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<h1 class="page-title">PEP 311 – Simplified Global Interpreter Lock Acquisition for Extensions</h1>
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<dl class="rfc2822 field-list simple">
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<dt class="field-odd">Author<span class="colon">:</span></dt>
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<dd class="field-odd">Mark Hammond <mhammond at skippinet.com.au></dd>
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<dt class="field-even">Status<span class="colon">:</span></dt>
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<dd class="field-even"><abbr title="Accepted and implementation complete, or no longer active">Final</abbr></dd>
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<dt class="field-odd">Type<span class="colon">:</span></dt>
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<dd class="field-odd"><abbr title="Normative PEP with a new feature for Python, implementation change for CPython or interoperability standard for the ecosystem">Standards Track</abbr></dd>
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<dt class="field-even">Created<span class="colon">:</span></dt>
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<dd class="field-even">05-Feb-2003</dd>
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<dt class="field-odd">Python-Version<span class="colon">:</span></dt>
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<dd class="field-odd">2.3</dd>
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<dt class="field-even">Post-History<span class="colon">:</span></dt>
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<dd class="field-even">05-Feb-2003, 14-Feb-2003, 19-Apr-2003</dd>
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</dl>
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<hr class="docutils" />
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<section id="contents">
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<details><summary>Table of Contents</summary><ul class="simple">
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<li><a class="reference internal" href="#abstract">Abstract</a></li>
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<li><a class="reference internal" href="#rationale">Rationale</a></li>
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<li><a class="reference internal" href="#limitations-and-exclusions">Limitations and Exclusions</a></li>
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<li><a class="reference internal" href="#proposal">Proposal</a></li>
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<li><a class="reference internal" href="#design-and-implementation">Design and Implementation</a></li>
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<li><a class="reference internal" href="#implementation">Implementation</a></li>
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<li><a class="reference internal" href="#references">References</a></li>
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<li><a class="reference internal" href="#copyright">Copyright</a></li>
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</ul>
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</details></section>
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<section id="abstract">
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<h2><a class="toc-backref" href="#abstract" role="doc-backlink">Abstract</a></h2>
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<p>This PEP proposes a simplified API for access to the Global
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Interpreter Lock (GIL) for Python extension modules.
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Specifically, it provides a solution for authors of complex
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multi-threaded extensions, where the current state of Python
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(i.e., the state of the GIL is unknown.</p>
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<p>This PEP proposes a new API, for platforms built with threading
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support, to manage the Python thread state. An implementation
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strategy is proposed, along with an initial, platform independent
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implementation.</p>
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</section>
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<section id="rationale">
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<h2><a class="toc-backref" href="#rationale" role="doc-backlink">Rationale</a></h2>
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<p>The current Python interpreter state API is suitable for simple,
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single-threaded extensions, but quickly becomes incredibly complex
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for non-trivial, multi-threaded extensions.</p>
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<p>Currently Python provides two mechanisms for dealing with the GIL:</p>
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<ul class="simple">
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<li><code class="docutils literal notranslate"><span class="pre">Py_BEGIN_ALLOW_THREADS</span></code> and <code class="docutils literal notranslate"><span class="pre">Py_END_ALLOW_THREADS</span></code> macros.
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These macros are provided primarily to allow a simple Python
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extension that already owns the GIL to temporarily release it
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while making an “external” (ie, non-Python), generally
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expensive, call. Any existing Python threads that are blocked
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waiting for the GIL are then free to run. While this is fine
|
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for extensions making calls from Python into the outside world,
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it is no help for extensions that need to make calls into Python
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when the thread state is unknown.</li>
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<li><code class="docutils literal notranslate"><span class="pre">PyThreadState</span></code> and <code class="docutils literal notranslate"><span class="pre">PyInterpreterState</span></code> APIs.
