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<!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/git/checkouts/tokio-377c595163f99a10/dfe252d/tokio/src/sync/batch_semaphore.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>batch_semaphore.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(not(feature = <span class="string">&quot;sync&quot;</span>), allow(unreachable_pub, dead_code))]
</span><span class="doccomment">//! # Implementation Details.
//!
//! The semaphore is implemented using an intrusive linked list of waiters. An
//! atomic counter tracks the number of available permits. If the semaphore does
//! not contain the required number of permits, the task attempting to acquire
//! permits places its waker at the end of a queue. When new permits are made
//! available (such as by releasing an initial acquisition), they are assigned
//! to the task at the front of the queue, waking that task if its requested
//! number of permits is met.
//!
//! Because waiters are enqueued at the back of the linked list and dequeued
//! from the front, the semaphore is fair. Tasks trying to acquire large numbers
//! of permits at a time will always be woken eventually, even if many other
//! tasks are acquiring smaller numbers of permits. This means that in a
//! use-case like tokio&#39;s read-write lock, writers will not be starved by
//! readers.
</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::sync::atomic::AtomicUsize;
<span class="kw">use </span><span class="kw">crate</span>::loom::sync::{Mutex, MutexGuard};
<span class="kw">use </span><span class="kw">crate</span>::util::linked_list::{<span class="self">self</span>, LinkedList};
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span><span class="kw">use </span><span class="kw">crate</span>::util::trace;
<span class="kw">use </span><span class="kw">crate</span>::util::WakeList;
<span class="kw">use </span>std::future::Future;
<span class="kw">use </span>std::marker::PhantomPinned;
<span class="kw">use </span>std::pin::Pin;
<span class="kw">use </span>std::ptr::NonNull;
<span class="kw">use </span>std::sync::atomic::Ordering::<span class="kw-2">*</span>;
<span class="kw">use </span>std::task::Poll::<span class="kw-2">*</span>;
<span class="kw">use </span>std::task::{Context, Poll, Waker};
<span class="kw">use </span>std::{cmp, fmt};
<span class="doccomment">/// An asynchronous counting semaphore which permits waiting on multiple permits at once.
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>Semaphore {
waiters: Mutex&lt;Waitlist&gt;,
<span class="doccomment">/// The current number of available permits in the semaphore.
</span>permits: AtomicUsize,
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span>resource_span: tracing::Span,
}
<span class="kw">struct </span>Waitlist {
queue: LinkedList&lt;Waiter, &lt;Waiter <span class="kw">as </span>linked_list::Link&gt;::Target&gt;,
closed: bool,
}
<span class="doccomment">/// Error returned from the [`Semaphore::try_acquire`] function.
///
/// [`Semaphore::try_acquire`]: crate::sync::Semaphore::try_acquire
</span><span class="attribute">#[derive(Debug, PartialEq, Eq)]
</span><span class="kw">pub enum </span>TryAcquireError {
<span class="doccomment">/// The semaphore has been [closed] and cannot issue new permits.
///
/// [closed]: crate::sync::Semaphore::close
</span>Closed,
<span class="doccomment">/// The semaphore has no available permits.
</span>NoPermits,
}
<span class="doccomment">/// Error returned from the [`Semaphore::acquire`] function.
///
/// An `acquire` operation can only fail if the semaphore has been
/// [closed].
///
/// [closed]: crate::sync::Semaphore::close
/// [`Semaphore::acquire`]: crate::sync::Semaphore::acquire
</span><span class="attribute">#[derive(Debug)]
</span><span class="kw">pub struct </span>AcquireError(());
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>Acquire&lt;<span class="lifetime">&#39;a</span>&gt; {
node: Waiter,
semaphore: <span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>Semaphore,
num_permits: u32,
queued: bool,
}
<span class="doccomment">/// An entry in the wait queue.
</span><span class="kw">struct </span>Waiter {
<span class="doccomment">/// The current state of the waiter.
///
/// This is either the number of remaining permits required by
/// the waiter, or a flag indicating that the waiter is not yet queued.
</span>state: AtomicUsize,
<span class="doccomment">/// The waker to notify the task awaiting permits.
///
/// # Safety
///
/// This may only be accessed while the wait queue is locked.
</span>waker: UnsafeCell&lt;<span class="prelude-ty">Option</span>&lt;Waker&gt;&gt;,
<span class="doccomment">/// Intrusive linked-list pointers.
///
/// # Safety
///
/// This may only be accessed while the wait queue is locked.
