It's memory safe, but it's not necessarily free of race conditions! It's not only C extensions that release the GIL, the Python interpreter itself releases the GIL after a certain number of instructions so that other threads can make progress. See https://docs.python.org/3/library/sys.html#sys.getswitchinte....
Certain operations that look atomic to the user are actually comprised of multiple bytecode instructions. Now, if you are unlucky, the interpreter decides to release the GIL and yield to another thread exactly during such instructions. You won't get a segfault, but you might get unexpected results.
