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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - lldb-server can deadlock in ack-mode"
href="https://bugs.llvm.org/show_bug.cgi?id=37294">37294</a>
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<th>Summary</th>
<td>lldb-server can deadlock in ack-mode
</td>
</tr>
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<th>Product</th>
<td>lldb
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>All Bugs
</td>
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<th>Assignee</th>
<td>lldb-dev@lists.llvm.org
</td>
</tr>
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<th>Reporter</th>
<td>labath@google.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>lldb-server is a single-threaded event-driven app and uses the MainLoop class
to manage events and their actions. It's two main sources of events are a
network socket (requests from the client) and SEGV signals (events in the
inferior).
Normally everything works fine, if we have a very fast client (or a heavily
loaded system) which tries to communicate with lldb-server in ack-mode, things
can break down. The sequence of events if approximately like this:
1. inferior stops for some reason (event A)
2. lldb-server sends a stop-reply packet
3. lldb-server waits for acknowledgement from the client.
- now if the client is really fast, he will manage to also send a regular
packet in addition to a acknowledgement
4. GetAck will read both of these things, but process only the acknowledgement,
and store the rest in a side buffer
5. event A processing done, mainloop calls ppoll()
- at this point, we should wake up and process the client request, however,
ppoll blocks because as far as it is concerned, the event has already been
processed
- so server blocks waiting for data, and client blocks waiting for the server
to respond - deadlock.
I have a couple of ideas how to fix this, but none of them seemed completely
appealing. This issue only affects the lldb-server gtests, because they are the
only ones running in ack-mode. Given that this was more of an omission than
explicit intent, I am going to switch the tests to noack-mode as a stop-gap
measure, to make sure the tests are not flaky.</pre>
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