///
/// TODO: Ideally, we would be able to use loom to enforce that
/// this isn&#39;t accessed concurrently. However, it is difficult to
/// use a `UnsafeCell` here, since the `Link` trait requires _returning_
/// references to `Pointers`, and `UnsafeCell` requires that checked access
/// take place inside a closure. We should consider changing `Pointers` to
/// use `UnsafeCell` internally.
</span>pointers: linked_list::Pointers&lt;Waiter&gt;,
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span>ctx: trace::AsyncOpTracingCtx,
<span class="doccomment">/// Should not be `Unpin`.
</span>_p: PhantomPinned,
}
<span class="macro">generate_addr_of_methods! </span>{
<span class="kw">impl</span>&lt;&gt; Waiter {
<span class="kw">unsafe fn </span>addr_of_pointers(<span class="self">self</span>: NonNull&lt;<span class="self">Self</span>&gt;) -&gt; NonNull&lt;linked_list::Pointers&lt;Waiter&gt;&gt; {
<span class="kw-2">&amp;</span><span class="self">self</span>.pointers
}
}
}
<span class="kw">impl </span>Semaphore {
<span class="doccomment">/// The maximum number of permits which a semaphore can hold.
///
/// Note that this reserves three bits of flags in the permit counter, but
/// we only actually use one of them. However, the previous semaphore
/// implementation used three bits, so we will continue to reserve them to
/// avoid a breaking change if additional flags need to be added in the
/// future.
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">const </span>MAX_PERMITS: usize = std::usize::MAX &gt;&gt; <span class="number">3</span>;
<span class="kw">const </span>CLOSED: usize = <span class="number">1</span>;
<span class="comment">// The least-significant bit in the number of permits is reserved to use
// as a flag indicating that the semaphore has been closed. Consequently
// PERMIT_SHIFT is used to leave that bit for that purpose.
</span><span class="kw">const </span>PERMIT_SHIFT: usize = <span class="number">1</span>;
<span class="doccomment">/// Creates a new semaphore with the initial number of permits
///
/// Maximum number of permits on 32-bit platforms is `1&lt;&lt;29`.
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>new(permits: usize) -&gt; <span class="self">Self </span>{
<span class="macro">assert!</span>(
permits &lt;= <span class="self">Self</span>::MAX_PERMITS,
<span class="string">&quot;a semaphore may not have more than MAX_PERMITS permits ({})&quot;</span>,
<span class="self">Self</span>::MAX_PERMITS
);
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span><span class="kw">let </span>resource_span = {
<span class="kw">let </span>resource_span = <span class="macro">tracing::trace_span!</span>(
<span class="string">&quot;runtime.resource&quot;</span>,
concrete_type = <span class="string">&quot;Semaphore&quot;</span>,
kind = <span class="string">&quot;Sync&quot;</span>,
is_internal = <span class="bool-val">true
</span>);
resource_span.in_scope(|| {
<span class="macro">tracing::trace!</span>(
target: <span class="string">&quot;runtime::resource::state_update&quot;</span>,
permits = permits,
permits.op = <span class="string">&quot;override&quot;</span>,
)
});
resource_span
};
<span class="self">Self </span>{
permits: AtomicUsize::new(permits &lt;&lt; <span class="self">Self</span>::PERMIT_SHIFT),
waiters: Mutex::new(Waitlist {
queue: LinkedList::new(),
closed: <span class="bool-val">false</span>,
}),
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span>resource_span,
}
}
<span class="doccomment">/// Creates a new semaphore with the initial number of permits.
///
/// Maximum number of permits on 32-bit platforms is `1&lt;&lt;29`.
///
/// If the specified number of permits exceeds the maximum permit amount
/// Then the value will get clamped to the maximum number of permits.
</span><span class="attribute">#[cfg(all(feature = <span class="string">&quot;parking_lot&quot;</span>, not(all(loom, test))))]
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">const fn </span>const_new(<span class="kw-2">mut </span>permits: usize) -&gt; <span class="self">Self </span>{
<span class="comment">// NOTE: assertions and by extension panics are still being worked on: https://github.com/rust-lang/rust/issues/74925
// currently we just clamp the permit count when it exceeds the max
</span>permits &amp;= <span class="self">Self</span>::MAX_PERMITS;
<span class="self">Self </span>{
permits: AtomicUsize::new(permits &lt;&lt; <span class="self">Self</span>::PERMIT_SHIFT),
waiters: Mutex::const_new(Waitlist {
queue: LinkedList::new(),
closed: <span class="bool-val">false</span>,
}),
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span>resource_span: tracing::Span::none(),
}
}
<span class="doccomment">/// Returns the current number of available permits.
