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<li class="toctree-l2"><a class="reference internal" href="../chapter4/3sv39-implementation-1.html">实现 SV39 多级页表机制（上）</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter4/4sv39-implementation-2.html">实现 SV39 多级页表机制（下）</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter4/7exercise.html">chapter4练习</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter5/1process.html">与进程有关的重要系统调用</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter5/2core-data-structures.html">进程管理的核心数据结构</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter6/2fs-implementation-1.html">简易文件系统 easy-fs (上)</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter6/2fs-implementation-2.html">简易文件系统 easy-fs (下)</a></li>
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</li>
|
||
</ul>
|
||
<p class="caption" role="heading"><span class="caption-text">附录</span></p>
|
||
<ul>
|
||
<li class="toctree-l1 has-children"><a class="reference internal" href="../appendix-a/index.html">附录 A：Rust 系统编程资料</a><input class="toctree-checkbox" id="toctree-checkbox-17" name="toctree-checkbox-17" role="switch" type="checkbox"/><label for="toctree-checkbox-17"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul class="simple">
|
||
</ul>
|
||
</li>
|
||
<li class="toctree-l1 has-children"><a class="reference internal" href="../appendix-b/index.html">附录 B：常见工具的使用方法</a><input class="toctree-checkbox" id="toctree-checkbox-18" name="toctree-checkbox-18" role="switch" type="checkbox"/><label for="toctree-checkbox-18"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul class="simple">
|
||
</ul>
|
||
</li>
|
||
<li class="toctree-l1 has-children"><a class="reference internal" href="../appendix-c/index.html">附录 C：深入机器模式：RustSBI</a><input class="toctree-checkbox" id="toctree-checkbox-19" name="toctree-checkbox-19" role="switch" type="checkbox"/><label for="toctree-checkbox-19"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul class="simple">
|
||
</ul>
|
||
</li>
|
||
<li class="toctree-l1"><a class="reference internal" href="../appendix-d/index.html">附录 D：RISC-V相关信息</a></li>
|
||
</ul>
|
||
<p class="caption" role="heading"><span class="caption-text">开发注记</span></p>
|
||
<ul>
|
||
<li class="toctree-l1 has-children"><a class="reference internal" href="../setup-sphinx.html">修改和构建本项目</a><input class="toctree-checkbox" id="toctree-checkbox-20" name="toctree-checkbox-20" role="switch" type="checkbox"/><label for="toctree-checkbox-20"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul class="simple">
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<li class="toctree-l1 has-children"><a class="reference internal" href="../rest-example.html">reStructuredText 基本语法</a><input class="toctree-checkbox" id="toctree-checkbox-21" name="toctree-checkbox-21" role="switch" type="checkbox"/><label for="toctree-checkbox-21"><div class="visually-hidden">Toggle child pages in navigation</div><i class="icon"><svg><use href="#svg-arrow-right"></use></svg></i></label><ul class="simple">
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<div class="section" id="id1">
|
||
<h1>条件变量机制<a class="headerlink" href="#id1" title="永久链接至标题">¶</a></h1>
|
||
<div class="section" id="id2">
|
||
<h2>本节导读<a class="headerlink" href="#id2" title="永久链接至标题">¶</a></h2>
|
||
<p>到目前为止，我们已经了解了操作系统提供的互斥锁和信号量。但应用程序在使用这两者时需要非常小心，
|
||
如果使用不当，就会产生效率低下、竞态条件、死锁或者其他一些不可预测的情况。为了简化编程、避免错误，
|
||
计算机科学家针对某些情况设计了一种更高层的同步互斥原语。具体而言，在有些情况下，
|
||
线程需要检查某一条件（condition）满足之后，才会继续执行。</p>
|
||
<p>我们来看一个例子，有两个线程 first 和 second 在运行，线程 first 会把全局变量 A 设置为
|
||
1，而线程 second 在 <code class="docutils literal notranslate"><span class="pre">A</span> <span class="pre">!=</span> <span class="pre">0</span></code> 的条件满足后，才能继续执行，如下面的伪代码所示：</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="k">static</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">A</span>: <span class="kt">usize</span> <span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
|
||
<span class="linenos"> 2</span><span class="k">unsafe</span><span class="w"> </span><span class="k">fn</span> <span class="nf">first</span><span class="p">()</span><span class="w"> </span>-> <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos"> 3</span><span class="w"> </span><span class="n">A</span><span class="o">=</span><span class="mi">1</span><span class="p">;</span><span class="w"></span>
|
||
