The GIL and PyEval_RestoreThread

[Peter Ebden]

Hi all,

I've been working on embedding Python and have an interesting case around
locking with PyEval_RestoreThread which wasn't quite doing what I expect,
hoping someone can explain what I should expect here.

I have a little example (I'm running this in parallel from two different
threads; I have some more C code for that but I don't think it's super
interesting):

void run_python(PyThreadState* thread) {
  LOG("Restoring thread %p...", thread);
  PyEval_RestoreThread(thread);
  LOG("Restored thread %p", thread);
  PyRun_SimpleString("import time; print('sleeping'); time.sleep(3.0)");
  LOG("Saving thread...");
  PyThreadState* saved_thread = PyEval_SaveThread();
  LOG("Saved thread %p", saved_thread);
}

This produces output like
11:46:48.110058893: Restoring thread 0xabc480...
11:46:48.110121656: Restored thread 0xabc480
11:46:48.110166060: Restoring thread 0xabc480...
sleeping
11:46:48.110464194: Restored thread 0xabc480
sleeping
11:46:51.111307541: Saving thread...
11:46:51.111361075: Saved thread 0xabc480
11:46:51.113116633: Saving thread...
11:46:51.113177605: Saved thread 0xabc480

The thing that surprises me is that both threads seem to be able to pass
PyEval_RestoreThread before either reaches the corresponding
PyEval_SaveThread call, which I wasn't expecting to happen; I assumed that
since RestoreThread acquires the GIL, that thread state would remain locked
until it's released.

I understand that the system occasionally switches threads, which I guess
might well happen with that time.sleep() call, but I wasn't expecting the
same thread to become usable somewhere else. Maybe I am just confusing
things by approaching the same Python thread from multiple OS threads
concurrently and should be managing my own locking around that?

Thanks in advance,

Peter

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