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>available_permits(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; usize {
<span class="self">self</span>.permits.load(Acquire) &gt;&gt; <span class="self">Self</span>::PERMIT_SHIFT
}
<span class="doccomment">/// Adds `added` new permits to the semaphore.
///
/// The maximum number of permits is `usize::MAX &gt;&gt; 3`, and this function will panic if the limit is exceeded.
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>release(<span class="kw-2">&amp;</span><span class="self">self</span>, added: usize) {
<span class="kw">if </span>added == <span class="number">0 </span>{
<span class="kw">return</span>;
}
<span class="comment">// Assign permits to the wait queue
</span><span class="self">self</span>.add_permits_locked(added, <span class="self">self</span>.waiters.lock());
}
<span class="doccomment">/// Closes the semaphore. This prevents the semaphore from issuing new
/// permits and notifies all pending waiters.
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>close(<span class="kw-2">&amp;</span><span class="self">self</span>) {
<span class="kw">let </span><span class="kw-2">mut </span>waiters = <span class="self">self</span>.waiters.lock();
<span class="comment">// If the semaphore&#39;s permits counter has enough permits for an
// unqueued waiter to acquire all the permits it needs immediately,
// it won&#39;t touch the wait list. Therefore, we have to set a bit on
// the permit counter as well. However, we must do this while
// holding the lock --- otherwise, if we set the bit and then wait
// to acquire the lock we&#39;ll enter an inconsistent state where the
// permit counter is closed, but the wait list is not.
</span><span class="self">self</span>.permits.fetch_or(<span class="self">Self</span>::CLOSED, Release);
waiters.closed = <span class="bool-val">true</span>;
<span class="kw">while let </span><span class="prelude-val">Some</span>(<span class="kw-2">mut </span>waiter) = waiters.queue.pop_back() {
<span class="kw">let </span>waker = <span class="kw">unsafe </span>{ waiter.as_mut().waker.with_mut(|waker| (<span class="kw-2">*</span>waker).take()) };
<span class="kw">if let </span><span class="prelude-val">Some</span>(waker) = waker {
waker.wake();
}
}
}
<span class="doccomment">/// Returns true if the semaphore is closed.
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>is_closed(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
<span class="self">self</span>.permits.load(Acquire) &amp; <span class="self">Self</span>::CLOSED == <span class="self">Self</span>::CLOSED
}
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>try_acquire(<span class="kw-2">&amp;</span><span class="self">self</span>, num_permits: u32) -&gt; <span class="prelude-ty">Result</span>&lt;(), TryAcquireError&gt; {
<span class="macro">assert!</span>(
num_permits <span class="kw">as </span>usize &lt;= <span class="self">Self</span>::MAX_PERMITS,
<span class="string">&quot;a semaphore may not have more than MAX_PERMITS permits ({})&quot;</span>,
<span class="self">Self</span>::MAX_PERMITS
);
<span class="kw">let </span>num_permits = (num_permits <span class="kw">as </span>usize) &lt;&lt; <span class="self">Self</span>::PERMIT_SHIFT;
<span class="kw">let </span><span class="kw-2">mut </span>curr = <span class="self">self</span>.permits.load(Acquire);
<span class="kw">loop </span>{
<span class="comment">// Has the semaphore closed?
</span><span class="kw">if </span>curr &amp; <span class="self">Self</span>::CLOSED == <span class="self">Self</span>::CLOSED {
<span class="kw">return </span><span class="prelude-val">Err</span>(TryAcquireError::Closed);
}
<span class="comment">// Are there enough permits remaining?
</span><span class="kw">if </span>curr &lt; num_permits {
<span class="kw">return </span><span class="prelude-val">Err</span>(TryAcquireError::NoPermits);
}
<span class="kw">let </span>next = curr - num_permits;
<span class="kw">match </span><span class="self">self</span>.permits.compare_exchange(curr, next, AcqRel, Acquire) {
<span class="prelude-val">Ok</span>(<span class="kw">_</span>) =&gt; {
<span class="comment">// TODO: Instrument once issue has been solved}
</span><span class="kw">return </span><span class="prelude-val">Ok</span>(());
}
<span class="prelude-val">Err</span>(actual) =&gt; curr = actual,
}
}
}
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>acquire(<span class="kw-2">&amp;</span><span class="self">self</span>, num_permits: u32) -&gt; Acquire&lt;<span class="lifetime">&#39;_</span>&gt; {
Acquire::new(<span class="self">self</span>, num_permits)
}
<span class="doccomment">/// Release `rem` permits to the semaphore&#39;s wait list, starting from the
/// end of the queue.
///
/// If `rem` exceeds the number of permits needed by the wait list, the
/// remainder are assigned back to the semaphore.