<span class="linenos"> 4</span><span class="w"> </span><span class="o">..</span><span class="p">.</span><span class="w"></span>
|
||
<span class="linenos"> 5</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos"> 6</span>
|
||
<span class="linenos"> 7</span><span class="k">unsafe</span><span class="w"> </span><span class="k">fn</span> <span class="nf">second</span><span class="p">()</span><span class="w"> </span>-> <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos"> 8</span><span class="w"> </span><span class="k">while</span><span class="w"> </span><span class="n">A</span><span class="o">==</span><span class="mi">0</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos"> 9</span><span class="w"> </span><span class="c1">// 忙等或睡眠等待 A==1</span>
|
||
<span class="linenos">10</span><span class="w"> </span><span class="p">};</span><span class="w"></span>
|
||
<span class="linenos">11</span><span class="w"> </span><span class="c1">//继续执行相关事务</span>
|
||
<span class="linenos">12</span><span class="p">}</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<p>在上面的例子中，如果线程 second 先执行，会忙等在 while 循环中，在操作系统的调度下，线程
|
||
first 会执行并把 A 赋值为 1 后，然后线程 second 再次执行时，就会跳出 while 循环，进行接下来的工作。
|
||
配合互斥锁，可以正确完成上述带条件的同步流程，如下面的伪代码所示：</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="k">static</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">A</span>: <span class="kt">usize</span> <span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
|
||
<span class="linenos"> 2</span><span class="k">unsafe</span><span class="w"> </span><span class="k">fn</span> <span class="nf">first</span><span class="p">()</span><span class="w"> </span>-> <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos"> 3</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">lock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos"> 4</span><span class="w"> </span><span class="n">A</span><span class="o">=</span><span class="mi">1</span><span class="p">;</span><span class="w"></span>
|
||
<span class="linenos"> 5</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">unlock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos"> 6</span><span class="w"> </span><span class="o">..</span><span class="p">.</span><span class="w"></span>
|
||
<span class="linenos"> 7</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos"> 8</span>
|
||
<span class="linenos"> 9</span><span class="k">unsafe</span><span class="w"> </span><span class="k">fn</span> <span class="nf">second</span><span class="p">()</span><span class="w"> </span>-> <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">10</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">lock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">11</span><span class="w"> </span><span class="k">while</span><span class="w"> </span><span class="n">A</span><span class="o">==</span><span class="mi">0</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">12</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">unlock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">13</span><span class="w"> </span><span class="c1">// give other thread a chance to lock</span>
|
||
<span class="linenos">14</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">lock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">15</span><span class="w"> </span><span class="p">};</span><span class="w"></span>
|
||
<span class="linenos">16</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">unlock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">17</span><span class="w"> </span><span class="c1">//继续执行相关事务</span>
|
||
<span class="linenos">18</span><span class="p">}</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<p>这种实现能执行，但效率低下，因为线程 second 会忙等检查，浪费处理器时间。我们希望有某种方式让线程
|
||
second 休眠，直到等待的条件满足，再继续执行。于是，我们可以写出如下的代码：</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="k">static</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">A</span>: <span class="kt">usize</span> <span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
|
||
<span class="linenos"> 2</span><span class="k">unsafe</span><span class="w"> </span><span class="k">fn</span> <span class="nf">first</span><span class="p">()</span><span class="w"> </span>-> <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos"> 3</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">lock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos"> 4</span><span class="w"> </span><span class="n">A</span><span class="o">=</span><span class="mi">1</span><span class="p">;</span><span class="w"></span>
