I don't see segfaults anymore, but there's still an unsafe dereference of gil_last_holder inside take_gil:

    /* Wait on the appropriate GIL depending on thread's classification */
    if (!tstate->cpu_bound) {
      /* We are I/O bound.  If the current thread is CPU-bound, force it off now! */
      if (gil_last_holder->cpu_bound) {
	SET_GIL_DROP_REQUEST();
      }

You're still accessing a location that may have been free()'d previously: while it will work most of the time (that's why I said it's not as risky), if the page gets unmapped between the time the current thread is deleted and the next thread takes over, you'll get a segfault. And that's undefined behaviour anyway ;-)