</span><span class="kw">fn </span>add_permits_locked(<span class="kw-2">&amp;</span><span class="self">self</span>, <span class="kw-2">mut </span>rem: usize, waiters: MutexGuard&lt;<span class="lifetime">&#39;_</span>, Waitlist&gt;) {
<span class="kw">let </span><span class="kw-2">mut </span>wakers = WakeList::new();
<span class="kw">let </span><span class="kw-2">mut </span>lock = <span class="prelude-val">Some</span>(waiters);
<span class="kw">let </span><span class="kw-2">mut </span>is_empty = <span class="bool-val">false</span>;
<span class="kw">while </span>rem &gt; <span class="number">0 </span>{
<span class="kw">let </span><span class="kw-2">mut </span>waiters = lock.take().unwrap_or_else(|| <span class="self">self</span>.waiters.lock());
<span class="lifetime">&#39;inner</span>: <span class="kw">while </span>wakers.can_push() {
<span class="comment">// Was the waiter assigned enough permits to wake it?
</span><span class="kw">match </span>waiters.queue.last() {
<span class="prelude-val">Some</span>(waiter) =&gt; {
<span class="kw">if </span>!waiter.assign_permits(<span class="kw-2">&amp;mut </span>rem) {
<span class="kw">break </span><span class="lifetime">&#39;inner</span>;
}
}
<span class="prelude-val">None </span>=&gt; {
is_empty = <span class="bool-val">true</span>;
<span class="comment">// If we assigned permits to all the waiters in the queue, and there are
// still permits left over, assign them back to the semaphore.
</span><span class="kw">break </span><span class="lifetime">&#39;inner</span>;
}
};
<span class="kw">let </span><span class="kw-2">mut </span>waiter = waiters.queue.pop_back().unwrap();
<span class="kw">if let </span><span class="prelude-val">Some</span>(waker) =
<span class="kw">unsafe </span>{ waiter.as_mut().waker.with_mut(|waker| (<span class="kw-2">*</span>waker).take()) }
{
wakers.push(waker);
}
}
<span class="kw">if </span>rem &gt; <span class="number">0 </span>&amp;&amp; is_empty {
<span class="kw">let </span>permits = rem;
<span class="macro">assert!</span>(
permits &lt;= <span class="self">Self</span>::MAX_PERMITS,
<span class="string">&quot;cannot add more than MAX_PERMITS permits ({})&quot;</span>,
<span class="self">Self</span>::MAX_PERMITS
);
<span class="kw">let </span>prev = <span class="self">self</span>.permits.fetch_add(rem &lt;&lt; <span class="self">Self</span>::PERMIT_SHIFT, Release);
<span class="kw">let </span>prev = prev &gt;&gt; <span class="self">Self</span>::PERMIT_SHIFT;
<span class="macro">assert!</span>(
prev + permits &lt;= <span class="self">Self</span>::MAX_PERMITS,
<span class="string">&quot;number of added permits ({}) would overflow MAX_PERMITS ({})&quot;</span>,
rem,
<span class="self">Self</span>::MAX_PERMITS
);
<span class="comment">// add remaining permits back
</span><span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span><span class="self">self</span>.resource_span.in_scope(|| {
<span class="macro">tracing::trace!</span>(
target: <span class="string">&quot;runtime::resource::state_update&quot;</span>,
permits = rem,
permits.op = <span class="string">&quot;add&quot;</span>,
)
});
rem = <span class="number">0</span>;
}
drop(waiters); <span class="comment">// release the lock
</span>wakers.wake_all();
}
<span class="macro">assert_eq!</span>(rem, <span class="number">0</span>);
}
<span class="kw">fn </span>poll_acquire(
<span class="kw-2">&amp;</span><span class="self">self</span>,
cx: <span class="kw-2">&amp;mut </span>Context&lt;<span class="lifetime">&#39;_</span>&gt;,
num_permits: u32,
node: Pin&lt;<span class="kw-2">&amp;mut </span>Waiter&gt;,
queued: bool,
) -&gt; Poll&lt;<span class="prelude-ty">Result</span>&lt;(), AcquireError&gt;&gt; {
<span class="kw">let </span><span class="kw-2">mut </span>acquired = <span class="number">0</span>;
<span class="kw">let </span>needed = <span class="kw">if </span>queued {
node.state.load(Acquire) &lt;&lt; <span class="self">Self</span>::PERMIT_SHIFT
} <span class="kw">else </span>{
(num_permits <span class="kw">as </span>usize) &lt;&lt; <span class="self">Self</span>::PERMIT_SHIFT
};
<span class="kw">let </span><span class="kw-2">mut </span>lock = <span class="prelude-val">None</span>;
<span class="comment">// First, try to take the requested number of permits from the
// semaphore.