|
||
<span class="linenos"> 5</span><span class="w"> </span><span class="n">wakup</span><span class="p">(</span><span class="n">second</span><span class="p">);</span><span class="w"></span>
|
||
<span class="linenos"> 6</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">unlock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos"> 7</span><span class="w"> </span><span class="o">..</span><span class="p">.</span><span class="w"></span>
|
||
<span class="linenos"> 8</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos"> 9</span>
|
||
<span class="linenos">10</span><span class="k">unsafe</span><span class="w"> </span><span class="k">fn</span> <span class="nf">second</span><span class="p">()</span><span class="w"> </span>-> <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">11</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">lock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">12</span><span class="w"> </span><span class="k">while</span><span class="w"> </span><span class="n">A</span><span class="o">==</span><span class="mi">0</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">13</span><span class="w"> </span><span class="n">wait</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">14</span><span class="w"> </span><span class="p">};</span><span class="w"></span>
|
||
<span class="linenos">15</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">unlock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">16</span><span class="w"> </span><span class="c1">//继续执行相关事务</span>
|
||
<span class="linenos">17</span><span class="p">}</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<p>粗略地看，这样就可以实现睡眠等待了。但请同学仔细想想，当线程 second 在睡眠的时候， <code class="docutils literal notranslate"><span class="pre">mutex</span></code>
|
||
是否已经上锁了？ 确实，线程 second 是带着上锁的 <code class="docutils literal notranslate"><span class="pre">mutex</span></code> 进入等待睡眠状态的。
|
||
如果这两个线程的调度顺序是先执行线程 second，再执行线程first，那么线程 second 会先睡眠且拥有
|
||
<code class="docutils literal notranslate"><span class="pre">mutex</span></code> 的锁；当线程 first 执行时，会由于没有 <code class="docutils literal notranslate"><span class="pre">mutex</span></code> 的锁而进入等待锁的睡眠状态。
|
||
结果就是两个线程都睡了，都执行不下去，这就出现了 <strong>死锁</strong> 。</p>
|
||
<p>这里需要解决的两个关键问题： <strong>如何等待一个条件？</strong> 和 <strong>在条件为真时如何向等待线程发出信号</strong> 。
|
||
我们的计算机科学家给出了 <strong>管程（Monitor）</strong> 和 <strong>条件变量（Condition Variables）</strong>
|
||
这种巧妙的方法。接下来，我们就会深入讲解条件变量的设计与实现。</p>
|
||
</div>
|
||
<div class="section" id="id3">
|
||
<h2>条件变量的基本思路<a class="headerlink" href="#id3" title="永久链接至标题">¶</a></h2>
|
||
<p>管程有一个很重要的特性，即任一时刻只能有一个活跃线程调用管程中的过程，
|
||
这一特性使线程在调用执行管程中过程时能保证互斥，这样线程就可以放心地访问共享变量。
|
||
管程是编程语言的组成部分，编译器知道其特殊性，因此可以采用与其他过程调用不同的方法来处理对管程的调用.
|
||
因为是由编译器而非程序员来生成互斥相关的代码，所以出错的可能性要小。</p>
|
||
<p>管程虽然借助编译器提供了一种实现互斥的简便途径，但这还不够，还需要一种线程间的沟通机制。
|
||
首先是等待机制：由于线程在调用管程中某个过程时，发现某个条件不满足，那就在无法继续运行而被阻塞。
|
||
其次是唤醒机制：另外一个线程可以在调用管程的过程中，把某个条件设置为真，并且还需要有一种机制，
|
||
及时唤醒等待条件为真的阻塞线程。为了避免管程中同时有两个活跃线程，
|
||
我们需要一定的规则来约定线程发出唤醒操作的行为。目前有三种典型的规则方案：</p>
|
||
<ul class="simple">
|
||
<li><p>Hoare 语义：线程发出唤醒操作后，马上阻塞自己，让新被唤醒的线程运行。注：此时唤醒线程的执行位置还在管程中。</p></li>
|
||
<li><p>Hansen 语义：是执行唤醒操作的线程必须立即退出管程，即唤醒操作只可能作为一个管程过程的最后一条语句。
|
||
注：此时唤醒线程的执行位置离开了管程。</p></li>
|
||
<li><p>Mesa 语义：唤醒线程在发出行唤醒操作后继续运行，并且只有它退出管程之后，才允许等待的线程开始运行。
|
||
注：此时唤醒线程的执行位置还在管程中。</p></li>
|
||
</ul>
|
||
<p>一般开发者会采纳 Brinch Hansen 的建议，因为它在概念上更简单，并且更容易实现。这种沟通机制的具体实现就是
|
||
<strong>条件变量</strong> 和对应的操作：wait 和 signal。线程使用条件变量来等待一个条件变成真。
|
||
条件变量其实是一个线程等待队列，当条件不满足时，线程通过执行条件变量的 wait
|
||
操作就可以把自己加入到等待队列中，睡眠等待（waiting）该条件。另外某个线程，当它改变条件为真后，
|
||
就可以通过条件变量的 signal 操作来唤醒一个或者多个等待的线程（通过在该条件上发信号），让它们继续执行。</p>
|
||
<p>早期提出的管程是基于 Concurrent Pascal 来设计的，其他语言如 C 和 Rust 等，并没有在语言上支持这种机制。
|
||
我们还是可以用手动加入互斥锁的方式来代替编译器，就可以在 C 和 Rust 的基础上实现原始的管程机制了。
|
||
在目前的 C 语言应用开发中，实际上也是这么做的。这样，我们就可以用互斥锁和条件变量，
|
||
来重现上述的同步互斥例子：</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="k">static</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">A</span>: <span class="kt">usize</span> <span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
|
||
<span class="linenos"> 2</span><span class="k">unsafe</span><span class="w"> </span><span class="k">fn</span> <span class="nf">first</span><span class="p">()</span><span class="w"> </span>-> <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos"> 3</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">lock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos"> 4</span><span class="w"> </span><span class="n">A</span><span class="o">=</span><span class="mi">1</span><span class="p">;</span><span class="w"></span>
