Locked vs. Lock-Free Atomic Variables: Where\'s the Lock and What are the Implications?

Lock-Free Atomic Variables vs. Locked Atomic Variables

In the world of multithreaded programming, atomic variables play a crucial role in ensuring data integrity. However, not all atomic variables are created equal. While some are lock-free, guaranteeing concurrent access without the need for synchronization primitives, others rely on locks.

Where is the Lock for Locked Atomic Variables?

When an atomic variable is not lock-free, it must employ a lock to enforce exclusive access to its data. This lock is typically stored as a separate data structure, such as a hash table of mutexes. The key for the hash table is usually the address of the atomic variable, enabling efficient lookup when multiple instances of the variable coexist.

Implications of Locks in Atomic Variables with Multiple Instances

When multiple instances of a locked atomic variable exist, the shared lock will serialize access to the variable's data. This means that if one thread holds the lock to modify the variable, all other threads attempting to access it will be blocked until the lock is released.

Collision Resolution

Hash collisions can occur when multiple atomic variables map to the same hash bucket in the lock table. In such cases, the contending threads will compete for the same lock, potentially leading to increased contention and reduced performance. However, this is usually not a significant issue, as collisions are relatively rare and performance is generally acceptable.

Deadlock Avoidance

Atomic variables with locks do not suffer from the risk of deadlocks because the std::atomic API ensures that no operation attempts to take locks on multiple variables concurrently. This design prevents cyclic dependencies and guarantees correct concurrent behavior.

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