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The Python GIL problem could been solved in Python3 by just moving all global interpreter instance variables into a single struct and changing the C API to take a pointer to this interpreter struct as the first argument in all C API functions. No thread local variable hacks need be used and all interpreter instances could have been completely independent of each other, allowing for seamless and efficient share-nothing multi-threading. Python3 was not intended to be 100% compatible with Python2 anyway, so it was a perfect opportunity to fix this long standing design bug. But this opportunity was lost.

Strangely, the Google V8 team repeated Python's API design error and also relies on global interpreter variables and thread-local variables to swap interpreter contexts, or "isolates" in V8-speak. Although my understanding is that the V8 team recognizes the problem and are (slowly) addressing it and changing their APIs accordingly with each successive release.