| <!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/crossbeam-epoch-0.9.14/src/internal.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>internal.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="../../crossbeam_epoch/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="../../crossbeam_epoch/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="doccomment">//! The global data and participant for garbage collection. |
| //! |
| //! # Registration |
| //! |
| //! In order to track all participants in one place, we need some form of participant |
| //! registration. When a participant is created, it is registered to a global lock-free |
| //! singly-linked list of registries; and when a participant is leaving, it is unregistered from the |
| //! list. |
| //! |
| //! # Pinning |
| //! |
| //! Every participant contains an integer that tells whether the participant is pinned and if so, |
| //! what was the global epoch at the time it was pinned. Participants also hold a pin counter that |
| //! aids in periodic global epoch advancement. |
| //! |
| //! When a participant is pinned, a `Guard` is returned as a witness that the participant is pinned. |
| //! Guards are necessary for performing atomic operations, and for freeing/dropping locations. |
| //! |
| //! # Thread-local bag |
| //! |
| //! Objects that get unlinked from concurrent data structures must be stashed away until the global |
| //! epoch sufficiently advances so that they become safe for destruction. Pointers to such objects |
| //! are pushed into a thread-local bag, and when it becomes full, the bag is marked with the current |
| //! global epoch and pushed into the global queue of bags. We store objects in thread-local storages |
| //! for amortizing the synchronization cost of pushing the garbages to a global queue. |
| //! |
| //! # Global queue |
| //! |
| //! Whenever a bag is pushed into a queue, the objects in some bags in the queue are collected and |
| //! destroyed along the way. This design reduces contention on data structures. The global queue |
| //! cannot be explicitly accessed: the only way to interact with it is by calling functions |
| //! `defer()` that adds an object to the thread-local bag, or `collect()` that manually triggers |
| //! garbage collection. |
| //! |
| //! Ideally each instance of concurrent data structure may have its own queue that gets fully |
| //! destroyed as soon as the data structure gets dropped. |
| |
| </span><span class="kw">use </span><span class="kw">crate</span>::primitive::cell::UnsafeCell; |
| <span class="kw">use </span><span class="kw">crate</span>::primitive::sync::atomic; |
| <span class="kw">use </span>core::cell::Cell; |
| <span class="kw">use </span>core::mem::{<span class="self">self</span>, ManuallyDrop}; |
| <span class="kw">use </span>core::num::Wrapping; |
| <span class="kw">use </span>core::sync::atomic::Ordering; |
| <span class="kw">use </span>core::{fmt, ptr}; |
| |
| <span class="kw">use </span>crossbeam_utils::CachePadded; |
| <span class="kw">use </span>memoffset::offset_of; |
| |
| <span class="kw">use </span><span class="kw">crate</span>::atomic::{Owned, Shared}; |
| <span class="kw">use </span><span class="kw">crate</span>::collector::{Collector, LocalHandle}; |
| <span class="kw">use </span><span class="kw">crate</span>::deferred::Deferred; |
| <span class="kw">use </span><span class="kw">crate</span>::epoch::{AtomicEpoch, Epoch}; |
| <span class="kw">use </span><span class="kw">crate</span>::guard::{unprotected, Guard}; |
| <span class="kw">use </span><span class="kw">crate</span>::sync::list::{Entry, IsElement, IterError, List}; |
| <span class="kw">use </span><span class="kw">crate</span>::sync::queue::Queue; |
| |
| <span class="doccomment">/// Maximum number of objects a bag can contain. |
| </span><span class="attribute">#[cfg(not(any(crossbeam_sanitize, miri)))] |
| </span><span class="kw">const </span>MAX_OBJECTS: usize = <span class="number">64</span>; |
| <span class="comment">// Makes it more likely to trigger any potential data races. |
| </span><span class="attribute">#[cfg(any(crossbeam_sanitize, miri))] |
