[zaphirplane]

You mean libraries with c extensions

[glandium]

No, even libraries without. Picking something random in the python 3.7 standard library: collections.OrderedDict.__setitem__ doesn't look thread-safe when it updates its linked list. EDIT: well, in fact, the data race in that one is there whether there is a GIL or not...

[xapata]

Just FYI, there are few remaining use cases for OrderedDict, now that built-in dict keeps insertion order.

[ehsankia]

Yes, getting rid of the GIL breaks most c extensions, many of which are very important.