</span><span class="kw">let </span><span class="kw-2">mut </span>curr = <span class="self">self</span>.permits.load(Acquire);
<span class="kw">let </span><span class="kw-2">mut </span>waiters = <span class="kw">loop </span>{
<span class="comment">// Has the semaphore closed?
</span><span class="kw">if </span>curr &amp; <span class="self">Self</span>::CLOSED &gt; <span class="number">0 </span>{
<span class="kw">return </span>Ready(<span class="prelude-val">Err</span>(AcquireError::closed()));
}
<span class="kw">let </span><span class="kw-2">mut </span>remaining = <span class="number">0</span>;
<span class="kw">let </span>total = curr
.checked_add(acquired)
.expect(<span class="string">&quot;number of permits must not overflow&quot;</span>);
<span class="kw">let </span>(next, acq) = <span class="kw">if </span>total &gt;= needed {
<span class="kw">let </span>next = curr - (needed - acquired);
(next, needed &gt;&gt; <span class="self">Self</span>::PERMIT_SHIFT)
} <span class="kw">else </span>{
remaining = (needed - acquired) - curr;
(<span class="number">0</span>, curr &gt;&gt; <span class="self">Self</span>::PERMIT_SHIFT)
};
<span class="kw">if </span>remaining &gt; <span class="number">0 </span>&amp;&amp; lock.is_none() {
<span class="comment">// No permits were immediately available, so this permit will
// (probably) need to wait. We&#39;ll need to acquire a lock on the
// wait queue before continuing. We need to do this _before_ the
// CAS that sets the new value of the semaphore&#39;s `permits`
// counter. Otherwise, if we subtract the permits and then
// acquire the lock, we might miss additional permits being
// added while waiting for the lock.
</span>lock = <span class="prelude-val">Some</span>(<span class="self">self</span>.waiters.lock());
}
<span class="kw">match </span><span class="self">self</span>.permits.compare_exchange(curr, next, AcqRel, Acquire) {
<span class="prelude-val">Ok</span>(<span class="kw">_</span>) =&gt; {
acquired += acq;
<span class="kw">if </span>remaining == <span class="number">0 </span>{
<span class="kw">if </span>!queued {
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span><span class="self">self</span>.resource_span.in_scope(|| {
<span class="macro">tracing::trace!</span>(
target: <span class="string">&quot;runtime::resource::state_update&quot;</span>,
permits = acquired,
permits.op = <span class="string">&quot;sub&quot;</span>,
);
<span class="macro">tracing::trace!</span>(
target: <span class="string">&quot;runtime::resource::async_op::state_update&quot;</span>,
permits_obtained = acquired,
permits.op = <span class="string">&quot;add&quot;</span>,
)
});
<span class="kw">return </span>Ready(<span class="prelude-val">Ok</span>(()));
} <span class="kw">else if </span>lock.is_none() {
<span class="kw">break </span><span class="self">self</span>.waiters.lock();
}
}
<span class="kw">break </span>lock.expect(<span class="string">&quot;lock must be acquired before waiting&quot;</span>);
}
<span class="prelude-val">Err</span>(actual) =&gt; curr = actual,
}
};
<span class="kw">if </span>waiters.closed {
<span class="kw">return </span>Ready(<span class="prelude-val">Err</span>(AcquireError::closed()));
}
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span><span class="self">self</span>.resource_span.in_scope(|| {
<span class="macro">tracing::trace!</span>(
target: <span class="string">&quot;runtime::resource::state_update&quot;</span>,
permits = acquired,
permits.op = <span class="string">&quot;sub&quot;</span>,
)
});
<span class="kw">if </span>node.assign_permits(<span class="kw-2">&amp;mut </span>acquired) {
<span class="self">self</span>.add_permits_locked(acquired, waiters);
<span class="kw">return </span>Ready(<span class="prelude-val">Ok</span>(()));
}
<span class="macro">assert_eq!</span>(acquired, <span class="number">0</span>);
<span class="comment">// Otherwise, register the waker &amp; enqueue the node.
</span>node.waker.with_mut(|waker| {
<span class="comment">// Safety: the wait list is locked, so we may modify the waker.
</span><span class="kw">let </span>waker = <span class="kw">unsafe </span>{ <span class="kw-2">&amp;mut *</span>waker };
<span class="comment">// Do we need to register the new waker?
