With current git master, configured --with-valgrind --with-pydebug, I get:

==31074== Command: /home/nikratio/clones/cpython/python /home/nikratio/in-progress/pyfuse3/test/../examples/hello.py /tmp/pytest-of-nikratio/pytest-11/test_hello_hello_py_0
==31074== 
==31074== Syscall param epoll_ctl(event) points to uninitialised byte(s)
==31074==    at 0x584906A: epoll_ctl (syscall-template.S:84)
==31074==    by 0xBDAA493: pyepoll_internal_ctl (selectmodule.c:1392)
==31074==    by 0xBDAA59F: select_epoll_register_impl (selectmodule.c:1438)
==31074==    by 0xBDAA5F8: select_epoll_register (selectmodule.c.h:599)
==31074==    by 0x174E15: _PyMethodDef_RawFastCallKeywords (call.c:658)
==31074==    by 0x300BCA: _PyMethodDescr_FastCallKeywords (descrobject.c:290)
==31074==    by 0x21FC05: call_function (ceval.c:4611)
==31074==    by 0x22B5E7: _PyEval_EvalFrameDefault (ceval.c:3183)
==31074==    by 0x2206FF: PyEval_EvalFrameEx (ceval.c:533)
==31074==    by 0x173B61: function_code_fastcall (call.c:285)
==31074==    by 0x174737: _PyFunction_FastCallKeywords (call.c:410)
==31074==    by 0x21FDF4: call_function (ceval.c:4634)
==31074==  Address 0xffeffeb4c is on thread 1's stack
==31074==  in frame #1, created by pyepoll_internal_ctl (selectmodule.c:1379)

To reproduce:

$ python-dev -m pip install --user pyfuse3  # for dependencies
$ git clone https://github.com/libfuse/pyfuse3.git
$ valgrind --trace-children=yes "--trace-children-skip=*mount*" python-dev -m pytest test/


pyfuse3 provides a C extension module, but I believe the problem is in the interpreter core as the stacktrace does not include anything from the extension.