| </span><span class="kw">const </span>MAX_OBJECTS: usize = <span class="number">4</span>; |
| |
| <span class="doccomment">/// A bag of deferred functions. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>Bag { |
| <span class="doccomment">/// Stashed objects. |
| </span>deferreds: [Deferred; MAX_OBJECTS], |
| len: usize, |
| } |
| |
| <span class="doccomment">/// `Bag::try_push()` requires that it is safe for another thread to execute the given functions. |
| </span><span class="kw">unsafe impl </span>Send <span class="kw">for </span>Bag {} |
| |
| <span class="kw">impl </span>Bag { |
| <span class="doccomment">/// Returns a new, empty bag. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>new() -> <span class="self">Self </span>{ |
| <span class="self">Self</span>::default() |
| } |
| |
| <span class="doccomment">/// Returns `true` if the bag is empty. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>is_empty(<span class="kw-2">&</span><span class="self">self</span>) -> bool { |
| <span class="self">self</span>.len == <span class="number">0 |
| </span>} |
| |
| <span class="doccomment">/// Attempts to insert a deferred function into the bag. |
| /// |
| /// Returns `Ok(())` if successful, and `Err(deferred)` for the given `deferred` if the bag is |
| /// full. |
| /// |
| /// # Safety |
| /// |
| /// It should be safe for another thread to execute the given function. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">unsafe fn </span>try_push(<span class="kw-2">&mut </span><span class="self">self</span>, deferred: Deferred) -> <span class="prelude-ty">Result</span><(), Deferred> { |
| <span class="kw">if </span><span class="self">self</span>.len < MAX_OBJECTS { |
| <span class="self">self</span>.deferreds[<span class="self">self</span>.len] = deferred; |
| <span class="self">self</span>.len += <span class="number">1</span>; |
| <span class="prelude-val">Ok</span>(()) |
| } <span class="kw">else </span>{ |
| <span class="prelude-val">Err</span>(deferred) |
| } |
| } |
| |
| <span class="doccomment">/// Seals the bag with the given epoch. |
| </span><span class="kw">fn </span>seal(<span class="self">self</span>, epoch: Epoch) -> SealedBag { |
| SealedBag { epoch, _bag: <span class="self">self </span>} |
| } |
| } |
| |
| <span class="kw">impl </span>Default <span class="kw">for </span>Bag { |
| <span class="kw">fn </span>default() -> <span class="self">Self </span>{ |
| Bag { |
| len: <span class="number">0</span>, |
| deferreds: [Deferred::NO_OP; MAX_OBJECTS], |
| } |
| } |
| } |
| |
| <span class="kw">impl </span>Drop <span class="kw">for </span>Bag { |
| <span class="kw">fn </span>drop(<span class="kw-2">&mut </span><span class="self">self</span>) { |
| <span class="comment">// Call all deferred functions. |
| </span><span class="kw">for </span>deferred <span class="kw">in </span><span class="kw-2">&mut </span><span class="self">self</span>.deferreds[..<span class="self">self</span>.len] { |
| <span class="kw">let </span>no_op = Deferred::NO_OP; |
| <span class="kw">let </span>owned_deferred = mem::replace(deferred, no_op); |
| owned_deferred.call(); |
| } |
| } |
| } |
| |
| <span class="comment">// can't #[derive(Debug)] because Debug is not implemented for arrays 64 items long |
| </span><span class="kw">impl </span>fmt::Debug <span class="kw">for </span>Bag { |
| <span class="kw">fn </span>fmt(<span class="kw-2">&</span><span class="self">self</span>, f: <span class="kw-2">&mut </span>fmt::Formatter<<span class="lifetime">'_</span>>) -> fmt::Result { |
| f.debug_struct(<span class="string">"Bag"</span>) |
| .field(<span class="string">"deferreds"</span>, &&<span class="self">self</span>.deferreds[..<span class="self">self</span>.len]) |
| .finish() |
| } |
| } |
| |
| <span class="doccomment">/// A pair of an epoch and a bag. |
| </span><span class="attribute">#[derive(Default, Debug)] |
| </span><span class="kw">struct </span>SealedBag { |
| epoch: Epoch, |
| _bag: Bag, |
| } |
| |
| <span class="doccomment">/// It is safe to share `SealedBag` because `is_expired` only inspects the epoch. |
| </span><span class="kw">unsafe impl </span>Sync <span class="kw">for </span>SealedBag {} |
| |
