Why only std::atomic_flag is guaranteed to be lock-free?

Question

From C++ Concurrency in Action:

difference between std::atomic and std::atomic_flag is that std::atomic may not be lock-free; the implementation may have to acquire a mutex internally in order to ensure the atomicity of the operations

I wonder why. If atomic_flag is guaranteed to be lock-free, why isn't it guaranteed for atomic<bool> as well? Is this because of the member function compare_exchange_weak? I know that some machines lack a single compare-and-exchange instruction, is that the reason?

Answer 1

First of all, you are perfectly allowed to have something like std::atomic<very_nontrivial_large_structure>, so std::atomic as such cannot generally be guaranteed to be lock-free (although most specializations for trivial types like bool or int probably could, on most systems). But that is somewhat unrelated.

The exact reasoning why atomic_flag and nothing else must be lock-free is given in the Note in N2427/29.3:

Hence the operations must be address-free. No other type requires lock-free operations, and hence the atomic_flag type is the minimum hardware-implemented type needed to conform to this standard. The remaining types can be emulated with atomic_flag, though with less than ideal properties.

In other words, it's the minimum thing that must be guaranteed on every platform, so it's possible to implement the standard correctly.

Answer 2

The standard does not garantee atomic objects are lock-free. On a platform that doesn't provide lock-free atomic operations for a type T, std::atomic<T> objects may be implemented using a mutex, which wouldn't be lock-free. In that case, any containers using these objects in their implementation would not be lock-free either.

The standard provide an opportunity to check if an std::atomic<T> variable is lock-free: you can use var.is_lock_free() or atomic_is_lock_free(&var). For basic types such as int, there is also macros provided (e.g. ATOMIC_INT_LOCK_FREE) which specify if lock-free atomic access to that type is available.

std::atomic_flag is an atomic boolean type. Almost always for boolean type it's not needed to use mutex or another way for synchronization.