|
||
<span class="linenos"> 5</span><span class="w"> </span><span class="n">condvar</span><span class="p">.</span><span class="n">wakup</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos"> 6</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">unlock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos"> 7</span><span class="w"> </span><span class="o">..</span><span class="p">.</span><span class="w"></span>
|
||
<span class="linenos"> 8</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos"> 9</span>
|
||
<span class="linenos">10</span><span class="k">unsafe</span><span class="w"> </span><span class="k">fn</span> <span class="nf">second</span><span class="p">()</span><span class="w"> </span>-> <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">11</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">lock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">12</span><span class="w"> </span><span class="k">while</span><span class="w"> </span><span class="n">A</span><span class="o">==</span><span class="mi">0</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">13</span><span class="w"> </span><span class="n">condvar</span><span class="p">.</span><span class="n">wait</span><span class="p">(</span><span class="n">mutex</span><span class="p">);</span><span class="w"> </span><span class="c1">//在睡眠等待之前，需要释放mutex</span>
|
||
<span class="linenos">14</span><span class="w"> </span><span class="p">};</span><span class="w"></span>
|
||
<span class="linenos">15</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">unlock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">16</span><span class="w"> </span><span class="c1">//继续执行相关事务</span>
|
||
<span class="linenos">17</span><span class="p">}</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<p>有了上面的介绍，我们就可以实现条件变量的基本逻辑了。下面是条件变量的 wait 和 signal 操作的伪代码：</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos">1</span><span class="k">fn</span> <span class="nf">wait</span><span class="p">(</span><span class="n">mutex</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">2</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">unlock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">3</span><span class="w"> </span><span class="o"><</span><span class="n">block</span><span class="w"> </span><span class="n">and</span><span class="w"> </span><span class="n">enqueue</span><span class="w"> </span><span class="n">the</span><span class="w"> </span><span class="n">thread</span><span class="o">></span><span class="p">;</span><span class="w"></span>
|
||
<span class="linenos">4</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">lock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">5</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">6</span>
|
||
<span class="linenos">7</span><span class="k">fn</span> <span class="nf">signal</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">8</span><span class="w"> </span><span class="o"><</span><span class="n">unblock</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="n">thread</span><span class="o">></span><span class="p">;</span><span class="w"></span>
|
||
<span class="linenos">9</span><span class="p">}</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<p>条件变量的wait操作包含三步，1. 释放锁；2. 把自己挂起；3. 被唤醒后，再获取锁。条件变量的 signal
|
||
操作只包含一步：找到挂在条件变量上睡眠的线程，把它唤醒。</p>
|
||
<p>注意，条件变量不像信号量那样有一个整型计数值的成员变量，所以条件变量也不能像信号量那样有读写计数值的能力。
|
||
如果一个线程向一个条件变量发送唤醒操作，但是在该条件变量上并没有等待的线程，则唤醒操作实际上什么也没做。</p>
|
||
</div>
|
||
<div class="section" id="id4">
|
||
<h2>实现条件变量<a class="headerlink" href="#id4" title="永久链接至标题">¶</a></h2>
|
||
<div class="section" id="condvar">
|
||
<h3>使用 condvar 系统调用<a class="headerlink" href="#condvar" title="永久链接至标题">¶</a></h3>
|
||
<p>我们通过例子来看看如何实际使用条件变量。下面是面向应用程序对条件变量系统调用的简单使用，
|
||
可以看到对它的使用与上一节介绍的信号量系统调用类似。 在这个例子中，主线程先创建了初值为 1
|
||
的互斥锁和一个条件变量，然后再创建两个线程 First 和 Second。线程 First 会先睡眠 10ms，而当线程
|
||
Second 执行时，会由于条件不满足执行条件变量的 wait 操作而等待睡眠；当线程 First 醒来后，通过设置
|
||
A 为 1，让线程 second 等待的条件满足，然后会执行条件变量的 signal 操作，从而能够唤醒线程 Second。
|
||
这样线程 First 和线程 Second 就形成了一种稳定的同步与互斥关系。</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="k">static</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">A</span>: <span class="kt">usize</span> <span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="c1">//全局变量</span>
