> If so is there any way relying on these can break data race freedom (which as I understand Rust guarantees)?
It’s not clear what your question is.
You can use unsafe to write unsound code which suffers from data races.
If you use safe Rust, then you should not suffer from data races (and fences only add constraints, so they can’t add data races, they’re tools to avoid those), although bugs are always possible and sometimes purportedly safe stdlib APIs are found to be unsound (that’s one of the few things for which the rust project allows breaking backwards compatibility, though even that depends on the blast radius e.g. std::mem::uninitialized is still there, just deprecated, even though there’s almost no way to use it soundly).
It’s not clear what your question is.
You can use unsafe to write unsound code which suffers from data races.
If you use safe Rust, then you should not suffer from data races (and fences only add constraints, so they can’t add data races, they’re tools to avoid those), although bugs are always possible and sometimes purportedly safe stdlib APIs are found to be unsound (that’s one of the few things for which the rust project allows breaking backwards compatibility, though even that depends on the blast radius e.g. std::mem::uninitialized is still there, just deprecated, even though there’s almost no way to use it soundly).