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These API functions allow an extension/embedded application to
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acquire the GIL, but suffer from a serious boot-strapping
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problem - they require you to know the state of the Python
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interpreter and of the GIL before they can be used. One
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particular problem is for extension authors that need to deal
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with threads never before seen by Python, but need to call
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Python from this thread. It is very difficult, delicate and
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error prone to author an extension where these “new” threads
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always know the exact state of the GIL, and therefore can
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reliably interact with this API.</li>
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</ul>
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<p>For these reasons, the question of how such extensions should
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interact with Python is quickly becoming a FAQ. The main impetus
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for this PEP, a thread on python-dev <a class="footnote-reference brackets" href="#id2" id="id1">[1]</a>, immediately identified
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the following projects with this exact issue:</p>
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<ul class="simple">
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<li>The win32all extensions</li>
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<li>Boost</li>
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<li>ctypes</li>
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<li>Python-GTK bindings</li>
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<li>Uno</li>
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<li>PyObjC</li>
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<li>Mac toolbox</li>
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<li>PyXPCOM</li>
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</ul>
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<p>Currently, there is no reasonable, portable solution to this
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problem, forcing each extension author to implement their own
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hand-rolled version. Further, the problem is complex, meaning
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many implementations are likely to be incorrect, leading to a
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variety of problems that will often manifest simply as “Python has
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hung”.</p>
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<p>While the biggest problem in the existing thread-state API is the
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lack of the ability to query the current state of the lock, it is
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felt that a more complete, simplified solution should be offered
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to extension authors. Such a solution should encourage authors to
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provide error-free, complex extension modules that take full
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advantage of Python’s threading mechanisms.</p>
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</section>
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<section id="limitations-and-exclusions">
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<h2><a class="toc-backref" href="#limitations-and-exclusions" role="doc-backlink">Limitations and Exclusions</a></h2>
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<p>This proposal identifies a solution for extension authors with
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complex multi-threaded requirements, but that only require a
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single “PyInterpreterState”. There is no attempt to cater for
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||
extensions that require multiple interpreter states. At the time
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||
of writing, no extension has been identified that requires
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multiple PyInterpreterStates, and indeed it is not clear if that
|
||
facility works correctly in Python itself.</p>
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<p>This API will not perform automatic initialization of Python, or
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initialize Python for multi-threaded operation. Extension authors
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||
must continue to call <code class="docutils literal notranslate"><span class="pre">Py_Initialize()</span></code>, and for multi-threaded
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||
applications, <code class="docutils literal notranslate"><span class="pre">PyEval_InitThreads()</span></code>. The reason for this is that
|
||
the first thread to call <code class="docutils literal notranslate"><span class="pre">PyEval_InitThreads()</span></code> is nominated as the
|
||
“main thread” by Python, and so forcing the extension author to
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||