|
||
<span class="linenos"> 2</span>
|
||
<span class="linenos"> 3</span><span class="k">const</span><span class="w"> </span><span class="n">CONDVAR_ID</span>: <span class="kt">usize</span> <span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
|
||
<span class="linenos"> 4</span><span class="k">const</span><span class="w"> </span><span class="n">MUTEX_ID</span>: <span class="kt">usize</span> <span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"></span>
|
||
<span class="linenos"> 5</span>
|
||
<span class="linenos"> 6</span><span class="k">unsafe</span><span class="w"> </span><span class="k">fn</span> <span class="nf">first</span><span class="p">()</span><span class="w"> </span>-> <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos"> 7</span><span class="w"> </span><span class="n">sleep</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span><span class="w"></span>
|
||
<span class="linenos"> 8</span><span class="w"> </span><span class="fm">println!</span><span class="p">(</span><span class="s">"First work, Change A --> 1 and wakeup Second"</span><span class="p">);</span><span class="w"></span>
|
||
<span class="linenos"> 9</span><span class="w"> </span><span class="n">mutex_lock</span><span class="p">(</span><span class="n">MUTEX_ID</span><span class="p">);</span><span class="w"></span>
|
||
<span class="linenos">10</span><span class="w"> </span><span class="n">A</span><span class="o">=</span><span class="mi">1</span><span class="p">;</span><span class="w"></span>
|
||
<span class="hll"><span class="linenos">11</span><span class="w"> </span><span class="n">condvar_signal</span><span class="p">(</span><span class="n">CONDVAR_ID</span><span class="p">);</span><span class="w"></span>
|
||
</span><span class="linenos">12</span><span class="w"> </span><span class="n">mutex_unlock</span><span class="p">(</span><span class="n">MUTEX_ID</span><span class="p">);</span><span class="w"></span>
|
||
<span class="linenos">13</span><span class="w"> </span><span class="o">..</span><span class="p">.</span><span class="w"></span>
|
||
<span class="linenos">14</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">15</span><span class="k">unsafe</span><span class="w"> </span><span class="k">fn</span> <span class="nf">second</span><span class="p">()</span><span class="w"> </span>-> <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">16</span><span class="w"> </span><span class="fm">println!</span><span class="p">(</span><span class="s">"Second want to continue,but need to wait A=1"</span><span class="p">);</span><span class="w"></span>
|
||
<span class="linenos">17</span><span class="w"> </span><span class="n">mutex_lock</span><span class="p">(</span><span class="n">MUTEX_ID</span><span class="p">);</span><span class="w"></span>
|
||
<span class="linenos">18</span><span class="w"> </span><span class="k">while</span><span class="w"> </span><span class="n">A</span><span class="o">==</span><span class="mi">0</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="hll"><span class="linenos">19</span><span class="w"> </span><span class="n">condvar_wait</span><span class="p">(</span><span class="n">CONDVAR_ID</span><span class="p">,</span><span class="w"> </span><span class="n">MUTEX_ID</span><span class="p">);</span><span class="w"></span>
|
||
</span><span class="linenos">20</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">21</span><span class="w"> </span><span class="n">mutex_unlock</span><span class="p">(</span><span class="n">MUTEX_ID</span><span class="p">);</span><span class="w"></span>
|
||
<span class="linenos">22</span><span class="w"> </span><span class="o">..</span><span class="p">.</span><span class="w"></span>
|
||
<span class="linenos">23</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">24</span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">main</span><span class="p">()</span><span class="w"> </span>-> <span class="kt">i32</span> <span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">25</span><span class="w"> </span><span class="c1">// create condvar & mutex</span>
|
||
<span class="hll"><span class="linenos">26</span><span class="w"> </span><span class="fm">assert_eq!</span><span class="p">(</span><span class="n">condvar_create</span><span class="p">()</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="kt">usize</span><span class="p">,</span><span class="w"> </span><span class="n">CONDVAR_ID</span><span class="p">);</span><span class="w"></span>
