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This is an automatically generated e-mail. To reply, visit:
<a href="https://reviewboard.asterisk.org/r/2826/">https://reviewboard.asterisk.org/r/2826/</a>
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<p style="margin-top: 0;">On September 19th, 2013, 10:06 p.m. UTC, <b>rmudgett</b> wrote:</p>
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<a href="https://reviewboard.asterisk.org/r/2826/diff/3/?file=45654#file45654line73" style="color: black; font-weight: bold; text-decoration: underline;">/branches/1.8/main/lock.c</a>
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(Diff revision 3)
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="tb"> </span><span class="cm">/* It's a bit painful to lock a global mutex for every access to the</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="cm"><span class="tb"> </span> * reentrancy structure, but it's necessary to ensure that we don't</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="cm"><span class="tb"> </span> * double-allocate the structure or double-initialize the reentr_mutex.</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="cm"><span class="tb"> </span> *</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="cm"><span class="tb"> </span> * If you'd like to replace this with a double-checked lock, be sure to</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="cm"><span class="tb"> </span> * properly volatile-ize everything to avoid optimizer bugs.</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="cm"><span class="tb"> </span> *</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="cm"><span class="tb"> </span> * We also have to use the underlying pthread calls for manipulating</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="cm"><span class="tb"> </span> * the mutex, because this is called from the Asterisk mutex code.</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="cm"><span class="tb"> </span> */</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="tb"> </span><span class="n">pthread_mutex_lock</span><span class="p">(</span><span class="o">&</span><span class="n">reentrancy_lock</span><span class="p">.</span><span class="n">mutex</span><span class="p">);</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="tb"> </span><span class="k">if</span> <span class="p">(</span><span class="o">*</span><span class="n">plt</span><span class="p">)</span> <span class="p">{</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="tb"> </span><span class="tb"> </span><span class="n">pthread_mutex_unlock</span><span class="p">(</span><span class="o">&</span><span class="n">reentrancy_lock</span><span class="p">.</span><span class="n">mutex</span><span class="p">);</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="tb"> </span><span class="tb"> </span><span class="k">return</span> <span class="o">*</span><span class="n">plt</span><span class="p">;</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="tb"> </span><span class="p">}</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="tb"> </span><span class="n">lt</span> <span class="o">=</span> <span class="o">*</span><span class="n">plt</span> <span class="o">=</span> <span class="n">ast_std_calloc</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span> <span class="k">sizeof</span><span class="p">(</span><span class="o">*</span><span class="n">lt</span><span class="p">));</span></pre></td>
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<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">You could reduce the pain of the global lock here by:
if *plt
return *plt
lock global reentrancy lock
if *plt
unlock global reentrancy lock
return *plt
get memory for plt and initialize it
*plt = initialized
unlock global reentrancy lock
return *plt</pre>
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<p>On September 19th, 2013, 10:09 p.m. UTC, <b>David Lee</b> wrote:</p>
<blockquote style="margin-left: 1em; border-left: 2px solid #d0d0d0; padding-left: 10px;">
<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">The optimizer throws a wrench into that. Hence my comment about a double-checked lock.</pre>
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<p>On September 21st, 2013, 4:17 a.m. UTC, <b>rmudgett</b> wrote:</p>
<blockquote style="margin-left: 1em; border-left: 2px solid #d0d0d0; padding-left: 10px;">
<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">Instead of checking if *plt is not NULL and returning it since setting a pointer cannot always be done atomically. Use a trace initialized flag that can be set atomically before returning *plt and do the double-checked lock.</pre>
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<p>On September 23rd, 2013, 2:19 p.m. UTC, <b>David Lee</b> wrote:</p>
<blockquote style="margin-left: 1em; border-left: 2px solid #d0d0d0; padding-left: 10px;">
<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">Optimizer still has wrench-throwing capabilities. Even if the flag can
be set atomically, the compiler is allowed to reorder instructions, so
it could be set before the pointer update.</pre>
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<p>On September 23rd, 2013, 8:14 p.m. UTC, <b>rmudgett</b> wrote:</p>
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<pre style="white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">The optimizer cannot violate code sequence points without breaking the language. How the function is coded will determine what the optimizer can do.</pre>
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<pre style="margin-left: 1em; white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">Instead of using the pointer as a flag variable which cannot be changed atomically, use a volatile int as the flag variable. If you still have concernes then you can use ast_atomic_fetchadd_int() to examine and change the int flag.</pre>
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<p>- rmudgett</p>
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<p>On September 5th, 2013, 6:24 p.m. UTC, David Lee wrote:</p>
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<div>Review request for Asterisk Developers and Matt Jordan.</div>
<div>By David Lee.</div>
<p style="color: grey;"><i>Updated Sept. 5, 2013, 6:24 p.m.</i></p>
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<b style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Bugs: </b>
<a href="https://issues.asterisk.org/jira/browse/ASTERISK-19463">ASTERISK-19463</a>
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<b style="color: #575012; font-size: 10pt;">Repository: </b>
Asterisk
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<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Description </h1>
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<pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">This patch corrects a consistency issue with debug threads that I
noticed while fixing ASTERISK-22455.
The initialization of a mutex's lock tracking structure was not
protected in a critical section. This is fine for any mutex that is
explicitly initialized, but a static mutex may have its lock tracking
double-initialized if multiple threads attempt the first lock
simultaneously.
This patch creates a global mutex to properly serialize the
initialization of the lock tracking structure for a mutex. It also
changes lock.c to properly handle allocation failures of the lock
tracking structure.</pre>
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<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Testing </h1>
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<pre style="margin: 0; padding: 0; white-space: pre-wrap; white-space: -moz-pre-wrap; white-space: -pre-wrap; white-space: -o-pre-wrap; word-wrap: break-word;">I propose we setup Bamboo to run the TestSuite with DEBUG_THREADS
enabled on this branch nightly for a few weeks.</pre>
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<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Diffs</b> </h1>
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<li>/branches/1.8/main/lock.c <span style="color: grey">(398421)</span></li>
<li>/branches/1.8/include/asterisk/lock.h <span style="color: grey">(398421)</span></li>
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<p><a href="https://reviewboard.asterisk.org/r/2826/diff/" style="margin-left: 3em;">View Diff</a></p>
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