If any Python devs are out there reading: my understanding is that removing the GIL itself isn't the hard part so much as removing the GIL while satisfying certain constraints deemed necessary by GvR and/or the rest of the community. I know some of those constraints relate to compatibility with existing C extensions -- but there must be others too?

The reason I ask is Larry's attempt buffered ref counting surely has implications for single-threaded code that maybe relies on the existing semantics -- e.g. a program like this may no longer reliably print "Deallocated!":

  Python 2.7.13 (default, Mar  5 2017, 00:33:10) 
  [GCC 6.3.0 20170205] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>> class Foo(object):
  ...     def __del__(self):
  ...             print 'Deallocated!'
  ... 
  >>> foo = Foo()
  >>> foo = None
  Deallocated!
  >>> 

A bad example in some ways since in this particular case we could wait for all ref counting operations to be processed before letting the interpreter exit, but hopefully my point is still clear.

Similarly, what about multi-threaded Python code that isn't written to operate in a GIL-free environment -- absent locks, atomic reads/writes, etc.? At best, you might expect some bad results. At worst, segfaults.

Are these all bridges that need to be crossed once a realistic solution to the core GIL removal issue is proposed? As glad as I am that folks are still thinking hard about this problem, I'm personally sort of pessimistic that the GIL can be killed off without a policy change wrt backward compatibility. Still, I do sort of wonder if some rules of engagement wrt departures from existing semantics might help drive a solution.