Taking a lock only needs to be an acquire operation and a compiler barrier for other lock operations. Using seq_cst or acq_rel semantics is stronger than needed. From my reading and discussions with people from WG21 the current argument for why taking a lock only requires acq semantics is that a compiler optimization that transforms a non-deadlocking program into a potentially deadlocking program is not allowed. There's an interesting twitter thread where we discuss this I can't find anymore :(.
Taking a lock only needs to be an acquire operation and a compiler barrier for other lock operations. Using seq_cst or acq_rel semantics is stronger than needed. From my reading and discussions with people from WG21 the current argument for why taking a lock only requires acq semantics is that a compiler optimization that transforms a non-deadlocking program into a potentially deadlocking program is not allowed. There's an interesting twitter thread where we discuss this I can't find anymore :(.