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<main>
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<h1 id="基于-send-和-sync-的线程安全"><a class="header" href="#基于-send-和-sync-的线程安全">基于 Send 和 Sync 的线程安全</a></h1>
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<p>为何 Rc、RefCell 和裸指针不可以在多线程间使用？如何让裸指针可以在多线程使用？我们一起来探寻下这些问题的答案。</p>
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<h2 id="无法用于多线程的rc"><a class="header" href="#无法用于多线程的rc">无法用于多线程的<code>Rc</code></a></h2>
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<p>先来看一段多线程使用<code>Rc</code>的代码:</p>
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<pre class="playground"><code class="language-rust edition2021">use std::thread;
|
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|
use std::rc::Rc;
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|
fn main() {
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let v = Rc::new(5);
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|
let t = thread::spawn(move || {
|
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println!("{}",v);
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});
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t.join().unwrap();
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}</code></pre>
|
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<p>以上代码将<code>v</code>的所有权通过<code>move</code>转移到子线程中，看似正确实则会报错：</p>
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<pre><code class="language-console">error[E0277]: `Rc<i32>` cannot be sent between threads safely
|
|
|
------ 省略部分报错 --------
|
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|
= help: within `[closure@src/main.rs:5:27: 7:6]`, the trait `Send` is not implemented for `Rc<i32>`
|
|
|
</code></pre>
|
|
|
<p>表面原因是<code>Rc</code>无法在线程间安全的转移，实际是编译器给予我们的那句帮助: <code>the trait `Send` is not implemented for `Rc<i32>` </code>(<code>Rc<i32></code>未实现<code>Send</code>特征), 那么此处的<code>Send</code>特征又是何方神圣？</p>
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|
<h2 id="rc-和-arc-源码对比"><a class="header" href="#rc-和-arc-源码对比">Rc 和 Arc 源码对比</a></h2>
|
|
|
<p>在介绍<code>Send</code>特征之前，再来看看<code>Arc</code>为何可以在多线程使用，玄机在于两者的源码实现上：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>// Rc源码片段
|
|
|
impl<T: ?Sized> !marker::Send for Rc<T> {}
|
|
|
impl<T: ?Sized> !marker::Sync for Rc<T> {}
|
|
|
|
|
|
// Arc源码片段
|
|
|
unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
|
|
|
unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p><code>!</code>代表移除特征的相应实现，上面代码中<code>Rc<T></code>的<code>Send</code>和<code>Sync</code>特征被特地移除了实现，而<code>Arc<T></code>则相反，实现了<code>Sync + Send</code>，再结合之前的编译器报错，大概可以明白了：<code>Send</code>和<code>Sync</code>是在线程间安全使用一个值的关键。</p>
|
|
|
<h2 id="send-和-sync"><a class="header" href="#send-和-sync">Send 和 Sync</a></h2>
|
|
|
<p><code>Send</code>和<code>Sync</code>是 Rust 安全并发的重中之重，但是实际上它们只是标记特征(marker trait，该特征未定义任何行为，因此非常适合用于标记), 来看看它们的作用：</p>
|
|
|
<ul>
|
|
|