| <span class="kw">impl </span>SealedBag { |
| <span class="doccomment">/// Checks if it is safe to drop the bag w.r.t. the given global epoch. |
| </span><span class="kw">fn </span>is_expired(<span class="kw-2">&</span><span class="self">self</span>, global_epoch: Epoch) -> bool { |
| <span class="comment">// A pinned participant can witness at most one epoch advancement. Therefore, any bag that |
| // is within one epoch of the current one cannot be destroyed yet. |
| </span>global_epoch.wrapping_sub(<span class="self">self</span>.epoch) >= <span class="number">2 |
| </span>} |
| } |
| |
| <span class="doccomment">/// The global data for a garbage collector. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>Global { |
| <span class="doccomment">/// The intrusive linked list of `Local`s. |
| </span>locals: List<Local>, |
| |
| <span class="doccomment">/// The global queue of bags of deferred functions. |
| </span>queue: Queue<SealedBag>, |
| |
| <span class="doccomment">/// The global epoch. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) epoch: CachePadded<AtomicEpoch>, |
| } |
| |
| <span class="kw">impl </span>Global { |
| <span class="doccomment">/// Number of bags to destroy. |
| </span><span class="kw">const </span>COLLECT_STEPS: usize = <span class="number">8</span>; |
| |
| <span class="doccomment">/// Creates a new global data for garbage collection. |
| </span><span class="attribute">#[inline] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>new() -> <span class="self">Self </span>{ |
| <span class="self">Self </span>{ |
| locals: List::new(), |
| queue: Queue::new(), |
| epoch: CachePadded::new(AtomicEpoch::new(Epoch::starting())), |
| } |
| } |
| |
| <span class="doccomment">/// Pushes the bag into the global queue and replaces the bag with a new empty bag. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>push_bag(<span class="kw-2">&</span><span class="self">self</span>, bag: <span class="kw-2">&mut </span>Bag, guard: <span class="kw-2">&</span>Guard) { |
| <span class="kw">let </span>bag = mem::replace(bag, Bag::new()); |
| |
| atomic::fence(Ordering::SeqCst); |
| |
| <span class="kw">let </span>epoch = <span class="self">self</span>.epoch.load(Ordering::Relaxed); |
| <span class="self">self</span>.queue.push(bag.seal(epoch), guard); |
| } |
| |
| <span class="doccomment">/// Collects several bags from the global queue and executes deferred functions in them. |
| /// |
| /// Note: This may itself produce garbage and in turn allocate new bags. |
| /// |
| /// `pin()` rarely calls `collect()`, so we want the compiler to place that call on a cold |
| /// path. In other words, we want the compiler to optimize branching for the case when |
| /// `collect()` is not called. |
| </span><span class="attribute">#[cold] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>collect(<span class="kw-2">&</span><span class="self">self</span>, guard: <span class="kw-2">&</span>Guard) { |
| <span class="kw">let </span>global_epoch = <span class="self">self</span>.try_advance(guard); |
| |
| <span class="kw">let </span>steps = <span class="kw">if </span><span class="macro">cfg!</span>(crossbeam_sanitize) { |
| usize::max_value() |
| } <span class="kw">else </span>{ |
| <span class="self">Self</span>::COLLECT_STEPS |
| }; |
| |
| <span class="kw">for _ in </span><span class="number">0</span>..steps { |
| <span class="kw">match </span><span class="self">self</span>.queue.try_pop_if( |
| <span class="kw-2">&</span>|sealed_bag: <span class="kw-2">&</span>SealedBag| sealed_bag.is_expired(global_epoch), |
| guard, |
| ) { |
| <span class="prelude-val">None </span>=> <span class="kw">break</span>, |
| <span class="prelude-val">Some</span>(sealed_bag) => drop(sealed_bag), |
| } |
| } |
| } |
| |
| <span class="doccomment">/// Attempts to advance the global epoch. |
| /// |
| /// The global epoch can advance only if all currently pinned participants have been pinned in |
| /// the current epoch. |
| /// |
| /// Returns the current global epoch. |
| /// |
| /// `try_advance()` is annotated `#[cold]` because it is rarely called. |
