The POSIX standard requires a full memory barrier for the underlying operations. The problem is that synchronization is expensive: not only because of the memor

The POSIX standard requires a full memory barrier for the underlying operations. The problem is that synchronization is expensive: not only because of the memory barriers, which have to include the most expensive ones, but also because locking and unlocking operations need to access main memory directly, bypassing all caches. The problem is, in any event, not the synchronization, but the common path -- for which double-checked locking tries to avoid synchronization --, which needs at a minimum a load-load barrier. That can be done efficiently on TSO architectures (where a load-load barrier is a no-op), but is not guaranteed to be fast on other architectures.