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caml_release_runtime_system导致的死锁问题
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Jun 28, 2023
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<p>给出一个<code>test.ml</code>:</p>
<figure class="highlight ocaml"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="type">Printf</span>.eprintf <span class="string">"hello from OCaml\n%!"</span></span><br></pre></td></tr></table></figure>
<p>和<code>test-ocaml-5.c</code>:</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><stdio.h></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><stdlib.h></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><caml/misc.h></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><caml/callback.h></span></span></span><br><span class="line"><span class="meta">#<span class="keyword">include</span> <span class="string"><caml/threads.h></span></span></span><br><span class="line"></span><br><span class="line"><span class="type">int</span></span><br><span class="line"><span class="title function_">main</span> <span class="params">(<span class="type">int</span> argc, <span class="type">char</span> *argv[])</span></span><br><span class="line"> {</span><br><span class="line"> <span class="built_in">fprintf</span> (<span class="built_in">stderr</span>, <span class="string">"starting up ...\n"</span>);</span><br><span class="line"> caml_startup (argv);</span><br><span class="line"> <span class="built_in">fprintf</span> (<span class="built_in">stderr</span>, <span class="string">"acquiring ...\n"</span>);</span><br><span class="line"> caml_acquire_runtime_system ();</span><br><span class="line"> <span class="built_in">fprintf</span> (<span class="built_in">stderr</span>, <span class="string">"acquired\n"</span>);</span><br><span class="line"> <span class="comment">// here is where I would be calling an OCaml callback</span></span><br><span class="line"> <span class="comment">// omitted for simplicity</span></span><br><span class="line"> caml_release_runtime_system ();</span><br><span class="line"> <span class="built_in">exit</span> (<span class="number">0</span>);</span><br><span class="line"> }</span><br></pre></td></tr></table></figure>
<p>然后用OCaml5的ocaml native compiler编译一下（这里我用的是<code>ocaml-variants.5.0.0+options</code>）：<br><code>ocamlopt -g test-ocaml-5.c test.ml -o test-ocaml-5</code></p>
<p>运行<code>test-ocaml-5</code> 会出现:</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line">starting up ...</span><br><span class="line">hello from OCaml</span><br><span class="line">acquiring ...</span><br><span class="line">Fatal error: Fatal error during lock: Resource deadlock avoided</span><br><span class="line"></span><br><span class="line">[1] 8998 IOT instruction (core dumped) ./test-ocaml-5</span><br></pre></td></tr></table></figure>
<p>这里我尝试了一下 4.14.0 和 4.14.1 ， 都没有出现这个情况，而如果用<code>threads</code>编译的话:<br><code>ocamlopt -g -I +unix unix.cmxa -I +threads threads.cmxa test-ocaml-5.c test.ml -o test-ocaml-5</code></p>
<p>无论在5.0还是4.14.x，都会挂起。</p>
<p>出现这个问题的一个可能原因是，在<code>test-ocaml-5.c</code>中，我在开头调用了<code>caml_startup()</code>，这会让当前线程获取锁，而<code>caml_acquire_runtime_system()</code>会再次获取它，<code>caml_acquire_runtime_system()</code> 应该在<code>caml_release_runtime_system（）</code>之后调用：</p>
<figure class="highlight ocaml"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">int</span></span><br><span class="line">main (<span class="built_in">int</span> argc, <span class="built_in">char</span> *argv<span class="literal">[]</span>)</span><br><span class="line"> {</span><br><span class="line"> fprintf (stderr, <span class="string">"starting up ...\n"</span>);</span><br><span class="line"> caml_startup (argv);</span><br><span class="line"> fprintf (stderr, <span class="string">"acquiring ...\n"</span>);</span><br><span class="line"> caml_release_runtime_system <span class="literal">()</span>;</span><br><span class="line"> caml_acquire_runtime_system <span class="literal">()</span>;</span><br><span class="line"> fprintf (stderr, <span class="string">"acquired\n"</span>);</span><br><span class="line"> exit (<span class="number">0</span>);</span><br><span class="line"> }</span><br></pre></td></tr></table></figure>
<p>运行结果为:</p>
<figure class="highlight plaintext"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">starting up ...</span><br><span class="line">hello from OCaml</span><br><span class="line">acquiring ...</span><br><span class="line">acquired</span><br></pre></td></tr></table></figure>
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