| </span><span class="attribute">#[cold] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>try_advance(<span class="kw-2">&</span><span class="self">self</span>, guard: <span class="kw-2">&</span>Guard) -> Epoch { |
| <span class="kw">let </span>global_epoch = <span class="self">self</span>.epoch.load(Ordering::Relaxed); |
| atomic::fence(Ordering::SeqCst); |
| |
| <span class="comment">// TODO(stjepang): `Local`s are stored in a linked list because linked lists are fairly |
| // easy to implement in a lock-free manner. However, traversal can be slow due to cache |
| // misses and data dependencies. We should experiment with other data structures as well. |
| </span><span class="kw">for </span>local <span class="kw">in </span><span class="self">self</span>.locals.iter(guard) { |
| <span class="kw">match </span>local { |
| <span class="prelude-val">Err</span>(IterError::Stalled) => { |
| <span class="comment">// A concurrent thread stalled this iteration. That thread might also try to |
| // advance the epoch, in which case we leave the job to it. Otherwise, the |
| // epoch will not be advanced. |
| </span><span class="kw">return </span>global_epoch; |
| } |
| <span class="prelude-val">Ok</span>(local) => { |
| <span class="kw">let </span>local_epoch = local.epoch.load(Ordering::Relaxed); |
| |
| <span class="comment">// If the participant was pinned in a different epoch, we cannot advance the |
| // global epoch just yet. |
| </span><span class="kw">if </span>local_epoch.is_pinned() && local_epoch.unpinned() != global_epoch { |
| <span class="kw">return </span>global_epoch; |
| } |
| } |
| } |
| } |
| atomic::fence(Ordering::Acquire); |
| |
| <span class="comment">// All pinned participants were pinned in the current global epoch. |
| // Now let's advance the global epoch... |
| // |
| // Note that if another thread already advanced it before us, this store will simply |
| // overwrite the global epoch with the same value. This is true because `try_advance` was |
| // called from a thread that was pinned in `global_epoch`, and the global epoch cannot be |
| // advanced two steps ahead of it. |
| </span><span class="kw">let </span>new_epoch = global_epoch.successor(); |
| <span class="self">self</span>.epoch.store(new_epoch, Ordering::Release); |
| new_epoch |
| } |
| } |
| |
| <span class="doccomment">/// Participant for garbage collection. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>Local { |
| <span class="doccomment">/// A node in the intrusive linked list of `Local`s. |
| </span>entry: Entry, |
| |
| <span class="doccomment">/// The local epoch. |
| </span>epoch: AtomicEpoch, |
| |
| <span class="doccomment">/// A reference to the global data. |
| /// |
| /// When all guards and handles get dropped, this reference is destroyed. |
| </span>collector: UnsafeCell<ManuallyDrop<Collector>>, |
| |
| <span class="doccomment">/// The local bag of deferred functions. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) bag: UnsafeCell<Bag>, |
| |
| <span class="doccomment">/// The number of guards keeping this participant pinned. |
| </span>guard_count: Cell<usize>, |
| |
| <span class="doccomment">/// The number of active handles. |
| </span>handle_count: Cell<usize>, |
| |
| <span class="doccomment">/// Total number of pinnings performed. |
| /// |
| /// This is just an auxiliary counter that sometimes kicks off collection. |
| </span>pin_count: Cell<Wrapping<usize>>, |
| } |
| |
| <span class="comment">// Make sure `Local` is less than or equal to 2048 bytes. |
| // https://github.com/crossbeam-rs/crossbeam/issues/551 |
| </span><span class="attribute">#[cfg(not(any(crossbeam_sanitize, miri)))] </span><span class="comment">// `crossbeam_sanitize` and `miri` reduce the size of `Local` |
| </span><span class="attribute">#[test] |
| </span><span class="kw">fn </span>local_size() { |
| <span class="comment">// TODO: https://github.com/crossbeam-rs/crossbeam/issues/869 |
| // assert!( |
| // core::mem::size_of::<Local>() <= 2048, |
| // "An allocation of `Local` should be <= 2048 bytes." |
| // ); |
| </span>} |
| |
| <span class="kw">impl </span>Local { |
| <span class="doccomment">/// Number of pinnings after which a participant will execute some deferred functions from the |
