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This is an automatically generated e-mail. To reply, visit:
<a href="https://reviewboard.asterisk.org/r/2738/">https://reviewboard.asterisk.org/r/2738/</a>
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<p style="margin-top: 0;">On August 3rd, 2013, 3:29 a.m. UTC, <b>Matt Jordan</b> wrote:</p>
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<a href="https://reviewboard.asterisk.org/r/2738/diff/1/?file=43757#file43757line169" style="color: black; font-weight: bold; text-decoration: underline;">/trunk/main/stasis_cache_pattern.c</a>
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(Diff revision 1)
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<td colspan="2"><pre style="font-size: 8pt; line-height: 140%; margin: 0; ">void stasis_cp_single_unsubscribe(struct stasis_cp_single *one)</pre></td>
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<td bgcolor="#ffffff" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="kt">void</span> <span class="nf">stasis_cp_single_unsubscribe</span><span class="p">(</span><span class="k">struct</span> <span class="n">stasis_cp_single</span> <span class="o">*</span><span class="n">one</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="kt">void</span> <span class="nf">stasis_cp_single_unsubscribe</span><span class="p">(</span><span class="k">struct</span> <span class="n">stasis_cp_single</span> <span class="o">*</span><span class="n">one</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="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="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="tb"> </span><span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="n">one</span><span class="p">)</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="tb"> </span><span class="k">if</span> <span class="p">(</span><span class="o">!</span><span class="n">one</span><span class="p">)</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="tb"> </span><span class="tb"> </span><span class="k">return</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="tb"> </span><span class="tb"> </span><span class="k">return</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="tb"> </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="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">stasis_unsubscribe</span><span class="p">(</span><span class="n">one</span><span class="o">-></span><span class="n">forward_topic_to_all</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">one</span><span class="o">-></span><span class="n">forward_topic_to_all</span> <span class="o">=</span> <span class="nb">NULL</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">stasis_unsubscribe</span><span class="p">(</span><span class="n">one</span><span class="o">-></span><span class="n">forward_cached_to_all</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">one</span><span class="o">-></span><span class="n">forward_cached_to_all</span> <span class="o">=</span> <span class="nb">NULL</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="tb"> </span><span class="n">stasis_caching_unsubscribe</span><span class="p">(</span><span class="n">one</span><span class="o">-></span><span class="n">topic_cached</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="tb"> </span><span class="n">stasis_caching_unsubscribe</span><span class="p">(</span><span class="n">one</span><span class="o">-></span><span class="n">topic_cached</span><span class="p">);</span></pre></td>
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<td bgcolor="#ffc5ce" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="tb"> </span><span class="n">one</span><span class="o">-></span><span class="n">topic_cached</span> <span class="o">=</span> <span class="nb">NULL</span><span class="p">;</span></pre></td>
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<td bgcolor="#ffc5ce" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="tb"> </span><span class="n">stasis_unsubscribe</span><span class="p">(</span><span class="n">one</span><span class="o">-></span><span class="n">forward</span><span class="p">);</span></pre></td>
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<td bgcolor="#ffc5ce" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="tb"> </span><span class="n">one</span><span class="o">-></span><span class="n">forward</span> <span class="o">=</span> <span class="nb">NULL</span><span class="p">;</span></pre></td>
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<td bgcolor="#ffc5ce" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="tb"> </span><span class="n">stasis_unsubscribe</span><span class="p">(</span><span class="n">one</span><span class="o">-></span><span class="n">forward_cached</span><span class="p">);</span></pre></td>
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<td bgcolor="#ffc5ce" width="50%"><pre style="font-size: 8pt; line-height: 140%; margin: 0; "><span class="tb"> </span><span class="n">one</span><span class="o">-></span><span class="n">forward_cached</span> <span class="o">=</span> <span class="nb">NULL</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;">So earlier tonight I was getting a lot of crashes when dummy channels were disposed of. They were attempting to dispose of their caching topic only to find that that topic was already disposed of.
