Do Atomic Variables for Complex Data Structures Really Use Locks, and If So, How?

Atomic Variables and Locks

In the realm of multithreaded programming, atomic variables play a crucial role in ensuring consistent data manipulation. However, when it comes to complex data structures like foo with multiple elements, concerns arise regarding the presence of locks within atomic variables.

The Puzzle of Atomic Variables and Locks

Despite the presumptions that larger atomic types necessitate locks, observations suggest otherwise. The following code snippet demonstrates this phenomenon:

#include <iostream>
#include <atomic>

struct foo {
    double a;
    double b;
};

std::atomic<foo> var;

int main()
{
    std::cout << var.is_lock_free() << std::endl;
    std::cout << sizeof(foo) << std::endl;
    std::cout << sizeof(var) << std::endl;
}

The output of this code is:

0
16
16

As you can see, the is_lock_free() method returns 0 for the atomic variable var, yet its size remains identical to that of its underlying data structure foo. This has led to questions: Where is the lock stored, and how does it impact multiple instances of the atomic variable?

Unveiling the Lock Mechanism

The common implementation of locks within atomic variables involves a hash table of mutexes. The address of the atomic object serves as a key, assigning it to a unique lock. This hash function ensures that multiple atomic variables map to distinct locks, effectively isolating their access operations.

Potential Implications and Performance Considerations

Collisions within the hash table can result in multiple atomic objects sharing the same lock. While this does not compromise correctness, it can create performance bottlenecks. Instead of independent contention between different objects, multiple threads may compete for access to the shared lock.

Absence of Deadlocks

It's important to note that deadlocks are not possible in this context because std::atomic operations never attempt to acquire locks on multiple objects simultaneously. This design ensures that extra contention does not impact correctness but may affect performance.

Conclusion

Atomic variables employ a lock mechanism for complex data structures to maintain data integrity. These locks are typically implemented as a hash table of mutexes, with the atomic variable's address serving as the key. While shared locks may result in performance issues, deadlocks are prevented by the design of std::atomic functions.

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