| /// global queue. |
| </span><span class="kw">const </span>PINNINGS_BETWEEN_COLLECT: usize = <span class="number">128</span>; |
| |
| <span class="doccomment">/// Registers a new `Local` in the provided `Global`. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>register(collector: <span class="kw-2">&</span>Collector) -> LocalHandle { |
| <span class="kw">unsafe </span>{ |
| <span class="comment">// Since we dereference no pointers in this block, it is safe to use `unprotected`. |
| |
| </span><span class="kw">let </span>local = Owned::new(Local { |
| entry: Entry::default(), |
| epoch: AtomicEpoch::new(Epoch::starting()), |
| collector: UnsafeCell::new(ManuallyDrop::new(collector.clone())), |
| bag: UnsafeCell::new(Bag::new()), |
| guard_count: Cell::new(<span class="number">0</span>), |
| handle_count: Cell::new(<span class="number">1</span>), |
| pin_count: Cell::new(Wrapping(<span class="number">0</span>)), |
| }) |
| .into_shared(unprotected()); |
| collector.global.locals.insert(local, unprotected()); |
| LocalHandle { |
| local: local.as_raw(), |
| } |
| } |
| } |
| |
| <span class="doccomment">/// Returns a reference to the `Global` in which this `Local` resides. |
| </span><span class="attribute">#[inline] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>global(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="kw-2">&</span>Global { |
| <span class="kw-2">&</span><span class="self">self</span>.collector().global |
| } |
| |
| <span class="doccomment">/// Returns a reference to the `Collector` in which this `Local` resides. |
| </span><span class="attribute">#[inline] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>collector(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="kw-2">&</span>Collector { |
| <span class="self">self</span>.collector.with(|c| <span class="kw">unsafe </span>{ <span class="kw-2">&**</span>c }) |
| } |
| |
| <span class="doccomment">/// Returns `true` if the current participant is pinned. |
| </span><span class="attribute">#[inline] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>is_pinned(<span class="kw-2">&</span><span class="self">self</span>) -> bool { |
| <span class="self">self</span>.guard_count.get() > <span class="number">0 |
| </span>} |
| |
| <span class="doccomment">/// Adds `deferred` to the thread-local bag. |
| /// |
| /// # Safety |
| /// |
| /// It should be safe for another thread to execute the given function. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">unsafe fn </span>defer(<span class="kw-2">&</span><span class="self">self</span>, <span class="kw-2">mut </span>deferred: Deferred, guard: <span class="kw-2">&</span>Guard) { |
| <span class="kw">let </span>bag = <span class="self">self</span>.bag.with_mut(|b| <span class="kw-2">&mut *</span>b); |
| |
| <span class="kw">while let </span><span class="prelude-val">Err</span>(d) = bag.try_push(deferred) { |
| <span class="self">self</span>.global().push_bag(bag, guard); |
| deferred = d; |
| } |
| } |
| |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>flush(<span class="kw-2">&</span><span class="self">self</span>, guard: <span class="kw-2">&</span>Guard) { |
| <span class="kw">let </span>bag = <span class="self">self</span>.bag.with_mut(|b| <span class="kw">unsafe </span>{ <span class="kw-2">&mut *</span>b }); |
| |
| <span class="kw">if </span>!bag.is_empty() { |
| <span class="self">self</span>.global().push_bag(bag, guard); |
| } |
| |
| <span class="self">self</span>.global().collect(guard); |
| } |
| |
| <span class="doccomment">/// Pins the `Local`. |
| </span><span class="attribute">#[inline] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>pin(<span class="kw-2">&</span><span class="self">self</span>) -> Guard { |
| <span class="kw">let </span>guard = Guard { local: <span class="self">self </span>}; |
| |
| <span class="kw">let </span>guard_count = <span class="self">self</span>.guard_count.get(); |
| <span class="self">self</span>.guard_count.set(guard_count.checked_add(<span class="number">1</span>).unwrap()); |
| |
| <span class="kw">if </span>guard_count == <span class="number">0 </span>{ |
