Modern processors use something like https://en.m.wikipedia.org/wiki/MOESI_protocol https://en.m.wikipedia.org/wiki/MOESI_protocol to share data between process

Modern processors use something like https://en.m.wikipedia.org/wiki/MOESI_protocol https://en.m.wikipedia.org/wiki/MOESI_protocol to share data between processors. So basically, writing a shared line is expensive. Atomic reads are free, as are atomic writes to a cache line that no one else is using.