<li>实现<code>Send</code>的类型可以在线程间安全的传递其所有权</li>
|
|
|
<li>实现<code>Sync</code>的类型可以在线程间安全的共享(通过引用)</li>
|
|
|
</ul>
|
|
|
<p>这里还有一个潜在的依赖：一个类型要在线程间安全的共享的前提是，指向它的引用必须能在线程间传递。因为如果引用都不能被传递，我们就无法在多个线程间使用引用去访问同一个数据了。</p>
|
|
|
<p>由上可知，<strong>若类型 T 的引用<code>&T</code>是<code>Send</code>，则<code>T</code>是<code>Sync</code></strong>。</p>
|
|
|
<p>没有例子的概念讲解都是耍流氓，来看看<code>RwLock</code>的实现:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>unsafe impl<T: ?Sized + Send + Sync> Sync for RwLock<T> {}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>首先<code>RwLock</code>可以在线程间安全的共享，那它肯定是实现了<code>Sync</code>，但是我们的关注点不在这里。众所周知，<code>RwLock</code>可以并发的读，说明其中的值<code>T</code>必定也可以在线程间共享，那<code>T</code>必定要实现<code>Sync</code>。</p>
|
|
|
<p>果不其然，上述代码中，<code>T</code>的特征约束中就有一个<code>Sync</code>特征，那问题又来了，<code>Mutex</code>是不是相反？再来看看:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>unsafe impl<T: ?Sized + Send> Sync for Mutex<T> {}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>不出所料，<code>Mutex<T></code>中的<code>T</code>并没有<code>Sync</code>特征约束。</p>
|
|
|
<p>武学秘籍再好，不见生死也是花拳绣腿。同样的，我们需要通过实战来彻底掌握<code>Send</code>和<code>Sync</code>，但在实战之前，先来简单看看有哪些类型实现了它们。</p>
|
|
|
<h2 id="实现send和sync的类型"><a class="header" href="#实现send和sync的类型">实现<code>Send</code>和<code>Sync</code>的类型</a></h2>
|
|
|
<p>在 Rust 中，几乎所有类型都默认实现了<code>Send</code>和<code>Sync</code>，而且由于这两个特征都是可自动派生的特征(通过<code>derive</code>派生)，意味着一个复合类型(例如结构体), 只要它内部的所有成员都实现了<code>Send</code>或者<code>Sync</code>，那么它就自动实现了<code>Send</code>或<code>Sync</code>。</p>
|
|
|
<p>正是因为以上规则，Rust 中绝大多数类型都实现了<code>Send</code>和<code>Sync</code>，除了以下几个(事实上不止这几个，只不过它们比较常见):</p>
|
|
|
<ul>
|
|
|
<li>裸指针两者都没实现，因为它本身就没有任何安全保证</li>
|
|
|
<li><code>UnsafeCell</code>不是<code>Sync</code>，因此<code>Cell</code>和<code>RefCell</code>也不是</li>
|
|
|
<li><code>Rc</code>两者都没实现(因为内部的引用计数器不是线程安全的)</li>
|
|
|
</ul>
|
|
|
<p>当然，如果是自定义的复合类型，那没实现那哥俩的就较为常见了：<strong>只要复合类型中有一个成员不是<code>Send</code>或<code>Sync</code>，那么该复合类型也就不是<code>Send</code>或<code>Sync</code></strong>。</p>
|
|
|
<p><strong>手动实现 <code>Send</code> 和 <code>Sync</code> 是不安全的</strong>，通常并不需要手动实现 Send 和 Sync trait，实现者需要使用<code>unsafe</code>小心维护并发安全保证。</p>
|
|
|
<p>至此，相关的概念大家已经掌握，但是我敢肯定，对于这两个滑不溜秋的家伙，大家依然会非常模糊，不知道它们该如何使用。那么我们来一起看看如何让裸指针可以在线程间安全的使用。</p>
|
|
|
<h2 id="为裸指针实现send"><a class="header" href="#为裸指针实现send">为裸指针实现<code>Send</code></a></h2>
|
|
|
<p>上面我们提到裸指针既没实现<code>Send</code>，意味着下面代码会报错:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::thread;
|
|
|
fn main() {
|
|
|
let p = 5 as *mut u8;
|
|
|
let t = thread::spawn(move || {
|
|
|
println!("{:?}",p);
|
|
|
});
|
|
|
|
|
|
t.join().unwrap();
|
|
|
}</code></pre>
|
|
|
<p>报错跟之前无二： <code> `*mut u8` cannot be sent between threads safely</code>, 但是有一个问题，我们无法为其直接实现<code>Send</code>特征，好在可以用<a href="https://course.rs/advance/into-types/custom-type.html#newtype"><code>newtype</code>类型</a> :<code>struct MyBox(*mut u8);</code>。</p>
|
|
|