|
||
</span><span class="linenos">27</span><span class="w"> </span><span class="fm">assert_eq!</span><span class="p">(</span><span class="n">mutex_blocking_create</span><span class="p">()</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="kt">usize</span><span class="p">,</span><span class="w"> </span><span class="n">MUTEX_ID</span><span class="p">);</span><span class="w"></span>
|
||
<span class="linenos">28</span><span class="w"> </span><span class="c1">// create first, second threads</span>
|
||
<span class="linenos">29</span><span class="w"> </span><span class="o">..</span><span class="p">.</span><span class="w"></span>
|
||
<span class="linenos">30</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">31</span>
|
||
<span class="linenos">32</span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">condvar_create</span><span class="p">()</span><span class="w"> </span>-> <span class="kt">isize</span> <span class="p">{</span><span class="w"></span>
|
||
<span class="hll"><span class="linenos">33</span><span class="w"> </span><span class="n">sys_condvar_create</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span><span class="w"></span>
|
||
</span><span class="linenos">34</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">35</span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">condvar_signal</span><span class="p">(</span><span class="n">condvar_id</span>: <span class="kt">usize</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="hll"><span class="linenos">36</span><span class="w"> </span><span class="n">sys_condvar_signal</span><span class="p">(</span><span class="n">condvar_id</span><span class="p">);</span><span class="w"></span>
|
||
</span><span class="linenos">37</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">38</span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">condvar_wait</span><span class="p">(</span><span class="n">condvar_id</span>: <span class="kt">usize</span><span class="p">,</span><span class="w"> </span><span class="n">mutex_id</span>: <span class="kt">usize</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="hll"><span class="linenos">39</span><span class="w"> </span><span class="n">sys_condvar_wait</span><span class="p">(</span><span class="n">condvar_id</span><span class="p">,</span><span class="w"> </span><span class="n">mutex_id</span><span class="p">);</span><span class="w"></span>
|
||
</span><span class="linenos">40</span><span class="p">}</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<ul class="simple">
|
||
<li><p>第 26 行，创建了一个 ID 为 <code class="docutils literal notranslate"><span class="pre">CONDVAR_ID</span></code> 的条件量，对应第 33 行 <code class="docutils literal notranslate"><span class="pre">SYSCALL_CONDVAR_CREATE</span></code> 系统调用；</p></li>
|
||
<li><p>第 19 行，线程 Second 执行条件变量 <code class="docutils literal notranslate"><span class="pre">wait</span></code> 操作（对应第 39 行 <code class="docutils literal notranslate"><span class="pre">SYSCALL_CONDVAR_WAIT</span></code> 系统调用），
|
||
该线程将释放 <code class="docutils literal notranslate"><span class="pre">mutex</span></code> 锁并阻塞；</p></li>
|
||
<li><p>第 5 行，线程 First 执行条件变量 <code class="docutils literal notranslate"><span class="pre">signal</span></code> 操作（对应第 36 行 <code class="docutils literal notranslate"><span class="pre">SYSCALL_CONDVAR_SIGNAL</span></code> 系统调用），
|
||
会唤醒等待该条件变量的线程 Second。</p></li>
|
||
</ul>
|
||
</div>
|
||
<div class="section" id="id5">
|
||
<h3>实现 condvar 系统调用<a class="headerlink" href="#id5" title="永久链接至标题">¶</a></h3>
|
||
<p>操作系统如何实现条件变量系统调用呢？在线程的眼里，条件变量是一种每个线程能看到的共享资源，
|
||
且在一个进程中，可以存在多个不同条件变量资源，所以我们可以把所有的条件变量资源放在一起让进程来管理，
|
||
如下面代码第9行所示。这里需要注意的是： <code class="docutils literal notranslate"><span class="pre">condvar_list:</span> <span class="pre">Vec<Option<Arc<Condvar>>></span></code>
|
||
表示的是条件变量资源的列表。而 <code class="docutils literal notranslate"><span class="pre">Condvar</span></code> 是条件变量的内核数据结构，由等待队列组成。
|
||
操作系统需要显式地施加某种控制，来确定当一个线程执行 <code class="docutils literal notranslate"><span class="pre">wait</span></code> 操作和 <code class="docutils literal notranslate"><span class="pre">signal</span></code> 操作时，
|
||
