[pcwalton]

There are exceptions to this rule--i.e. there are ways to not really be thread-safe but to have things work anyway--but they fall in the category of "you have to really, really know your CPU and be willing to write processor-specific code that just happens to work", so you can basically ignore them.

My favorite is the libdispatch abuse of cpuid to flood the pipeline on Intel CPUs for this problem: https://www.mikeash.com/pyblog/friday-qa-2014-06-06-secrets-...

But really, take Coda's advice. If you aren't synchronizing your reads, you can basically just assume your code is broken.

[twic]

An interesting exception is Lamport's Bakery:

https://en.m.wikipedia.org/wiki/Lamport%27s_bakery_algorithm

Thread safety without synchronisation primitives. I'm not sure it's ever actually a good idea to use it, though.

EDIT: unless you count a fence as a synchronisation primitive.