| <!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/arc-swap-1.6.0/src/debt/mod.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>mod.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="../../../arc_swap/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="../../../arc_swap/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">//! Debt handling. |
| //! |
| //! A debt is a reference count of a smart pointer that is owed. This module provides a lock-free |
| //! storage for debts. |
| //! |
| //! Each thread has its own node with bunch of slots. Only that thread can allocate debts in there, |
| //! but others are allowed to inspect and pay them. The nodes form a linked list for the reason of |
| //! inspection. The nodes are never removed (even after the thread terminates), but if the thread |
| //! gives it up, another (new) thread can claim it. |
| //! |
| //! The writers walk the whole chain and pay the debts (by bumping the ref counts) of the just |
| //! removed pointer. |
| //! |
| //! Each node has some fast (but fallible) nodes and a fallback node, with different algorithms to |
| //! claim them (see the relevant submodules). |
| |
| </span><span class="kw">use </span>std::sync::atomic::AtomicUsize; |
| <span class="kw">use </span>std::sync::atomic::Ordering::<span class="kw-2">*</span>; |
| |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">use </span><span class="self">self</span>::list::{LocalNode, Node}; |
| <span class="kw">use </span><span class="kw">super</span>::RefCnt; |
| |
| <span class="kw">mod </span>fast; |
| <span class="kw">mod </span>helping; |
| <span class="kw">mod </span>list; |
| |
| <span class="doccomment">/// One debt slot. |
| /// |
| /// It may contain an „owed“ reference count. |
| </span><span class="attribute">#[derive(Debug)] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>Debt(<span class="kw">pub</span>(<span class="kw">crate</span>) AtomicUsize); |
| |
| <span class="kw">impl </span>Debt { |
| <span class="doccomment">/// The value of pointer `3` should be pretty safe, for two reasons: |
| /// |
| /// * It's an odd number, but the pointers we have are likely aligned at least to the word size, |
| /// because the data at the end of the `Arc` has the counters. |
| /// * It's in the very first page where NULL lives, so it's not mapped. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">const </span>NONE: usize = <span class="number">0b11</span>; |
| } |
| |
| <span class="kw">impl </span>Default <span class="kw">for </span>Debt { |
| <span class="kw">fn </span>default() -> <span class="self">Self </span>{ |
| Debt(AtomicUsize::new(<span class="self">Self</span>::NONE)) |
| } |
| } |
| |
| <span class="kw">impl </span>Debt { |
| <span class="doccomment">/// Tries to pay the given debt. |
| /// |
| /// If the debt is still there, for the given pointer, it is paid and `true` is returned. If it |
| /// is empty or if there's some other pointer, it is not paid and `false` is returned, meaning |
| /// the debt was paid previously by someone else. |
| /// |
| /// # Notes |
| /// |
| /// * It is possible that someone paid the debt and then someone else put a debt for the same |
| /// pointer in there. This is fine, as we'll just pay the debt for that someone else. |
| /// * This relies on the fact that the same pointer must point to the same object and |
| /// specifically to the same type ‒ the caller provides the type, it's destructor, etc. |
| /// * It also relies on the fact the same thing is not stuffed both inside an `Arc` and `Rc` or |
| /// something like that, but that sounds like a reasonable assumption. Someone storing it |
| /// through `ArcSwap<T>` and someone else with `ArcSwapOption<T>` will work. |
| </span><span class="attribute">#[inline] |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>pay<T: RefCnt>(<span class="kw-2">&</span><span class="self">self</span>, ptr: <span class="kw-2">*const </span>T::Base) -> bool { |
| <span class="self">self</span>.<span class="number">0 |
| </span><span class="comment">// If we don't change anything because there's something else, Relaxed is fine. |
| // |
| // The Release works as kind of Mutex. We make sure nothing from the debt-protected |
| // sections leaks below this point. |
| // |
| // Note that if it got paid already, it is inside the reference count. We don't |
| // necessarily observe that increment, but whoever destroys the pointer *must* see the |
| // up to date value, with all increments already counted in (the Arc takes care of that |
| // part). |
| </span>.compare_exchange(ptr <span class="kw">as </span>usize, <span class="self">Self</span>::NONE, Release, Relaxed) |
| .is_ok() |
| } |
| |
| <span class="doccomment">/// Pays all the debts on the given pointer and the storage. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>pay_all<T, R>(ptr: <span class="kw-2">*const </span>T::Base, storage_addr: usize, replacement: R) |
| <span class="kw">where |
| </span>T: RefCnt, |
| R: Fn() -> T, |
| { |
| LocalNode::with(|local| { |
| <span class="kw">let </span>val = <span class="kw">unsafe </span>{ T::from_ptr(ptr) }; |
| <span class="comment">// Pre-pay one ref count that can be safely put into a debt slot to pay it. |
| </span>T::inc(<span class="kw-2">&</span>val); |
| |
| Node::traverse::<(), <span class="kw">_</span>>(|node| { |
| <span class="comment">// Make the cooldown trick know we are poking into this node. |
| </span><span class="kw">let </span>_reservation = node.reserve_writer(); |
| |
| local.help(node, storage_addr, <span class="kw-2">&</span>replacement); |
| |
| <span class="kw">let </span>all_slots = node |
| .fast_slots() |
| .chain(std::iter::once(node.helping_slot())); |
| <span class="kw">for </span>slot <span class="kw">in </span>all_slots { |
| <span class="comment">// Note: Release is enough even here. That makes sure the increment is |
| // visible to whoever might acquire on this slot and can't leak below this. |
| // And we are the ones doing decrements anyway. |
| </span><span class="kw">if </span>slot.pay::<T>(ptr) { |
| <span class="comment">// Pre-pay one more, for another future slot |
| </span>T::inc(<span class="kw-2">&</span>val); |
| } |
| } |
| |
| <span class="prelude-val">None |
| </span>}); |
| <span class="comment">// Implicit dec by dropping val in here, pair for the above |
| </span>}) |
| } |
| } |
| |
| <span class="attribute">#[cfg(test)] |
| </span><span class="kw">mod </span>tests { |
| <span class="kw">use </span>std::sync::Arc; |
| |
| <span class="doccomment">/// Checks the assumption that arcs to ZSTs have different pointer values. |
| </span><span class="attribute">#[test] |
| </span><span class="kw">fn </span>arc_zst() { |
| <span class="kw">struct </span>A; |
| <span class="kw">struct </span>B; |
| |
| <span class="kw">let </span>a = Arc::new(A); |
| <span class="kw">let </span>b = Arc::new(B); |
| |
| <span class="kw">let </span>aref: <span class="kw-2">&</span>A = <span class="kw-2">&</span>a; |
| <span class="kw">let </span>bref: <span class="kw-2">&</span>B = <span class="kw-2">&</span>b; |
| |
| <span class="kw">let </span>aptr = aref <span class="kw">as </span><span class="kw-2">*const </span><span class="kw">_ as </span>usize; |
| <span class="kw">let </span>bptr = bref <span class="kw">as </span><span class="kw-2">*const </span><span class="kw">_ as </span>usize; |
| <span class="macro">assert_ne!</span>(aptr, bptr); |
| } |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../../" data-current-crate="arc_swap" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
