[spinlocker]

Looks like in C++ memory_order_acq_rel is treated like memory_order_acquire when it's a load and memory_order_release when it's a store. I would argue that this isn't really a difference in memory model but a difference in API.

[raphlinus]

I agree. Sorry for the misdirection - the panic is something I observed when I was experimenting with it, and I misinterpreted it.