</span><span class="kw">if </span>waker
.as_ref()
.map(|waker| !waker.will_wake(cx.waker()))
.unwrap_or(<span class="bool-val">true</span>)
{
<span class="kw-2">*</span>waker = <span class="prelude-val">Some</span>(cx.waker().clone());
}
});
<span class="comment">// If the waiter is not already in the wait queue, enqueue it.
</span><span class="kw">if </span>!queued {
<span class="kw">let </span>node = <span class="kw">unsafe </span>{
<span class="kw">let </span>node = Pin::into_inner_unchecked(node) <span class="kw">as </span><span class="kw-2">*mut </span><span class="kw">_</span>;
NonNull::new_unchecked(node)
};
waiters.queue.push_front(node);
}
Pending
}
}
<span class="kw">impl </span>fmt::Debug <span class="kw">for </span>Semaphore {
<span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, fmt: <span class="kw-2">&amp;mut </span>fmt::Formatter&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; fmt::Result {
fmt.debug_struct(<span class="string">&quot;Semaphore&quot;</span>)
.field(<span class="string">&quot;permits&quot;</span>, <span class="kw-2">&amp;</span><span class="self">self</span>.available_permits())
.finish()
}
}
<span class="kw">impl </span>Waiter {
<span class="kw">fn </span>new(
num_permits: u32,
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))] </span>ctx: trace::AsyncOpTracingCtx,
) -&gt; <span class="self">Self </span>{
Waiter {
waker: UnsafeCell::new(<span class="prelude-val">None</span>),
state: AtomicUsize::new(num_permits <span class="kw">as </span>usize),
pointers: linked_list::Pointers::new(),
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span>ctx,
_p: PhantomPinned,
}
}
<span class="doccomment">/// Assign permits to the waiter.
///
/// Returns `true` if the waiter should be removed from the queue
</span><span class="kw">fn </span>assign_permits(<span class="kw-2">&amp;</span><span class="self">self</span>, n: <span class="kw-2">&amp;mut </span>usize) -&gt; bool {
<span class="kw">let </span><span class="kw-2">mut </span>curr = <span class="self">self</span>.state.load(Acquire);
<span class="kw">loop </span>{
<span class="kw">let </span>assign = cmp::min(curr, <span class="kw-2">*</span>n);
<span class="kw">let </span>next = curr - assign;
<span class="kw">match </span><span class="self">self</span>.state.compare_exchange(curr, next, AcqRel, Acquire) {
<span class="prelude-val">Ok</span>(<span class="kw">_</span>) =&gt; {
<span class="kw-2">*</span>n -= assign;
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span><span class="self">self</span>.ctx.async_op_span.in_scope(|| {
<span class="macro">tracing::trace!</span>(
target: <span class="string">&quot;runtime::resource::async_op::state_update&quot;</span>,
permits_obtained = assign,
permits.op = <span class="string">&quot;add&quot;</span>,
);
});
<span class="kw">return </span>next == <span class="number">0</span>;
}
<span class="prelude-val">Err</span>(actual) =&gt; curr = actual,
}
}
}
}
<span class="kw">impl </span>Future <span class="kw">for </span>Acquire&lt;<span class="lifetime">&#39;_</span>&gt; {
<span class="kw">type </span>Output = <span class="prelude-ty">Result</span>&lt;(), AcquireError&gt;;
<span class="kw">fn </span>poll(<span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;, cx: <span class="kw-2">&amp;mut </span>Context&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; Poll&lt;<span class="self">Self</span>::Output&gt; {
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span><span class="kw">let </span>_resource_span = <span class="self">self</span>.node.ctx.resource_span.clone().entered();
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span><span class="kw">let </span>_async_op_span = <span class="self">self</span>.node.ctx.async_op_span.clone().entered();
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span><span class="kw">let </span>_async_op_poll_span = <span class="self">self</span>.node.ctx.async_op_poll_span.clone().entered();
<span class="kw">let </span>(node, semaphore, needed, queued) = <span class="self">self</span>.project();
<span class="comment">// First, ensure the current task has enough budget to proceed.
