } |
| } |
| |
| <span class="kw">impl </span>fmt::Debug <span class="kw">for </span>AtomicWaker { |
| <span class="kw">fn </span>fmt(<span class="kw-2">&</span><span class="self">self</span>, fmt: <span class="kw-2">&mut </span>fmt::Formatter<<span class="lifetime">'_</span>>) -> fmt::Result { |
| <span class="macro">write!</span>(fmt, <span class="string">"AtomicWaker"</span>) |
| } |
| } |
| |
| <span class="kw">unsafe impl </span>Send <span class="kw">for </span>AtomicWaker {} |
| <span class="kw">unsafe impl </span>Sync <span class="kw">for </span>AtomicWaker {} |
| |
| <span class="kw">trait </span>WakerRef { |
| <span class="kw">fn </span>wake(<span class="self">self</span>); |
| <span class="kw">fn </span>into_waker(<span class="self">self</span>) -> Waker; |
| } |
| |
| <span class="kw">impl </span>WakerRef <span class="kw">for </span>Waker { |
| <span class="kw">fn </span>wake(<span class="self">self</span>) { |
| <span class="self">self</span>.wake() |
| } |
| |
| <span class="kw">fn </span>into_waker(<span class="self">self</span>) -> Waker { |
| <span class="self">self |
| </span>} |
| } |
| |
| <span class="kw">impl </span>WakerRef <span class="kw">for </span><span class="kw-2">&</span>Waker { |
| <span class="kw">fn </span>wake(<span class="self">self</span>) { |
| <span class="self">self</span>.wake_by_ref() |
| } |
| |
| <span class="kw">fn </span>into_waker(<span class="self">self</span>) -> Waker { |
| <span class="self">self</span>.clone() |
| } |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../../../" data-current-crate="tokio" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
