[addaon]

> Does there exist any platform which has multithreading but not atomics?

Yes. Also, almost every platform I know that supports multi threading and atomics doesn’t support atomics between /all/ possible masters. Consider a microcontroller with, say, two Arm cores (multithreaded, atomic-supporting) and a DMA engine.

[lpribis]

Yes but "atomic" operations with the DMA engine are accomplished through interrupts (atomic) or memory mapped IO configuration (atomic).