如何让线程睡眠或唤醒线程。在这里， <code class="docutils literal notranslate"><span class="pre">wait</span></code> 操作是由 <code class="docutils literal notranslate"><span class="pre">Condvar</span></code> 的 <code class="docutils literal notranslate"><span class="pre">wait</span></code> 方法实现，而 <code class="docutils literal notranslate"><span class="pre">signal</span></code>
|
||
操作是由 <code class="docutils literal notranslate"><span class="pre">Condvar</span></code> 的 <code class="docutils literal notranslate"><span class="pre">signal</span></code> 方法实现。</p>
|
||
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="linenos"> 1</span><span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">ProcessControlBlock</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos"> 2</span><span class="w"> </span><span class="c1">// immutable</span>
|
||
<span class="linenos"> 3</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="n">pid</span>: <span class="nc">PidHandle</span><span class="p">,</span><span class="w"></span>
|
||
<span class="linenos"> 4</span><span class="w"> </span><span class="c1">// mutable</span>
|
||
<span class="linenos"> 5</span><span class="w"> </span><span class="n">inner</span>: <span class="nc">UPSafeCell</span><span class="o"><</span><span class="n">ProcessControlBlockInner</span><span class="o">></span><span class="p">,</span><span class="w"></span>
|
||
<span class="linenos"> 6</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos"> 7</span><span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">ProcessControlBlockInner</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos"> 8</span><span class="w"> </span><span class="o">..</span><span class="p">.</span><span class="w"></span>
|
||
<span class="hll"><span class="linenos"> 9</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="n">condvar_list</span>: <span class="nb">Vec</span><span class="o"><</span><span class="nb">Option</span><span class="o"><</span><span class="n">Arc</span><span class="o"><</span><span class="n">Condvar</span><span class="o">>>></span><span class="p">,</span><span class="w"></span>
|
||
</span><span class="linenos">10</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">11</span><span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">Condvar</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">12</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="n">inner</span>: <span class="nc">UPSafeCell</span><span class="o"><</span><span class="n">CondvarInner</span><span class="o">></span><span class="p">,</span><span class="w"></span>
|
||
<span class="linenos">13</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">14</span><span class="k">pub</span><span class="w"> </span><span class="k">struct</span> <span class="nc">CondvarInner</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="hll"><span class="linenos">15</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="n">wait_queue</span>: <span class="nc">VecDeque</span><span class="o"><</span><span class="n">Arc</span><span class="o"><</span><span class="n">TaskControlBlock</span><span class="o">>></span><span class="p">,</span><span class="w"></span>
|
||
</span><span class="linenos">16</span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">17</span><span class="k">impl</span><span class="w"> </span><span class="n">Condvar</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="hll"><span class="linenos">18</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">new</span><span class="p">()</span><span class="w"> </span>-> <span class="nc">Self</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
</span><span class="linenos">19</span><span class="w"> </span><span class="bp">Self</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">20</span><span class="w"> </span><span class="n">inner</span>: <span class="nc">unsafe</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">UPSafeCell</span>::<span class="n">new</span><span class="p">(</span><span class="w"></span>
|
||
<span class="linenos">21</span><span class="w"> </span><span class="n">CondvarInner</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">22</span><span class="w"> </span><span class="n">wait_queue</span>: <span class="nc">VecDeque</span>::<span class="n">new</span><span class="p">(),</span><span class="w"></span>
|
||
<span class="linenos">23</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">24</span><span class="w"> </span><span class="p">)},</span><span class="w"></span>
|
||
