| <!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/signal-hook-registry-1.4.1/src/half_lock.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>half_lock.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="../../signal_hook_registry/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="../../signal_hook_registry/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 half-lock structure |
| //! |
| //! We need a way to protect the structure with configured hooks ‒ a signal may happen in arbitrary |
| //! thread and needs to read them while another thread might be manipulating the structure. |
| //! |
| //! Under ordinary circumstances we would be happy to just use `Mutex<HashMap<c_int, _>>`. However, |
| //! as we use it in the signal handler, we are severely limited in what we can or can't use. So we |
| //! choose to implement kind of spin-look thing with atomics. |
| //! |
| //! In the reader it is always simply locked and then unlocked, making sure it doesn't disappear |
| //! while in use. |
| //! |
| //! The writer has a separate mutex (that prevents other writers; this is used outside of the |
| //! signal handler), makes a copy of the data and swaps an atomic pointer to the data structure. |
| //! But it waits until everything is unlocked (no signal handler has the old data) for dropping the |
| //! old instance. There's a generation trick to make sure that new signal locks another instance. |
| //! |
| //! The downside is, this is an active spin lock at the writer end. However, we assume than: |
| //! |
| //! * Signals are one time setup before we actually have threads. We just need to make *sure* we |
| //! are safe even if this is not true. |
| //! * Signals are rare, happening at the same time as the write even rarer. |
| //! * Signals are short, as there is mostly nothing allowed inside them anyway. |
| //! * Our tool box is severely limited. |
| //! |
| //! Therefore this is hopefully reasonable trade-off. |
| //! |
| //! # Atomic orderings |
| //! |
| //! The whole code uses SeqCst conservatively. Atomics are not used because of performance here and |
| //! are the minor price around signals anyway. But the comments state which orderings should be |
| //! enough in practice in case someone wants to get inspired (but do make your own check through |
| //! them anyway). |
| |
| </span><span class="kw">use </span>std::isize; |
| <span class="kw">use </span>std::marker::PhantomData; |
| <span class="kw">use </span>std::ops::Deref; |
| <span class="kw">use </span>std::sync::atomic::{<span class="self">self</span>, AtomicPtr, AtomicUsize, Ordering}; |
| <span class="kw">use </span>std::sync::{Mutex, MutexGuard, PoisonError}; |
| <span class="kw">use </span>std::thread; |
| |
| <span class="kw">use </span>libc; |
| |
| <span class="kw">const </span>YIELD_EVERY: usize = <span class="number">16</span>; |
| <span class="kw">const </span>MAX_GUARDS: usize = (isize::MAX) <span class="kw">as </span>usize; |
| |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>ReadGuard<<span class="lifetime">'a</span>, T: <span class="lifetime">'a</span>> { |
| data: <span class="kw-2">&</span><span class="lifetime">'a </span>T, |
| lock: <span class="kw-2">&</span><span class="lifetime">'a </span>AtomicUsize, |
| } |
| |
| <span class="kw">impl</span><<span class="lifetime">'a</span>, T> Deref <span class="kw">for </span>ReadGuard<<span class="lifetime">'a</span>, T> { |
| <span class="kw">type </span>Target = T; |
| <span class="kw">fn </span>deref(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="kw-2">&</span>T { |
| <span class="self">self</span>.data |
| } |
| } |
| |
| <span class="kw">impl</span><<span class="lifetime">'a</span>, T> Drop <span class="kw">for </span>ReadGuard<<span class="lifetime">'a</span>, T> { |
| <span class="kw">fn </span>drop(<span class="kw-2">&mut </span><span class="self">self</span>) { |
| <span class="comment">// We effectively unlock; Release would be enough. |
| </span><span class="self">self</span>.lock.fetch_sub(<span class="number">1</span>, Ordering::SeqCst); |
| } |
| } |
| |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>WriteGuard<<span class="lifetime">'a</span>, T: <span class="lifetime">'a</span>> { |
| _guard: MutexGuard<<span class="lifetime">'a</span>, ()>, |
| lock: <span class="kw-2">&</span><span class="lifetime">'a </span>HalfLock<T>, |
| data: <span class="kw-2">&</span><span class="lifetime">'a </span>T, |
| } |
| |
| <span class="kw">impl</span><<span class="lifetime">'a</span>, T> WriteGuard<<span class="lifetime">'a</span>, T> { |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>store(<span class="kw-2">&mut </span><span class="self">self</span>, val: T) { |
