[issue30747] _Py_atomic_* not actually atomic on Windows with MSVC

[Pär Björklund]

Pär Björklund added the comment:

Antoine said it best.

It's very hard to prove that this code is correct or incorrect as it requires multiple threads accessing the same variable and very specific timings to produce an actual issue.

My PR only solved half of the issue because I didn't really have a good idea for how to handle the load macro but I think it out today so I'll be updating the PR.

Would there be any interest of implementing them for MSVC/ARM as well? It's basically the same code so not much work, however I don't have a platform to actually test it.

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<http://bugs.python.org/issue30747>
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