</span><span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span><span class="kw">let </span>coop = <span class="macro">ready!</span>(<span class="macro">trace_poll_op!</span>(
<span class="string">&quot;poll_acquire&quot;</span>,
<span class="kw">crate</span>::runtime::coop::poll_proceed(cx),
));
<span class="attribute">#[cfg(not(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>)))]
</span><span class="kw">let </span>coop = <span class="macro">ready!</span>(<span class="kw">crate</span>::runtime::coop::poll_proceed(cx));
<span class="kw">let </span>result = <span class="kw">match </span>semaphore.poll_acquire(cx, needed, node, <span class="kw-2">*</span>queued) {
Pending =&gt; {
<span class="kw-2">*</span>queued = <span class="bool-val">true</span>;
Pending
}
Ready(r) =&gt; {
coop.made_progress();
r<span class="question-mark">?</span>;
<span class="kw-2">*</span>queued = <span class="bool-val">false</span>;
Ready(<span class="prelude-val">Ok</span>(()))
}
};
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span><span class="kw">return </span><span class="macro">trace_poll_op!</span>(<span class="string">&quot;poll_acquire&quot;</span>, result);
<span class="attribute">#[cfg(not(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>)))]
</span><span class="kw">return </span>result;
}
}
<span class="kw">impl</span>&lt;<span class="lifetime">&#39;a</span>&gt; Acquire&lt;<span class="lifetime">&#39;a</span>&gt; {
<span class="kw">fn </span>new(semaphore: <span class="kw-2">&amp;</span><span class="lifetime">&#39;a </span>Semaphore, num_permits: u32) -&gt; <span class="self">Self </span>{
<span class="attribute">#[cfg(any(not(tokio_unstable), not(feature = <span class="string">&quot;tracing&quot;</span>)))]
</span><span class="kw">return </span><span class="self">Self </span>{
node: Waiter::new(num_permits),
semaphore,
num_permits,
queued: <span class="bool-val">false</span>,
};
<span class="attribute">#[cfg(all(tokio_unstable, feature = <span class="string">&quot;tracing&quot;</span>))]
</span><span class="kw">return </span>semaphore.resource_span.in_scope(|| {
<span class="kw">let </span>async_op_span =
<span class="macro">tracing::trace_span!</span>(<span class="string">&quot;runtime.resource.async_op&quot;</span>, source = <span class="string">&quot;Acquire::new&quot;</span>);
<span class="kw">let </span>async_op_poll_span = async_op_span.in_scope(|| {
<span class="macro">tracing::trace!</span>(
target: <span class="string">&quot;runtime::resource::async_op::state_update&quot;</span>,
permits_requested = num_permits,
permits.op = <span class="string">&quot;override&quot;</span>,
);
<span class="macro">tracing::trace!</span>(
target: <span class="string">&quot;runtime::resource::async_op::state_update&quot;</span>,
permits_obtained = <span class="number">0usize</span>,
permits.op = <span class="string">&quot;override&quot;</span>,
);
<span class="macro">tracing::trace_span!</span>(<span class="string">&quot;runtime.resource.async_op.poll&quot;</span>)
});
<span class="kw">let </span>ctx = trace::AsyncOpTracingCtx {
async_op_span,
async_op_poll_span,
resource_span: semaphore.resource_span.clone(),
};
<span class="self">Self </span>{
node: Waiter::new(num_permits, ctx),
semaphore,
num_permits,
queued: <span class="bool-val">false</span>,
}
});
}
<span class="kw">fn </span>project(<span class="self">self</span>: Pin&lt;<span class="kw-2">&amp;mut </span><span class="self">Self</span>&gt;) -&gt; (Pin&lt;<span class="kw-2">&amp;mut </span>Waiter&gt;, <span class="kw-2">&amp;</span>Semaphore, u32, <span class="kw-2">&amp;mut </span>bool) {
<span class="kw">fn </span>is_unpin&lt;T: Unpin&gt;() {}
<span class="kw">unsafe </span>{
<span class="comment">// Safety: all fields other than `node` are `Unpin`
</span>is_unpin::&lt;<span class="kw-2">&amp;</span>Semaphore&gt;();
is_unpin::&lt;<span class="kw-2">&amp;mut </span>bool&gt;();
is_unpin::&lt;u32&gt;();
<span class="kw">let </span>this = <span class="self">self</span>.get_unchecked_mut();
(
Pin::new_unchecked(<span class="kw-2">&amp;mut </span>this.node),
this.semaphore,
this.num_permits,
<span class="kw-2">&amp;mut </span>this.queued,
)
}
}
}
<span class="kw">impl </span>Drop <span class="kw">for </span>Acquire&lt;<span class="lifetime">&#39;_</span>&gt; {
<span class="kw">fn </span>drop(<span class="kw-2">&amp;mut </span><span class="self">self</span>) {
<span class="comment">// If the future is completed, there is no node in the wait list, so we
// can skip acquiring the lock.
</span><span class="kw">if </span>!<span class="self">self</span>.queued {
<span class="kw">return</span>;
}
<span class="comment">// This is where we ensure safety. The future is being dropped,
// which means we must ensure that the waiter entry is no longer stored
// in the linked list.