| <span class="comment">// Move to the heap and convert to raw pointer for AtomicPtr. |
| </span><span class="kw">let </span>new = Box::into_raw(Box::new(val)); |
| |
| <span class="self">self</span>.data = <span class="kw">unsafe </span>{ <span class="kw-2">&*</span>new }; |
| |
| <span class="comment">// We can just put the new value in here safely, we worry only about dropping the old one. |
| // Release might (?) be enough, to "upload" the data. |
| </span><span class="kw">let </span>old = <span class="self">self</span>.lock.data.swap(new, Ordering::SeqCst); |
| |
| <span class="comment">// Now we make sure there's no reader having the old data. |
| </span><span class="self">self</span>.lock.write_barrier(); |
| |
| drop(<span class="kw">unsafe </span>{ Box::from_raw(old) }); |
| } |
| } |
| |
| <span class="kw">impl</span><<span class="lifetime">'a</span>, T> Deref <span class="kw">for </span>WriteGuard<<span class="lifetime">'a</span>, T> { |
| <span class="kw">type </span>Target = T; |
| <span class="kw">fn </span>deref(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="kw-2">&</span>T { |
| <span class="comment">// Protected by that mutex |
| </span><span class="self">self</span>.data |
| } |
| } |
| |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>HalfLock<T> { |
| <span class="comment">// We conceptually contain an instance of T |
| </span>_t: PhantomData<T>, |
| <span class="comment">// The actual data as a pointer. |
| </span>data: AtomicPtr<T>, |
| <span class="comment">// The generation of the data. Influences which slot of the lock counter we use. |
| </span>generation: AtomicUsize, |
| <span class="comment">// How many active locks are there? |
| </span>lock: [AtomicUsize; <span class="number">2</span>], |
| <span class="comment">// Mutex for the writers; only one writer. |
| </span>write_mutex: Mutex<()>, |
| } |
| |
| <span class="kw">impl</span><T> HalfLock<T> { |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>new(data: T) -> <span class="self">Self </span>{ |
| <span class="comment">// Move to the heap so we can safely point there. Then convert to raw pointer as AtomicPtr |
| // operates on raw pointers. The AtomicPtr effectively acts like Box for us semantically. |
| </span><span class="kw">let </span>ptr = Box::into_raw(Box::new(data)); |
| <span class="self">Self </span>{ |
| _t: PhantomData, |
| data: AtomicPtr::new(ptr), |
| generation: AtomicUsize::new(<span class="number">0</span>), |
| lock: [AtomicUsize::new(<span class="number">0</span>), AtomicUsize::new(<span class="number">0</span>)], |
| write_mutex: Mutex::new(()), |
| } |
| } |
| |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>read(<span class="kw-2">&</span><span class="self">self</span>) -> ReadGuard<T> { |
| <span class="comment">// Relaxed should be enough; we only pick one or the other slot and the writer observes |
| // that both were 0 at some time. So the actual value doesn't really matter for safety, |
| // only the changing improves the performance. |
| </span><span class="kw">let </span>gen = <span class="self">self</span>.generation.load(Ordering::SeqCst); |
| <span class="kw">let </span>lock = <span class="kw-2">&</span><span class="self">self</span>.lock[gen % <span class="number">2</span>]; |
| <span class="comment">// Effectively locking something, acquire should be enough. |
| </span><span class="kw">let </span>guard_cnt = lock.fetch_add(<span class="number">1</span>, Ordering::SeqCst); |
| |
| <span class="comment">// This is to prevent overflowing the counter in some degenerate cases, which could lead to |
| // UB (freeing data while still in use). However, as this data structure is used only |
| // internally and it's not possible to leak the guard and the guard itself takes some |
| // memory, it should be really impossible to trigger this case. Still, we include it from |
| // abundance of caution. |
| // |
| // This technically is not fully correct as enough threads being in between here and the |
| // abort below could still overflow it and it could get freed for some *other* thread, but |
| // that would mean having too many active threads to fit into RAM too and is even more |
| // absurd corner case than the above. |
| </span><span class="kw">if </span>guard_cnt > MAX_GUARDS { |
| <span class="kw">unsafe </span>{ libc::abort() }; |
| } |
| |
| <span class="comment">// Acquire should be enough; we need to "download" the data, paired with the swap on the |
| // same pointer. |
| </span><span class="kw">let </span>data = <span class="self">self</span>.data.load(Ordering::SeqCst); |
| <span class="comment">// Safe: |
