Can One Thread Read Stale Data from a C 11 Atomic Variable?

Concurrency: Understanding Atomic and Volatile in C 11 Memory Model

Consider a global variable accessed by multiple threads running concurrently on different cores. Each thread can both write to and read from the variable. Can one thread read outdated information from an atomic variable?

Volatile and Atomic: A Distinction

While atomic implies atomic access, volatile alone does not. Volatile finds use in situations like memory-mapped I/O and signal handling. It becomes redundant when paired with std::atomic. Furthermore, it bears no relevance to atomic access or inter-thread memory order.

Memory Ordering with Atomic Variables

The std::atomic library in C 11 offers an atomic integer type,

 std::atomic<int> ai; 

The visibility and sequencing limitations depend on the memory ordering provided for operations:

• Sequence Consistent Ordering (std::memory_order_seq_cst): Maintains a single total ordering for all operations across all variables. It doesn't guarantee that stale values will be absent, but it ensures the value obtained determines and is determined by the position of the operation in the ordering.
• Relaxed Ordering (std::memory_order_relaxed): Removes any ordering constraints, allowing for optimal speed but risking unpredictable execution order.

Read-Modify-Write (RMW) Operations: Guaranteeing Freshness

RMW operations, like exchange(), compare_exchange_strong(), and fetch_add(), offer a guarantee: they always act on the most recent value. In a series of calls to ai.fetch_add(1) from multiple threads, no duplicate or missing numbers will be returned. However, the threads may see the values at different points in time.

Caution and Conclusion

Understanding atomic operations is crucial. Thorough research and code review are recommended before using them in production code. Locks often provide a more accessible and nearly as efficient approach for concurrency.

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