| <span class="kw">let </span>global_epoch = <span class="self">self</span>.global().epoch.load(Ordering::Relaxed); |
| <span class="kw">let </span>new_epoch = global_epoch.pinned(); |
| |
| <span class="comment">// Now we must store `new_epoch` into `self.epoch` and execute a `SeqCst` fence. |
| // The fence makes sure that any future loads from `Atomic`s will not happen before |
| // this store. |
| </span><span class="kw">if </span><span class="macro">cfg!</span>(all( |
| any(target_arch = <span class="string">"x86"</span>, target_arch = <span class="string">"x86_64"</span>), |
| not(miri) |
| )) { |
| <span class="comment">// HACK(stjepang): On x86 architectures there are two different ways of executing |
| // a `SeqCst` fence. |
| // |
| // 1. `atomic::fence(SeqCst)`, which compiles into a `mfence` instruction. |
| // 2. `_.compare_exchange(_, _, SeqCst, SeqCst)`, which compiles into a `lock cmpxchg` |
| // instruction. |
| // |
| // Both instructions have the effect of a full barrier, but benchmarks have shown |
| // that the second one makes pinning faster in this particular case. It is not |
| // clear that this is permitted by the C++ memory model (SC fences work very |
| // differently from SC accesses), but experimental evidence suggests that this |
| // works fine. Using inline assembly would be a viable (and correct) alternative, |
| // but alas, that is not possible on stable Rust. |
| </span><span class="kw">let </span>current = Epoch::starting(); |
| <span class="kw">let </span>res = <span class="self">self</span>.epoch.compare_exchange( |
| current, |
| new_epoch, |
| Ordering::SeqCst, |
| Ordering::SeqCst, |
| ); |
| <span class="macro">debug_assert!</span>(res.is_ok(), <span class="string">"participant was expected to be unpinned"</span>); |
| <span class="comment">// We add a compiler fence to make it less likely for LLVM to do something wrong |
| // here. Formally, this is not enough to get rid of data races; practically, |
| // it should go a long way. |
| </span>atomic::compiler_fence(Ordering::SeqCst); |
| } <span class="kw">else </span>{ |
| <span class="self">self</span>.epoch.store(new_epoch, Ordering::Relaxed); |
| atomic::fence(Ordering::SeqCst); |
| } |
| |
| <span class="comment">// Increment the pin counter. |
| </span><span class="kw">let </span>count = <span class="self">self</span>.pin_count.get(); |
| <span class="self">self</span>.pin_count.set(count + Wrapping(<span class="number">1</span>)); |
| |
| <span class="comment">// After every `PINNINGS_BETWEEN_COLLECT` try advancing the epoch and collecting |
| // some garbage. |
| </span><span class="kw">if </span>count.<span class="number">0 </span>% <span class="self">Self</span>::PINNINGS_BETWEEN_COLLECT == <span class="number">0 </span>{ |
| <span class="self">self</span>.global().collect(<span class="kw-2">&</span>guard); |
| } |
| } |
| |
| guard |
| } |
| |
| <span class="doccomment">/// Unpins the `Local`. |
| </span><span class="attribute">#[inline] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>unpin(<span class="kw-2">&</span><span class="self">self</span>) { |
| <span class="kw">let </span>guard_count = <span class="self">self</span>.guard_count.get(); |
| <span class="self">self</span>.guard_count.set(guard_count - <span class="number">1</span>); |
| |
| <span class="kw">if </span>guard_count == <span class="number">1 </span>{ |
| <span class="self">self</span>.epoch.store(Epoch::starting(), Ordering::Release); |
| |
| <span class="kw">if </span><span class="self">self</span>.handle_count.get() == <span class="number">0 </span>{ |
| <span class="self">self</span>.finalize(); |
| } |
| } |
| } |
| |
| <span class="doccomment">/// Unpins and then pins the `Local`. |
| </span><span class="attribute">#[inline] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>repin(<span class="kw-2">&</span><span class="self">self</span>) { |
| <span class="kw">let </span>guard_count = <span class="self">self</span>.guard_count.get(); |
| |
| <span class="comment">// Update the local epoch only if there's only one guard. |
| </span><span class="kw">if </span>guard_count == <span class="number">1 </span>{ |
