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Reproducer:
Code for native_ext.cpp:
#define PY_SSIZE_T_CLEAN
#include <Python.h>
#include <assert.h>
#include <pthread.h>
#include <malloc.h>
#pragma GCC push_options
#pragma GCC optimize ("O0")
PyObject*
run_simple(PyObject*, PyObject*)
{
	PyGILState_STATE gstate;
	gstate = PyGILState_Ensure();
	PyGILState_Release(gstate);
 Py_RETURN_NONE;
}
static PyMethodDef methods[] = {
 {"run_simple", run_simple, METH_NOARGS, "Execute a chain of native functions"},
 {NULL, NULL, 0, NULL},
};
#if PY_MAJOR_VERSION >= 3
static struct PyModuleDef moduledef = {PyModuleDef_HEAD_INIT, "native_ext", "", -1, methods};
PyMODINIT_FUNC
PyInit_native_ext(void)
{
 return PyModule_Create(&moduledef);
}
#else
PyMODINIT_FUNC
initnative_ext(void)
{
 Py_InitModule("native_ext", methods);
}
#endif
Code for setup.py:
import os
from distutils.core import Extension
from distutils.core import setup
ROOT = os.path.realpath(os.path.dirname(__file__))
setup(
 name="native_ext",
 version="0.0",
 ext_modules=[
 Extension(
 "native_ext",
 language="c++",
 sources=[os.path.join(ROOT, "native_ext.cpp")],
 ),
 ],
 zip_safe=False,
)
Compile extension:
python setup.py build_ext --inplace 
Execute the following script:
from test.support import run_in_subinterp
run_in_subinterp("""
import sys
sys.path.append(".")
import native_ext
native_ext.run_simple()
""")
The script deadlocks