<p>还记得之前的规则吗：复合类型中有一个成员没实现<code>Send</code>，该复合类型就不是<code>Send</code>，因此我们需要手动为它实现:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::thread;
|
|
|
|
|
|
#[derive(Debug)]
|
|
|
struct MyBox(*mut u8);
|
|
|
unsafe impl Send for MyBox {}
|
|
|
fn main() {
|
|
|
let p = MyBox(5 as *mut u8);
|
|
|
let t = thread::spawn(move || {
|
|
|
println!("{:?}",p);
|
|
|
});
|
|
|
|
|
|
t.join().unwrap();
|
|
|
}</code></pre>
|
|
|
<p>此时，我们的指针已经可以欢快的在多线程间撒欢，以上代码很简单，但有一点需要注意：<code>Send</code>和<code>Sync</code>是<code>unsafe</code>特征，实现时需要用<code>unsafe</code>代码块包裹。</p>
|
|
|
<h2 id="为裸指针实现sync"><a class="header" href="#为裸指针实现sync">为裸指针实现<code>Sync</code></a></h2>
|
|
|
<p>由于<code>Sync</code>是多线程间共享一个值，大家可能会想这么实现：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::thread;
|
|
|
fn main() {
|
|
|
let v = 5;
|
|
|
let t = thread::spawn(|| {
|
|
|
println!("{:?}",&v);
|
|
|
});
|
|
|
|
|
|
t.join().unwrap();
|
|
|
}</code></pre>
|
|
|
<p>关于这种用法，在多线程章节也提到过，线程如果直接去借用其它线程的变量，会报错:<code>closure may outlive the current function,</code>, 原因在于编译器无法确定主线程<code>main</code>和子线程<code>t</code>谁的生命周期更长，特别是当两个线程都是子线程时，没有任何人知道哪个子线程会先结束，包括编译器！</p>
|
|
|
<p>因此我们得配合<code>Arc</code>去使用:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::thread;
|
|
|
use std::sync::Arc;
|
|
|
use std::sync::Mutex;
|
|
|
|
|
|
#[derive(Debug)]
|
|
|
struct MyBox(*const u8);
|
|
|
unsafe impl Send for MyBox {}
|
|
|
|
|
|
fn main() {
|
|
|
let b = &MyBox(5 as *const u8);
|
|
|
let v = Arc::new(Mutex::new(b));
|
|
|
let t = thread::spawn(move || {
|
|
|
let _v1 = v.lock().unwrap();
|
|
|
});
|
|
|
|
|
|
t.join().unwrap();
|
|
|
}</code></pre>
|
|
|
<p>上面代码将智能指针<code>v</code>的所有权转移给新线程，同时<code>v</code>包含了一个引用类型<code>b</code>，当在新的线程中试图获取内部的引用时，会报错：</p>
|
|
|
<pre><code class="language-console">error[E0277]: `*const u8` cannot be shared between threads safely
|
|
|
--> src/main.rs:25:13
|
|
|
|
|
|
|
25 | let t = thread::spawn(move || {
|
|
|
| ^^^^^^^^^^^^^ `*const u8` cannot be shared between threads safely
|
|
|
|
|
|
|
= help: within `MyBox`, the trait `Sync` is not implemented for `*const u8`
|
|
|
</code></pre>
|
|
|
<p>因为我们访问的引用实际上还是对主线程中的数据的借用，转移进来的仅仅是外层的智能指针引用。要解决很简单，为<code>MyBox</code>实现<code>Sync</code>:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>unsafe impl Sync for MyBox {}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
|
|
|
<p>通过上面的两个裸指针的例子，我们了解了如何实现<code>Send</code>和<code>Sync</code>，以及如何只实现<code>Send</code>而不实现<code>Sync</code>，简单总结下：</p>
|
|
|
<ol>
|
|
|
<li>实现<code>Send</code>的类型可以在线程间安全的传递其所有权, 实现<code>Sync</code>的类型可以在线程间安全的共享(通过引用)</li>
|
|
|
<li>绝大部分类型都实现了<code>Send</code>和<code>Sync</code>，常见的未实现的有：裸指针、<code>Cell</code>、<code>RefCell</code>、<code>Rc</code> 等</li>
|
|
|
<li>可以为自定义类型实现<code>Send</code>和<code>Sync</code>，但是需要<code>unsafe</code>代码块</li>
|
|
|
<li>可以为部分 Rust 中的类型实现<code>Send</code>、<code>Sync</code>，但是需要使用<code>newtype</code>，例如文中的裸指针例子</li>
|
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</ol>
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