| <span class="kw">let </span>epoch = <span class="self">self</span>.epoch.load(Ordering::Relaxed); |
| <span class="kw">let </span>global_epoch = <span class="self">self</span>.global().epoch.load(Ordering::Relaxed).pinned(); |
| |
| <span class="comment">// Update the local epoch only if the global epoch is greater than the local epoch. |
| </span><span class="kw">if </span>epoch != global_epoch { |
| <span class="comment">// We store the new epoch with `Release` because we need to ensure any memory |
| // accesses from the previous epoch do not leak into the new one. |
| </span><span class="self">self</span>.epoch.store(global_epoch, Ordering::Release); |
| |
| <span class="comment">// However, we don't need a following `SeqCst` fence, because it is safe for memory |
| // accesses from the new epoch to be executed before updating the local epoch. At |
| // worse, other threads will see the new epoch late and delay GC slightly. |
| </span>} |
| } |
| } |
| |
| <span class="doccomment">/// Increments the handle count. |
| </span><span class="attribute">#[inline] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>acquire_handle(<span class="kw-2">&</span><span class="self">self</span>) { |
| <span class="kw">let </span>handle_count = <span class="self">self</span>.handle_count.get(); |
| <span class="macro">debug_assert!</span>(handle_count >= <span class="number">1</span>); |
| <span class="self">self</span>.handle_count.set(handle_count + <span class="number">1</span>); |
| } |
| |
| <span class="doccomment">/// Decrements the handle count. |
| </span><span class="attribute">#[inline] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>release_handle(<span class="kw-2">&</span><span class="self">self</span>) { |
| <span class="kw">let </span>guard_count = <span class="self">self</span>.guard_count.get(); |
| <span class="kw">let </span>handle_count = <span class="self">self</span>.handle_count.get(); |
| <span class="macro">debug_assert!</span>(handle_count >= <span class="number">1</span>); |
| <span class="self">self</span>.handle_count.set(handle_count - <span class="number">1</span>); |
| |
| <span class="kw">if </span>guard_count == <span class="number">0 </span>&& handle_count == <span class="number">1 </span>{ |
| <span class="self">self</span>.finalize(); |
| } |
| } |
| |
| <span class="doccomment">/// Removes the `Local` from the global linked list. |
| </span><span class="attribute">#[cold] |
| </span><span class="kw">fn </span>finalize(<span class="kw-2">&</span><span class="self">self</span>) { |
| <span class="macro">debug_assert_eq!</span>(<span class="self">self</span>.guard_count.get(), <span class="number">0</span>); |
| <span class="macro">debug_assert_eq!</span>(<span class="self">self</span>.handle_count.get(), <span class="number">0</span>); |
| |
| <span class="comment">// Temporarily increment handle count. This is required so that the following call to `pin` |
| // doesn't call `finalize` again. |
| </span><span class="self">self</span>.handle_count.set(<span class="number">1</span>); |
| <span class="kw">unsafe </span>{ |
| <span class="comment">// Pin and move the local bag into the global queue. It's important that `push_bag` |
| // doesn't defer destruction on any new garbage. |
| </span><span class="kw">let </span>guard = <span class="kw-2">&</span><span class="self">self</span>.pin(); |
| <span class="self">self</span>.global() |
| .push_bag(<span class="self">self</span>.bag.with_mut(|b| <span class="kw-2">&mut *</span>b), guard); |
| } |
| <span class="comment">// Revert the handle count back to zero. |
| </span><span class="self">self</span>.handle_count.set(<span class="number">0</span>); |
| |
| <span class="kw">unsafe </span>{ |
| <span class="comment">// Take the reference to the `Global` out of this `Local`. Since we're not protected |
| // by a guard at this time, it's crucial that the reference is read before marking the |
| // `Local` as deleted. |
| </span><span class="kw">let </span>collector: Collector = ptr::read(<span class="self">self</span>.collector.with(|c| <span class="kw-2">&*</span>(<span class="kw-2">*</span>c))); |
| |