| // * It did point to valid data when put in. |
| // * Protected by lock, so still valid. |
| </span><span class="kw">let </span>data = <span class="kw">unsafe </span>{ <span class="kw-2">&*</span>data }; |
| |
| ReadGuard { data, lock } |
| } |
| |
| <span class="kw">fn </span>update_seen(<span class="kw-2">&</span><span class="self">self</span>, seen_zero: <span class="kw-2">&mut </span>[bool; <span class="number">2</span>]) { |
| <span class="kw">for </span>(seen, slot) <span class="kw">in </span>seen_zero.iter_mut().zip(<span class="kw-2">&</span><span class="self">self</span>.lock) { |
| <span class="kw-2">*</span>seen = <span class="kw-2">*</span>seen || slot.load(Ordering::SeqCst) == <span class="number">0</span>; |
| } |
| } |
| |
| <span class="kw">fn </span>write_barrier(<span class="kw-2">&</span><span class="self">self</span>) { |
| <span class="comment">// Do a first check of seeing zeroes before we switch the generation. At least one of them |
| // should be zero by now, due to having drained the generation before leaving the previous |
| // writer. |
| </span><span class="kw">let </span><span class="kw-2">mut </span>seen_zero = [<span class="bool-val">false</span>; <span class="number">2</span>]; |
| <span class="self">self</span>.update_seen(<span class="kw-2">&mut </span>seen_zero); |
| <span class="comment">// By switching the generation to the other slot, we make sure the currently active starts |
| // draining while the other will start filling up. |
| </span><span class="self">self</span>.generation.fetch_add(<span class="number">1</span>, Ordering::SeqCst); <span class="comment">// Overflow is fine. |
| |
| </span><span class="kw">let </span><span class="kw-2">mut </span>iter = <span class="number">0usize</span>; |
| <span class="kw">while </span>!seen_zero.iter().all(|s| <span class="kw-2">*</span>s) { |
| iter = iter.wrapping_add(<span class="number">1</span>); |
| |
| <span class="comment">// Be somewhat less aggressive while looping, switch to the other threads if possible. |
| </span><span class="kw">if </span><span class="macro">cfg!</span>(not(miri)) { |
| <span class="kw">if </span>iter % YIELD_EVERY == <span class="number">0 </span>{ |
| thread::yield_now(); |
| } <span class="kw">else </span>{ |
| <span class="comment">// Replaced by hint::spin_loop, but we want to support older compiler |
| </span><span class="attribute">#[allow(deprecated)] |
| </span>atomic::spin_loop_hint(); |
| } |
| } |
| |
| <span class="self">self</span>.update_seen(<span class="kw-2">&mut </span>seen_zero); |
| } |
| } |
| |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>write(<span class="kw-2">&</span><span class="self">self</span>) -> WriteGuard<T> { |
| <span class="comment">// While it's possible the user code panics, our code in store doesn't and the data gets |
| // swapped atomically. So if it panics, nothing gets changed, therefore poisons are of no |
| // interest here. |
| </span><span class="kw">let </span>guard = <span class="self">self |
| </span>.write_mutex |
| .lock() |
| .unwrap_or_else(PoisonError::into_inner); |
| |
| <span class="comment">// Relaxed should be enough, as we are under the same mutex that was used to get the data |
| // in. |
| </span><span class="kw">let </span>data = <span class="self">self</span>.data.load(Ordering::SeqCst); |
| <span class="comment">// Safe: |
| // * Stored as valid data |
| // * Only this method, protected by mutex, can change the pointer, so it didn't go away. |
| </span><span class="kw">let </span>data = <span class="kw">unsafe </span>{ <span class="kw-2">&*</span>data }; |
| |
| WriteGuard { |
| data, |
| _guard: guard, |
| lock: <span class="self">self</span>, |
| } |
| } |
| } |
| |
| <span class="kw">impl</span><T> Drop <span class="kw">for </span>HalfLock<T> { |
| <span class="kw">fn </span>drop(<span class="kw-2">&mut </span><span class="self">self</span>) { |
| <span class="comment">// During drop we are sure there are no other borrows of the data so we are free to just |
| // drop it. Also, the drop impl won't be called in practice in our case, as it is used |
| // solely as a global variable, but we provide it for completeness and tests anyway. |
| // |
| // unsafe: the pointer in there is always valid, we just take the last instance out. |
| </span><span class="kw">unsafe </span>{ |
| <span class="comment">// Acquire should be enough. |
| </span><span class="kw">let </span>data = Box::from_raw(<span class="self">self</span>.data.load(Ordering::SeqCst)); |
| drop(data); |
| } |
| } |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="signal_hook_registry" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