specify the main thread (by requiring them to make this first call)
|
||
removes ambiguity. As <code class="docutils literal notranslate"><span class="pre">Py_Initialize()</span></code> must be called before
|
||
<code class="docutils literal notranslate"><span class="pre">PyEval_InitThreads()</span></code>, and as both of these functions currently
|
||
support being called multiple times, the burden this places on
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extension authors is considered reasonable.</p>
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||
<p>It is intended that this API be all that is necessary to acquire
|
||
the Python GIL. Apart from the existing, standard
|
||
<code class="docutils literal notranslate"><span class="pre">Py_BEGIN_ALLOW_THREADS</span></code> and <code class="docutils literal notranslate"><span class="pre">Py_END_ALLOW_THREADS</span></code> macros, it is
|
||
assumed that no additional thread state API functions will be used
|
||
by the extension. Extensions with such complicated requirements
|
||
are free to continue to use the existing thread state API.</p>
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||
</section>
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<section id="proposal">
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<h2><a class="toc-backref" href="#proposal" role="doc-backlink">Proposal</a></h2>
|
||
<p>This proposal recommends a new API be added to Python to simplify
|
||
the management of the GIL. This API will be available on all
|
||
platforms built with <code class="docutils literal notranslate"><span class="pre">WITH_THREAD</span></code> defined.</p>
|
||
<p>The intent is that assuming Python has correctly been initialized,
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an extension author be able to use a small, well-defined “prologue
|
||
dance”, at any time and on any thread, which will ensure Python
|
||
is ready to be used on that thread. After the extension has
|
||
finished with Python, it must also perform an “epilogue dance” to
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||
release any resources previously acquired. Ideally, these dances
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||
can be expressed in a single line.</p>
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<p>Specifically, the following new APIs are proposed:</p>
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<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">/*</span> <span class="n">Ensure</span> <span class="n">that</span> <span class="n">the</span> <span class="n">current</span> <span class="n">thread</span> <span class="ow">is</span> <span class="n">ready</span> <span class="n">to</span> <span class="n">call</span> <span class="n">the</span> <span class="n">Python</span>
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<span class="n">C</span> <span class="n">API</span><span class="p">,</span> <span class="n">regardless</span> <span class="n">of</span> <span class="n">the</span> <span class="n">current</span> <span class="n">state</span> <span class="n">of</span> <span class="n">Python</span><span class="p">,</span> <span class="ow">or</span> <span class="n">of</span> <span class="n">its</span>
|
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<span class="n">thread</span> <span class="n">lock</span><span class="o">.</span> <span class="n">This</span> <span class="n">may</span> <span class="n">be</span> <span class="n">called</span> <span class="k">as</span> <span class="n">many</span> <span class="n">times</span> <span class="k">as</span> <span class="n">desired</span>
|
||
<span class="n">by</span> <span class="n">a</span> <span class="n">thread</span> <span class="n">so</span> <span class="n">long</span> <span class="k">as</span> <span class="n">each</span> <span class="n">call</span> <span class="ow">is</span> <span class="n">matched</span> <span class="k">with</span> <span class="n">a</span> <span class="n">call</span> <span class="n">to</span>
|
||
<span class="n">PyGILState_Release</span><span class="p">()</span><span class="o">.</span> <span class="n">In</span> <span class="n">general</span><span class="p">,</span> <span class="n">other</span> <span class="n">thread</span><span class="o">-</span><span class="n">state</span> <span class="n">APIs</span> <span class="n">may</span>
|
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<span class="n">be</span> <span class="n">used</span> <span class="n">between</span> <span class="n">_Ensure</span><span class="p">()</span> <span class="ow">and</span> <span class="n">_Release</span><span class="p">()</span> <span class="n">calls</span><span class="p">,</span> <span class="n">so</span> <span class="n">long</span> <span class="k">as</span> <span class="n">the</span>
|
||
<span class="n">thread</span><span class="o">-</span><span class="n">state</span> <span class="ow">is</span> <span class="n">restored</span> <span class="n">to</span> <span class="n">its</span> <span class="n">previous</span> <span class="n">state</span> <span class="n">before</span> <span class="n">the</span> <span class="n">Release</span><span class="p">()</span><span class="o">.</span>
|
||
<span class="n">For</span> <span class="n">example</span><span class="p">,</span> <span class="n">normal</span> <span class="n">use</span> <span class="n">of</span> <span class="n">the</span> <span class="n">Py_BEGIN_ALLOW_THREADS</span><span class="o">/</span>
|
||
<span class="n">Py_END_ALLOW_THREADS</span> <span class="n">macros</span> <span class="n">are</span> <span class="n">acceptable</span><span class="o">.</span>
|
