Can Atomic Variables in C 11 Prevent Stale Reads in Multithreaded Environments?

Concurrency: Atomic and Volatile in C 11 Memory Model

Question:

In a multi-threaded environment with shared global variables, can one thread read a stale value from an atomic variable? How do atomic and volatile types differ in ensuring data integrity?

Answer:

Volatile vs. Atomic

Volatile does not guarantee atomic access. Its primary purpose is for memory-mapped I/O and signal handling. Using volatile with std::atomic is redundant.

Memory Ordering with Atomic Variables

The visibility of atomic variables depends on the memory ordering parameter used.

• std::memory_order_seq_cst: Provides a global order for all operations across all variables, ensuring that values are visible within "a reasonable period of time." However, stale values may still be returned.
• std::memory_order_relaxed: Relaxed memory ordering, where threads may not agree on the order of operations, leading to inconsistent results.

Ensuring Data Integrity

To ensure data integrity, read-modify-write (RMW) operations like exchange() and fetch_add() should be used. These operations always operate on the "latest" value, eliminating the risk of stale values.

Additional Considerations

• RMW operations do not force earlier changes to be visible quicker.
• Performance can vary depending on CPU store buffering, physical CPU distance, and cache coherency protocols.
• Working with atomic operations is complex, requiring careful code design and understanding of background material. In simpler cases, locks may be a more practical option.

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