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<body class="article">
<div id="header">
<h1>How does SPICE handle threading</h1>
<div id="toc">
<div id="toctitle">Table of Contents</div>
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<div id="content">
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>Lots of code runs in a single thread.</p></div>
<div class="paragraph"><p>Qemu usually calls SPICE from its main thread except on callbacks to code
already running in different threads.</p></div>
<div class="paragraph"><p>Channels (RedChannel/RedChannelClient code) run mostly on a single
thread except RedChannels on creation and destruction which MUST be
done in the main thread.
Lots of channels run in the main thread but currently CursorChannel and
DisplayChannel are run from within a thread created by RedWorker.
Note that different CursorChannel/DisplayChannel (they differ by id) run in
separate RedWorker threads (currently a single RedWorker thread runs a pair
of CursorChannel and DisplayChannel).</p></div>
<div class="paragraph"><p>RedChannelClient runs in the RedChannel thread. Creation is done in the same
thread while destruction can happen in different threads.</p></div>
<div class="paragraph"><p>A RedClient instance groups the various RedChannelClient connected to
the same remote SPICE client.
These RedChannelClient instances can run in separate threads:
MainChannelClient and most of the other RedChannelClient will be
running in the main thread, while the CursorChannelClient and the
DisplayChannelClient usually will be running from a different thread.</p></div>
<div class="paragraph"><p>Another important aspect of dealing with multiple threads are the dispatchers.
Dispatchers are used to send messages/request from one thread to another.
The main dispatcher is used to send requests to the main thread.
The Qxl uses a dispatcher to send requests to the RedWorker which will forward
to DisplayChannel/CursorChannel.</p></div>
<div class="paragraph"><p>One aspect to take into consideration is the event scheduling. SPICE uses some
<code>SpiceCoreInterface</code> to handle events. As the events will be handled from a
thread based on the core interface you have to use the correct core. Each
channel has an associated core interface which can be retrieved using
<code>red_channel_get_core_interface</code>. There’s also a main core interface you can get
using <code>reds_get_core_interface</code>. <code>reds_core_timer_*</code> and <code>reds_core_watch_*</code>
functions use the main core interface.
Even though multiple channel types run in the main thread and so could use
directly the main code interface, for coherence, rule simplicity and to allows
the code to be moved in a separate thread easily if needed, it’s advisable to
use the channel core interface (that will be the main core interface in this
case).
Currently character devices and not channel code runs in the main thread.</p></div>
</div>
</div>
<div class="sect1">
<h2 id="_reference_counting_and_ownership">1. Reference counting and ownership</h2>
<div class="sectionbody">
<div class="paragraph"><p>→ pointer</p></div>
<div class="paragraph"><p>⇒ pointer with owning (mostly GObject ref counting)</p></div>
<div class="paragraph"><p>RedChannelClient::client → RedClient</p></div>
<div class="paragraph"><p>RedClient::channels ⇒ RedChannelClient</p></div>
<div class="paragraph"><p>RedClient::mcc → MainChannelClient</p></div>
<div class="paragraph"><p>RedChannelClient::channel ⇒ RedChannel</p></div>
<div class="paragraph"><p>RedChannel::clients → RedChannelClient</p></div>
<div class="paragraph"><p>The RedClient represents a connection from a remote SPICE client,
RedClient::channels corresponding to the connections for the individual
channels. Their lifetime is tied to the TCP connection to this remote client.
When a disconnection is detected, the corresponding RedChannelClient are
removed from RedClient::channels and RedChannel::clients (the main owner of
RedChannelClient is RedClient).
When MainChannelClient is disconnected, all RedChannelClients connected to the
same RedClient are automatically disconnected.</p></div>
<div class="paragraph"><p>Who owns RedClient? RedClient is released when the MainChannelClient attached
to it is disconnected. In this case a request is scheduled in the main thread
(even if MainChannelClient runs on the main thread too) and
red_client_disconnect is called which calls red_client_destroy which uses
g_object_unref to free the object.</p></div>
<div class="paragraph"><p>Where is the MainChannelClient freed? On disconnection like other channel
clients, currently before RedClient which will have a dangling pointer.</p></div>
</div>
</div>
</div>
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Last updated
2020-02-27 11:26:12 GMT
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