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|
||
<span class="n">The</span> <span class="k">return</span> <span class="n">value</span> <span class="ow">is</span> <span class="n">an</span> <span class="n">opaque</span> <span class="s2">"handle"</span> <span class="n">to</span> <span class="n">the</span> <span class="n">thread</span> <span class="n">state</span> <span class="n">when</span>
|
||
<span class="n">PyGILState_Acquire</span><span class="p">()</span> <span class="n">was</span> <span class="n">called</span><span class="p">,</span> <span class="ow">and</span> <span class="n">must</span> <span class="n">be</span> <span class="n">passed</span> <span class="n">to</span>
|
||
<span class="n">PyGILState_Release</span><span class="p">()</span> <span class="n">to</span> <span class="n">ensure</span> <span class="n">Python</span> <span class="ow">is</span> <span class="n">left</span> <span class="ow">in</span> <span class="n">the</span> <span class="n">same</span> <span class="n">state</span><span class="o">.</span> <span class="n">Even</span>
|
||
<span class="n">though</span> <span class="n">recursive</span> <span class="n">calls</span> <span class="n">are</span> <span class="n">allowed</span><span class="p">,</span> <span class="n">these</span> <span class="n">handles</span> <span class="n">can</span> <span class="o">*</span><span class="ow">not</span><span class="o">*</span> <span class="n">be</span>
|
||
<span class="n">shared</span> <span class="o">-</span> <span class="n">each</span> <span class="n">unique</span> <span class="n">call</span> <span class="n">to</span> <span class="n">PyGILState_Ensure</span> <span class="n">must</span> <span class="n">save</span> <span class="n">the</span> <span class="n">handle</span>
|
||
<span class="k">for</span> <span class="n">its</span> <span class="n">call</span> <span class="n">to</span> <span class="n">PyGILState_Release</span><span class="o">.</span>
|
||
|
||
<span class="n">When</span> <span class="n">the</span> <span class="n">function</span> <span class="n">returns</span><span class="p">,</span> <span class="n">the</span> <span class="n">current</span> <span class="n">thread</span> <span class="n">will</span> <span class="n">hold</span> <span class="n">the</span> <span class="n">GIL</span><span class="o">.</span>
|
||
|
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<span class="n">Failure</span> <span class="ow">is</span> <span class="n">a</span> <span class="n">fatal</span> <span class="n">error</span><span class="o">.</span>
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<span class="o">*/</span>
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<span class="n">PyAPI_FUNC</span><span class="p">(</span><span class="n">PyGILState_STATE</span><span class="p">)</span> <span class="n">PyGILState_Ensure</span><span class="p">(</span><span class="n">void</span><span class="p">);</span>
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<span class="o">/*</span> <span class="n">Release</span> <span class="nb">any</span> <span class="n">resources</span> <span class="n">previously</span> <span class="n">acquired</span><span class="o">.</span> <span class="n">After</span> <span class="n">this</span> <span class="n">call</span><span class="p">,</span> <span class="n">Python</span><span class="s1">'s</span>
|
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<span class="n">state</span> <span class="n">will</span> <span class="n">be</span> <span class="n">the</span> <span class="n">same</span> <span class="k">as</span> <span class="n">it</span> <span class="n">was</span> <span class="n">prior</span> <span class="n">to</span> <span class="n">the</span> <span class="n">corresponding</span>
|
||
<span class="n">PyGILState_Acquire</span> <span class="n">call</span> <span class="p">(</span><span class="n">but</span> <span class="n">generally</span> <span class="n">this</span> <span class="n">state</span> <span class="n">will</span> <span class="n">be</span> <span class="n">unknown</span> <span class="n">to</span>
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||
<span class="n">the</span> <span class="n">caller</span><span class="p">,</span> <span class="n">hence</span> <span class="n">the</span> <span class="n">use</span> <span class="n">of</span> <span class="n">the</span> <span class="n">GILState</span> <span class="n">API</span><span class="o">.</span><span class="p">)</span>
|
||
|
||
<span class="n">Every</span> <span class="n">call</span> <span class="n">to</span> <span class="n">PyGILState_Ensure</span> <span class="n">must</span> <span class="n">be</span> <span class="n">matched</span> <span class="n">by</span> <span class="n">a</span> <span class="n">call</span> <span class="n">to</span>
|
||
<span class="n">PyGILState_Release</span> <span class="n">on</span> <span class="n">the</span> <span class="n">same</span> <span class="n">thread</span><span class="o">.</span>
|
||
<span class="o">*/</span>
|
||
<span class="n">PyAPI_FUNC</span><span class="p">(</span><span class="n">void</span><span class="p">)</span> <span class="n">PyGILState_Release</span><span class="p">(</span><span class="n">PyGILState_STATE</span><span class="p">);</span>
|
||
</pre></div>
|
||
</div>
|
||
<p>Common usage will be:</p>
|
||
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">void</span> <span class="n">SomeCFunction</span><span class="p">(</span><span class="n">void</span><span class="p">)</span>
|
||
<span class="p">{</span>
|
||
<span class="o">/*</span> <span class="n">ensure</span> <span class="n">we</span> <span class="n">hold</span> <span class="n">the</span> <span class="n">lock</span> <span class="o">*/</span>
|
||