<span class="linenos">25</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">26</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="hll"><span class="linenos">27</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">signal</span><span class="p">(</span><span class="o">&</span><span class="bp">self</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
</span><span class="linenos">28</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">inner</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">inner</span><span class="p">.</span><span class="n">exclusive_access</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">29</span><span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="nb">Some</span><span class="p">(</span><span class="n">task</span><span class="p">)</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">inner</span><span class="p">.</span><span class="n">wait_queue</span><span class="p">.</span><span class="n">pop_front</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
<span class="linenos">30</span><span class="w"> </span><span class="n">add_task</span><span class="p">(</span><span class="n">task</span><span class="p">);</span><span class="w"></span>
|
||
<span class="linenos">31</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">32</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="hll"><span class="linenos">33</span><span class="w"> </span><span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">wait</span><span class="p">(</span><span class="o">&</span><span class="bp">self</span><span class="p">,</span><span class="w"> </span><span class="n">mutex</span>:<span class="nc">Arc</span><span class="o"><</span><span class="k">dyn</span><span class="w"> </span><span class="n">Mutex</span><span class="o">></span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
|
||
</span><span class="linenos">34</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">unlock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">35</span><span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">inner</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="bp">self</span><span class="p">.</span><span class="n">inner</span><span class="p">.</span><span class="n">exclusive_access</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">36</span><span class="w"> </span><span class="n">inner</span><span class="p">.</span><span class="n">wait_queue</span><span class="p">.</span><span class="n">push_back</span><span class="p">(</span><span class="n">current_task</span><span class="p">().</span><span class="n">unwrap</span><span class="p">());</span><span class="w"></span>
|
||
<span class="linenos">37</span><span class="w"> </span><span class="nb">drop</span><span class="p">(</span><span class="n">inner</span><span class="p">);</span><span class="w"></span>
|
||
<span class="linenos">38</span><span class="w"> </span><span class="n">block_current_and_run_next</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">39</span><span class="w"> </span><span class="n">mutex</span><span class="p">.</span><span class="n">lock</span><span class="p">();</span><span class="w"></span>
|
||
<span class="linenos">40</span><span class="w"> </span><span class="p">}</span><span class="w"></span>
|
||
<span class="linenos">41</span><span class="p">}</span><span class="w"></span>
|
||
</pre></div>
|
||
</div>
|
||
<p>首先是核心数据结构：</p>
|
||
<ul class="simple">
|
||
<li><p>第 9 行，进程控制块中管理的条件变量列表。</p></li>
|
||
<li><p>第 15 行，条件变量的核心数据成员：等待队列。</p></li>
|
||
</ul>
|
||
<p>然后是重要的三个成员函数：</p>
|
||
<ul class="simple">
|
||
<li><p>第 18 行，创建条件变量，即创建了一个空的等待队列。</p></li>
|
||
<li><p>第 27 行，实现 <code class="docutils literal notranslate"><span class="pre">signal</span></code> 操作，将从条件变量的等待队列中弹出一个线程放入线程就绪队列。</p></li>
|
||
<li><p>第 33 行，实现 <code class="docutils literal notranslate"><span class="pre">wait</span></code> 操作，释放 <code class="docutils literal notranslate"><span class="pre">mutex</span></code> 互斥锁，将把当前线程放入条件变量的等待队列，
|
||
设置当前线程为挂起状态并选择新线程执行。在恢复执行后，再加上 <code class="docutils literal notranslate"><span class="pre">mutex</span></code> 互斥锁。</p></li>
|
||
</ul>
|
||
<p>Hansen, Per Brinch (1993). “Monitors and concurrent Pascal: a personal history”. HOPL-II:
|
||
The second ACM SIGPLAN conference on History of programming languages. History of Programming
|
||
Languages. New York, NY, USA: ACM. pp. 1–35. doi:10.1145/155360.155361. ISBN 0-89791-570-4.</p>
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