After a lot of ref count debugging, the best I could figure was that the reference to the caching topic held by one of the subscriptions was causing the caching topic destructor itself to fire when the subscription was fired. This made some "sense", as the subscription also holds a reference to the topic - but since the channel also holds a reference, I'm not sure how this is expected to work.
Then, with this patch, it all magically started working again. I can only surmise that the act of unsubscribing from the forwarding topics before unsubscribing from the caching topic resolves the issue.
The reference owning semantics between subscriptions and topics is *incredibly* complex. I'm not sure if there's a solution to this problem, but it feels like subscriptions going away should never impact the topic that was subscribed to so long as something still holds a reference to the topic.</pre>
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<p>On August 5th, 2013, 2:32 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;">Agreed, but that's an underlying problem with subscription/topic
references; not something introduced with this patch. This patch
'fixes the glitch', but doesn't address the underlying problem you
found.
Yes, the reference counting is complex. Unfortunately, we have cyclic
references betweeen subscriptions and topics, and that's something
that really befuddles a reference counting system. Add to that lots of
temporary references that happen while dispatching the unsubscribe
messages, and it gets really weird fast.
I'm sure there's something we can do to improve the situation, but
that's outside the scope of this patch.</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;">Agreed. I shouldn't have opened it as an "issue".
Since we are fixing a ref counting semantics issue through this patch, however, do you mind adding a more verbose description of what holds references to what in some portion of the Stasis code?
The only reference to the reference issue (hah, English language is funny) that I found is:
{quote}
* In order to stop receiving messages, call stasis_unsubscribe() with your \ref
* stasis_subscription. Due to cyclic references, the \ref
* stasis_subscription will not be freed until after it has been unsubscribed,
* and all other ao2_ref()'s have been cleaned up.
{quote}
That's good to know from an API usage standpoint, but the complexity involved with maintaining this might warrant a bit more of an explanation elsewhere.</pre>
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<p>- Matt</p>
<br />
<p>On August 2nd, 2013, 10:56 p.m. UTC, David Lee wrote:</p>
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<div>Review request for Asterisk Developers and kmoore.</div>
<div>By David Lee.</div>
<p style="color: grey;"><i>Updated Aug. 2, 2013, 10:56 p.m.</i></p>
<div style="margin-top: 1.5em;">
<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;">The Stasis changes in r395954 had an unanticipated side effect: messages
published directly to an _all topic does not get forwarded to the
corresponding caching topic.
This patch fixes that by changing how caching topics forward messages,
and how the caching pattern forwards are setup.
For the caching pattern, the all_topic is forwarded to the
all_topic_cached. This forwards messages published directly to the
all_topic to all_topic_cached.
In order to avoid duplicate messages on all_topic_cached, caching topics
were changed to no longer forward uncached messages. Subscribers to an
individual caching topic should only expect to receive cache updates,
and subscription change messages. Since individual caching topics are
new, this shouldn't be a problem.
There are a few minor changes to the pre-cache split behavior.
* For topics changed to use the caching pattern, the all_topic_cached
will forward snapshots in addition to cache updates. Since
subscribers by design ignore unexpected messages, this should be
fine.
* Caching topics that don't use the caching pattern no longer forward
non-cache updates. This makes no difference for the current caching
topics.
* mwi_topic_cached, channel_by_name_topic and
presence_state_topic_cached have no subscribers
* device_state_topic_cached's only subscriber only processes cache
udpates
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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;">Ran unit tests several times.</pre>
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<h1 style="color: #575012; font-size: 10pt; margin-top: 1.5em;">Diffs</b> </h1>
<ul style="margin-left: 3em; padding-left: 0;">
<li>/trunk/include/asterisk/stasis.h <span style="color: grey">(396153)</span></li>
<li>/trunk/main/stasis_cache.c <span style="color: grey">(396153)</span></li>
<li>/trunk/main/stasis_cache_pattern.c <span style="color: grey">(396153)</span></li>
<li>/trunk/tests/test_stasis.c <span style="color: grey">(396153)</span></li>
</ul>
<p><a href="https://reviewboard.asterisk.org/r/2738/diff/" style="margin-left: 3em;">View Diff</a></p>
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