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This is an automatically generated e-mail. To reply, visit:
<a href="https://reviewboard.asterisk.org/r/1564/">https://reviewboard.asterisk.org/r/1564/</a>
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<p style="margin-top: 0;">On November 3rd, 2011, 4:13 p.m., <b>schmidts</b> wrote:</p>
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<a href="https://reviewboard.asterisk.org/r/1564/diff/1/?file=21604#file21604line25312" style="color: black; font-weight: bold; text-decoration: underline;">/branches/1.8/channels/chan_sip.c</a>
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(Diff revision 1)
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<td colspan="4"><pre style="font-size: 8pt; line-height: 140%; margin: 0; ">static void *do_monitor(void *data)</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></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "> <span class="n">ao2_t_callback</span><span class="p">(</span><span class="n">dialogs</span><span class="p">,</span> <span class="n">OBJ_NODATA</span><span class="p">,</span> <span class="n">dialog_needdestroy</span><span class="p">,</span> <span class="o">&</span><span class="n">t</span><span class="p">,</span></pre></td>
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<td bgcolor="#ffffff" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "> <span class="s">"callback to remove dialogs w/needdestroy"</span><span class="p">);</span></pre></td>
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<td bgcolor="#ffffff" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "> <span class="s">"callback to remove dialogs w/needdestroy"</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="n">iter</span> <span class="o">=</span> <span class="n">ao2_iterator_init</span><span class="p">(</span><span class="n">dialogs_needdestroy</span><span class="p">,</span> <span class="n">AO2_ITERATOR_UNLINK</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="k">for</span> <span class="p">(;</span> <span class="p">(</span><span class="n">dialog</span> <span class="o">=</span> <span class="n">ao2_iterator_next</span><span class="p">(</span><span class="o">&</span><span class="n">iter</span><span class="p">));</span> <span class="n">ao2_ref</span><span class="p">(</span><span class="n">dialog</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</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="cm">/* Now destroy the found dialogs that need to be destroyed. */</span></pre></td>
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<td bgcolor="#c5ffc4" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "> <span class="n">dialog_unlink_all</span><span class="p">(</span><span class="n">dialog</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="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="n">ao2_iterator_destroy</span><span class="p">(</span><span class="o">&</span><span class="n">iter</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;">i prefer move the iter block above the callback for the dialog container.
with this you can make sure that there will not be a lock enabled before deleting the dialog. normally it takes between 1 ms to max. 1000 ms to come back here so this should be fast enough.
btw i think checking if dialogs_needdestroy container is empty should be faster than an iterator_init to an empty container and this will happens most of the time.</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;">I don't understand.
The iter block has to follow the ao2_callback of dialog_needdestroy. The dialog_needdestroy() function puts dialogs into the dialogs_needdestroy container rather than destroying the dialogs immediately. The following iter block then destroys all dialogs in the dialog_needdestroy container.
There are no locks held when dialog_unlink_all() is called here. The ao2_iterator_next() only holds the container lock while it is finding the next object.
However, testing ao2_container_count before iterating would be faster.</pre>
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<p>- rmudgett</p>
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<p>On November 3rd, 2011, 3:14 p.m., rmudgett wrote:</p>
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<div>Review request for Asterisk Developers, David Vossel and schmidts.</div>
<div>By rmudgett.</div>
<p style="color: grey;"><i>Updated Nov. 3, 2011, 3:14 p.m.</i></p>
<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;">Timing between dialog destruction and a MWI event sending a message could result in a deadlock.
Order of events causing deadlock:
1a) The event subscription system calls the registered callbacks with its list RWLOCK held.
1b) The SIP monitor checks for dialogs needing destruction. It does an ao2_callback that holds the dialogs container lock while searching for dialogs to destroy.
2a) The event subscription SIP callback needs to create a temporary dialog to send out the MWI notification. That temporary dialog needs to be inserted in the dialogs container so it must wait.
2b) The dialog search finds a dialog to destroy and as a result releases the last reference for a peer. The peer destructor attempts to get the subscription RWLOCK but must wait.
3) deadlock
This is based off of the original diff posted for review: https://reviewboard.asterisk.org/r/1557/
It directly addresses the comment dvossel posted to the original diff of https://reviewboard.asterisk.org/r/1557/</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;">It compiles. :)</pre>
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<div style="margin-top: 1.5em;">
<b style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Bugs: </b>
<a href="https://issues.asterisk.org/jira/browse/ASTERISK-18747">ASTERISK-18747</a>
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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/channels/chan_sip.c <span style="color: grey">(343338)</span></li>
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<p><a href="https://reviewboard.asterisk.org/r/1564/diff/" style="margin-left: 3em;">View Diff</a></p>
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