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bpo-20891: Py_Initialize() now creates the GIL (#4700)
The GIL is no longer created "on demand" to fix a race condition when PyGILState_Ensure() is called in a non-Python thread.
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Victor Stinner
GitHub
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commit
2914bb32e2
4 changed files with 43 additions and 52 deletions
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@ -687,15 +687,14 @@ This is so common that a pair of macros exists to simplify it::
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The :c:macro:`Py_BEGIN_ALLOW_THREADS` macro opens a new block and declares a
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hidden local variable; the :c:macro:`Py_END_ALLOW_THREADS` macro closes the
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block. These two macros are still available when Python is compiled without
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thread support (they simply have an empty expansion).
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block.
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When thread support is enabled, the block above expands to the following code::
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The block above expands to the following code::
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PyThreadState *_save;
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_save = PyEval_SaveThread();
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...Do some blocking I/O operation...
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... Do some blocking I/O operation ...
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PyEval_RestoreThread(_save);
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.. index::
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@ -818,36 +817,24 @@ code, or when embedding the Python interpreter:
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This is a no-op when called for a second time.
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.. versionchanged:: 3.7
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This function is now called by :c:func:`Py_Initialize()`, so you don't
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have to call it yourself anymore.
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.. versionchanged:: 3.2
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This function cannot be called before :c:func:`Py_Initialize()` anymore.
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.. index:: module: _thread
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.. note::
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When only the main thread exists, no GIL operations are needed. This is a
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common situation (most Python programs do not use threads), and the lock
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operations slow the interpreter down a bit. Therefore, the lock is not
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created initially. This situation is equivalent to having acquired the lock:
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when there is only a single thread, all object accesses are safe. Therefore,
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when this function initializes the global interpreter lock, it also acquires
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it. Before the Python :mod:`_thread` module creates a new thread, knowing
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that either it has the lock or the lock hasn't been created yet, it calls
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:c:func:`PyEval_InitThreads`. When this call returns, it is guaranteed that
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the lock has been created and that the calling thread has acquired it.
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It is **not** safe to call this function when it is unknown which thread (if
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any) currently has the global interpreter lock.
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This function is not available when thread support is disabled at compile time.
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.. c:function:: int PyEval_ThreadsInitialized()
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Returns a non-zero value if :c:func:`PyEval_InitThreads` has been called. This
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function can be called without holding the GIL, and therefore can be used to
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avoid calls to the locking API when running single-threaded. This function is
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not available when thread support is disabled at compile time.
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avoid calls to the locking API when running single-threaded.
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.. versionchanged:: 3.7
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The :term:`GIL` is now initialized by :c:func:`Py_Initialize()`.
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.. c:function:: PyThreadState* PyEval_SaveThread()
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@ -855,8 +842,7 @@ code, or when embedding the Python interpreter:
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Release the global interpreter lock (if it has been created and thread
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support is enabled) and reset the thread state to *NULL*, returning the
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previous thread state (which is not *NULL*). If the lock has been created,
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the current thread must have acquired it. (This function is available even
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when thread support is disabled at compile time.)
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the current thread must have acquired it.
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.. c:function:: void PyEval_RestoreThread(PyThreadState *tstate)
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@ -864,8 +850,7 @@ code, or when embedding the Python interpreter:
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Acquire the global interpreter lock (if it has been created and thread
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support is enabled) and set the thread state to *tstate*, which must not be
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*NULL*. If the lock has been created, the current thread must not have
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acquired it, otherwise deadlock ensues. (This function is available even
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when thread support is disabled at compile time.)
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acquired it, otherwise deadlock ensues.
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.. c:function:: PyThreadState* PyThreadState_Get()
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@ -957,7 +942,7 @@ example usage in the Python source distribution.
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This macro expands to ``{ PyThreadState *_save; _save = PyEval_SaveThread();``.
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Note that it contains an opening brace; it must be matched with a following
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:c:macro:`Py_END_ALLOW_THREADS` macro. See above for further discussion of this
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macro. It is a no-op when thread support is disabled at compile time.
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macro.
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.. c:macro:: Py_END_ALLOW_THREADS
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@ -965,29 +950,29 @@ example usage in the Python source distribution.
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This macro expands to ``PyEval_RestoreThread(_save); }``. Note that it contains
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a closing brace; it must be matched with an earlier
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:c:macro:`Py_BEGIN_ALLOW_THREADS` macro. See above for further discussion of
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this macro. It is a no-op when thread support is disabled at compile time.
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this macro.
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.. c:macro:: Py_BLOCK_THREADS
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This macro expands to ``PyEval_RestoreThread(_save);``: it is equivalent to
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:c:macro:`Py_END_ALLOW_THREADS` without the closing brace. It is a no-op when
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thread support is disabled at compile time.
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:c:macro:`Py_END_ALLOW_THREADS` without the closing brace.
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.. c:macro:: Py_UNBLOCK_THREADS
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This macro expands to ``_save = PyEval_SaveThread();``: it is equivalent to
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:c:macro:`Py_BEGIN_ALLOW_THREADS` without the opening brace and variable
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declaration. It is a no-op when thread support is disabled at compile time.
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declaration.
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Low-level API
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-------------
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All of the following functions are only available when thread support is enabled
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at compile time, and must be called only when the global interpreter lock has
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been created.
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All of the following functions must be called after :c:func:`Py_Initialize`.
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.. versionchanged:: 3.7
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:c:func:`Py_Initialize()` now initializes the :term:`GIL`.
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.. c:function:: PyInterpreterState* PyInterpreterState_New()
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@ -1068,8 +1053,7 @@ been created.
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If this thread already has the lock, deadlock ensues.
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:c:func:`PyEval_RestoreThread` is a higher-level function which is always
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available (even when thread support isn't enabled or when threads have
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not been initialized).
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available (even when threads have not been initialized).
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.. c:function:: void PyEval_ReleaseThread(PyThreadState *tstate)
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@ -1081,8 +1065,7 @@ been created.
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reported.
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:c:func:`PyEval_SaveThread` is a higher-level function which is always
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available (even when thread support isn't enabled or when threads have
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not been initialized).
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available (even when threads have not been initialized).
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.. c:function:: void PyEval_AcquireLock()
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