Do `std::atomic` Variables Use Locks, and If So, How?

The Location of Locks in std::atomic Variables

In the context of atomic variables, where data integrity is crucial, a popular misconception arises around the existence and storage of locks within such variables. Despite popular belief, atomic variables may utilize locks to maintain data consistency, especially when dealing with larger data structures that cannot be manipulated atomically by the CPU.

Lock Implementation

The common implementation for these locks is a hash table of mutexes or spinlocks, with the address of the atomic object serving as the key. This hash table resides within a shared memory space accessible to all threads operating on the atomic variable.

Lock Acquisition

When a thread attempts to modify the atomic variable, it first obtains the lock associated with the variable's address. This ensures that only one thread can access and modify the variable at a time, preventing data corruption or race conditions.

Collision Handling

It's important to note that hash collisions can occur, leading to situations where multiple atomic objects may share the same lock. While this is not a correctness issue, it can result in performance degradation as multiple threads contend for access to the shared lock.

Lock-Free Objects

In cases where the atomic variable is lock-free, no external lock is needed to maintain data consistency. This is typically the case for smaller data types that can be atomically manipulated by the CPU. The downside is that such implementations may be limited in their handling of larger, complex data structures.

Deadlock Prevention

It's worth emphasizing that std::atomic operations are specifically designed to prevent deadlocks. This is ensured by the fact that no std::atomic function attempts to lock more than one object at a time.

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