| <span class="comment">// Mark this node in the linked list as deleted. |
| </span><span class="self">self</span>.entry.delete(unprotected()); |
| |
| <span class="comment">// Finally, drop the reference to the global. Note that this might be the last reference |
| // to the `Global`. If so, the global data will be destroyed and all deferred functions |
| // in its queue will be executed. |
| </span>drop(collector); |
| } |
| } |
| } |
| |
| <span class="kw">impl </span>IsElement<Local> <span class="kw">for </span>Local { |
| <span class="kw">fn </span>entry_of(local: <span class="kw-2">&</span>Local) -> <span class="kw-2">&</span>Entry { |
| <span class="kw">let </span>entry_ptr = (local <span class="kw">as </span><span class="kw-2">*const </span>Local <span class="kw">as </span>usize + <span class="macro">offset_of!</span>(Local, entry)) <span class="kw">as </span><span class="kw-2">*const </span>Entry; |
| <span class="kw">unsafe </span>{ <span class="kw-2">&*</span>entry_ptr } |
| } |
| |
| <span class="kw">unsafe fn </span>element_of(entry: <span class="kw-2">&</span>Entry) -> <span class="kw-2">&</span>Local { |
| <span class="comment">// offset_of! macro uses unsafe, but it's unnecessary in this context. |
| </span><span class="attribute">#[allow(unused_unsafe)] |
| </span><span class="kw">let </span>local_ptr = (entry <span class="kw">as </span><span class="kw-2">*const </span>Entry <span class="kw">as </span>usize - <span class="macro">offset_of!</span>(Local, entry)) <span class="kw">as </span><span class="kw-2">*const </span>Local; |
| <span class="kw-2">&*</span>local_ptr |
| } |
| |
| <span class="kw">unsafe fn </span>finalize(entry: <span class="kw-2">&</span>Entry, guard: <span class="kw-2">&</span>Guard) { |
| guard.defer_destroy(Shared::from(<span class="self">Self</span>::element_of(entry) <span class="kw">as </span><span class="kw-2">*const </span><span class="kw">_</span>)); |
| } |
| } |
| |
| <span class="attribute">#[cfg(all(test, not(crossbeam_loom)))] |
| </span><span class="kw">mod </span>tests { |
| <span class="kw">use </span>std::sync::atomic::{AtomicUsize, Ordering}; |
| |
| <span class="kw">use super</span>::<span class="kw-2">*</span>; |
| |
| <span class="attribute">#[test] |
| </span><span class="kw">fn </span>check_defer() { |
| <span class="kw">static </span>FLAG: AtomicUsize = AtomicUsize::new(<span class="number">0</span>); |
| <span class="kw">fn </span>set() { |
| FLAG.store(<span class="number">42</span>, Ordering::Relaxed); |
| } |
| |
| <span class="kw">let </span>d = Deferred::new(set); |
| <span class="macro">assert_eq!</span>(FLAG.load(Ordering::Relaxed), <span class="number">0</span>); |
| d.call(); |
| <span class="macro">assert_eq!</span>(FLAG.load(Ordering::Relaxed), <span class="number">42</span>); |
| } |
| |
| <span class="attribute">#[test] |
| </span><span class="kw">fn </span>check_bag() { |
| <span class="kw">static </span>FLAG: AtomicUsize = AtomicUsize::new(<span class="number">0</span>); |
| <span class="kw">fn </span>incr() { |
| FLAG.fetch_add(<span class="number">1</span>, Ordering::Relaxed); |
| } |
| |
| <span class="kw">let </span><span class="kw-2">mut </span>bag = Bag::new(); |
| <span class="macro">assert!</span>(bag.is_empty()); |
| |
| <span class="kw">for _ in </span><span class="number">0</span>..MAX_OBJECTS { |
| <span class="macro">assert!</span>(<span class="kw">unsafe </span>{ bag.try_push(Deferred::new(incr)).is_ok() }); |
| <span class="macro">assert!</span>(!bag.is_empty()); |
| <span class="macro">assert_eq!</span>(FLAG.load(Ordering::Relaxed), <span class="number">0</span>); |
| } |
| |
| <span class="kw">let </span>result = <span class="kw">unsafe </span>{ bag.try_push(Deferred::new(incr)) }; |
| <span class="macro">assert!</span>(result.is_err()); |
| <span class="macro">assert!</span>(!bag.is_empty()); |
| <span class="macro">assert_eq!</span>(FLAG.load(Ordering::Relaxed), <span class="number">0</span>); |
| |
| drop(bag); |
| <span class="macro">assert_eq!</span>(FLAG.load(Ordering::Relaxed), MAX_OBJECTS); |
| } |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="crossbeam_epoch" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