<span class="n">PyGILState_STATE</span> <span class="n">state</span> <span class="o">=</span> <span class="n">PyGILState_Ensure</span><span class="p">();</span>
|
||
<span class="o">/*</span> <span class="n">Use</span> <span class="n">the</span> <span class="n">Python</span> <span class="n">API</span> <span class="o">*/</span>
|
||
<span class="o">...</span>
|
||
<span class="o">/*</span> <span class="n">Restore</span> <span class="n">the</span> <span class="n">state</span> <span class="n">of</span> <span class="n">Python</span> <span class="o">*/</span>
|
||
<span class="n">PyGILState_Release</span><span class="p">(</span><span class="n">state</span><span class="p">);</span>
|
||
<span class="p">}</span>
|
||
</pre></div>
|
||
</div>
|
||
</section>
|
||
<section id="design-and-implementation">
|
||
<h2><a class="toc-backref" href="#design-and-implementation" role="doc-backlink">Design and Implementation</a></h2>
|
||
<p>The general operation of <code class="docutils literal notranslate"><span class="pre">PyGILState_Ensure()</span></code> will be:</p>
|
||
<ul class="simple">
|
||
<li>assert Python is initialized.</li>
|
||
<li>Get a <code class="docutils literal notranslate"><span class="pre">PyThreadState</span></code> for the current thread, creating and saving
|
||
if necessary.</li>
|
||
<li>remember the current state of the lock (owned/not owned)</li>
|
||
<li>If the current state does not own the GIL, acquire it.</li>
|
||
<li>Increment a counter for how many calls to <code class="docutils literal notranslate"><span class="pre">PyGILState_Ensure</span></code> have been
|
||
made on the current thread.</li>
|
||
<li>return</li>
|
||
</ul>
|
||
<p>The general operation of <code class="docutils literal notranslate"><span class="pre">PyGILState_Release()</span></code> will be:</p>
|
||
<ul class="simple">
|
||
<li>assert our thread currently holds the lock.</li>
|
||
<li>If old state indicates lock was previously unlocked, release GIL.</li>
|
||
<li>Decrement the <code class="docutils literal notranslate"><span class="pre">PyGILState_Ensure</span></code> counter for the thread.</li>
|
||
<li>If counter == 0:<ul>
|
||
<li>release and delete the <code class="docutils literal notranslate"><span class="pre">PyThreadState</span></code>.</li>
|
||
<li>forget the <code class="docutils literal notranslate"><span class="pre">ThreadState</span></code> as being owned by the thread.</li>
|
||
</ul>
|
||
</li>
|
||
<li>return</li>
|
||
</ul>
|
||
<p>It is assumed that it is an error if two discrete <code class="docutils literal notranslate"><span class="pre">PyThreadStates</span></code>
|
||
are used for a single thread. Comments in <code class="docutils literal notranslate"><span class="pre">pystate.h</span></code> (“State
|
||
unique per thread”) support this view, although it is never
|
||
directly stated. Thus, this will require some implementation of
|
||
Thread Local Storage. Fortunately, a platform independent
|
||
implementation of Thread Local Storage already exists in the
|
||
Python source tree, in the SGI threading port. This code will be
|
||
integrated into the platform independent Python core, but in such
|
||
a way that platforms can provide a more optimal implementation if
|
||
desired.</p>
|
||
</section>
|
||
<section id="implementation">
|
||
<h2><a class="toc-backref" href="#implementation" role="doc-backlink">Implementation</a></h2>
|
||
<p>An implementation of this proposal can be found at
|
||
<a class="reference external" href="https://bugs.python.org/issue684256">https://bugs.python.org/issue684256</a></p>
|
||
</section>
|
||
<section id="references">
|
||
<h2><a class="toc-backref" href="#references" role="doc-backlink">References</a></h2>
|
||
<aside class="footnote-list brackets">
|
||
<aside class="footnote brackets" id="id2" role="doc-footnote">
|
||
<dt class="label" id="id2">[<a href="#id1">1</a>]</dt>
|
||
<dd>David Abrahams, Extension modules, Threading, and the GIL
|
||
<a class="reference external" href="https://mail.python.org/pipermail/python-dev/2002-December/031424.html">https://mail.python.org/pipermail/python-dev/2002-December/031424.html</a></aside>
|
||
</aside>
|
||
</section>
|
||
<section id="copyright">
|
||
<h2><a class="toc-backref" href="#copyright" role="doc-backlink">Copyright</a></h2>
|
||
<p>This document has been placed in the public domain.</p>
|
||
</section>
|
||
</section>
|
||
<hr class="docutils" />
|
||
<p>Source: <a class="reference external" href="https://github.com/python/peps/blob/main/peps/pep-0311.rst">https://github.com/python/peps/blob/main/peps/pep-0311.rst</a></p>
|
||
<p>Last modified: <a class="reference external" href="https://github.com/python/peps/commits/main/peps/pep-0311.rst">2023-09-09 17:39:29 GMT</a></p>
|
||
|
||
</article>
|
||
<nav id="pep-sidebar">
|
||
<h2>Contents</h2>
|
||
<ul>
|
||
<li><a class="reference internal" href="#abstract">Abstract</a></li>
|
||
<li><a class="reference internal" href="#rationale">Rationale</a></li>
|
||
<li><a class="reference internal" href="#limitations-and-exclusions">Limitations and Exclusions</a></li>
|
||
<li><a class="reference internal" href="#proposal">Proposal</a></li>
|
||
<li><a class="reference internal" href="#design-and-implementation">Design and Implementation</a></li>
|
||
<li><a class="reference internal" href="#implementation">Implementation</a></li>
|
||
<li><a class="reference internal" href="#references">References</a></li>
|
||
<li><a class="reference internal" href="#copyright">Copyright</a></li>
|
||
</ul>
|
||
|
||
<br>
|
||
<a id="source" href="https://github.com/python/peps/blob/main/peps/pep-0311.rst">Page Source (GitHub)</a>
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