</span><span class="kw">let </span><span class="kw-2">mut </span>waiters = <span class="self">self</span>.semaphore.waiters.lock();
<span class="comment">// remove the entry from the list
</span><span class="kw">let </span>node = NonNull::from(<span class="kw-2">&amp;mut </span><span class="self">self</span>.node);
<span class="comment">// Safety: we have locked the wait list.
</span><span class="kw">unsafe </span>{ waiters.queue.remove(node) };
<span class="kw">let </span>acquired_permits = <span class="self">self</span>.num_permits <span class="kw">as </span>usize - <span class="self">self</span>.node.state.load(Acquire);
<span class="kw">if </span>acquired_permits &gt; <span class="number">0 </span>{
<span class="self">self</span>.semaphore.add_permits_locked(acquired_permits, waiters);
}
}
}
<span class="comment">// Safety: the `Acquire` future is not `Sync` automatically because it contains
// a `Waiter`, which, in turn, contains an `UnsafeCell`. However, the
// `UnsafeCell` is only accessed when the future is borrowed mutably (either in
// `poll` or in `drop`). Therefore, it is safe (although not particularly
// _useful_) for the future to be borrowed immutably across threads.
</span><span class="kw">unsafe impl </span>Sync <span class="kw">for </span>Acquire&lt;<span class="lifetime">&#39;_</span>&gt; {}
<span class="comment">// ===== impl AcquireError ====
</span><span class="kw">impl </span>AcquireError {
<span class="kw">fn </span>closed() -&gt; AcquireError {
AcquireError(())
}
}
<span class="kw">impl </span>fmt::Display <span class="kw">for </span>AcquireError {
<span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, fmt: <span class="kw-2">&amp;mut </span>fmt::Formatter&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; fmt::Result {
<span class="macro">write!</span>(fmt, <span class="string">&quot;semaphore closed&quot;</span>)
}
}
<span class="kw">impl </span>std::error::Error <span class="kw">for </span>AcquireError {}
<span class="comment">// ===== impl TryAcquireError =====
</span><span class="kw">impl </span>TryAcquireError {
<span class="doccomment">/// Returns `true` if the error was caused by a closed semaphore.
</span><span class="attribute">#[allow(dead_code)] </span><span class="comment">// may be used later!
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>is_closed(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
<span class="macro">matches!</span>(<span class="self">self</span>, TryAcquireError::Closed)
}
<span class="doccomment">/// Returns `true` if the error was caused by calling `try_acquire` on a
/// semaphore with no available permits.
</span><span class="attribute">#[allow(dead_code)] </span><span class="comment">// may be used later!
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>is_no_permits(<span class="kw-2">&amp;</span><span class="self">self</span>) -&gt; bool {
<span class="macro">matches!</span>(<span class="self">self</span>, TryAcquireError::NoPermits)
}
}
<span class="kw">impl </span>fmt::Display <span class="kw">for </span>TryAcquireError {
<span class="kw">fn </span>fmt(<span class="kw-2">&amp;</span><span class="self">self</span>, fmt: <span class="kw-2">&amp;mut </span>fmt::Formatter&lt;<span class="lifetime">&#39;_</span>&gt;) -&gt; fmt::Result {
<span class="kw">match </span><span class="self">self </span>{
TryAcquireError::Closed =&gt; <span class="macro">write!</span>(fmt, <span class="string">&quot;semaphore closed&quot;</span>),
TryAcquireError::NoPermits =&gt; <span class="macro">write!</span>(fmt, <span class="string">&quot;no permits available&quot;</span>),
}
}
}
<span class="kw">impl </span>std::error::Error <span class="kw">for </span>TryAcquireError {}
<span class="doccomment">/// # Safety
///
/// `Waiter` is forced to be !Unpin.
</span><span class="kw">unsafe impl </span>linked_list::Link <span class="kw">for </span>Waiter {
<span class="kw">type </span>Handle = NonNull&lt;Waiter&gt;;
<span class="kw">type </span>Target = Waiter;
<span class="kw">fn </span>as_raw(handle: <span class="kw-2">&amp;</span><span class="self">Self</span>::Handle) -&gt; NonNull&lt;Waiter&gt; {
<span class="kw-2">*</span>handle
}
<span class="kw">unsafe fn </span>from_raw(ptr: NonNull&lt;Waiter&gt;) -&gt; NonNull&lt;Waiter&gt; {
ptr
}
<span class="kw">unsafe fn </span>pointers(target: NonNull&lt;Waiter&gt;) -&gt; NonNull&lt;linked_list::Pointers&lt;Waiter&gt;&gt; {
Waiter::addr_of_pointers(target)
}
}
</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>