How Can C 11's Atomic Operations Enable Lock-Free Queues Using ABA Counter Techniques?

Using the ABA counter technique, lock-free queues prevent deadlocks and improve concurrency by tracking object versions. In this technique, each object is assigned a counter that increments with every version change. However, implementing this counter using C 11 CAS presents a challenge as CAS only supports atomic comparisons and updates of a single value.

The solution lies in atomically modifying multiple values simultaneously. By placing the counter and the next pointer in adjacent memory, you can use std::atomic to generate a lock-free cmpxchg16b on x86-64. This operation atomically updates both values, preserving the ABA counter semantics.

Despite the lack of explicit inline assembly, this approach ensures correctness and avoids the use of slower library function calls. To enhance performance further, it's recommended to use a union to separate atomic operations on the pointer from those on the counter. This trick leverages the compiler's optimization capabilities to generate efficient code for reading the pointer alone.

To ensure efficiency and correctness, verify the following:

• Compilers generate efficient code for accessing only one union member.
• Union type-punning is supported or allowed (GNU dialect of C ).
• The object is aligned (16B for 64-bit, 8B for 32-bit).
• -mcx16 is used for x86-64 architectures to enable cmpxchg16b.
• Pointer uintptr_t atomic objects are lock-free (often guaranteed for x32 and 32-bit ABIs, but not for 16B objects).

The above is the detailed content of How Can C 11's Atomic Operations Enable Lock-Free Queues Using ABA Counter Techniques?. For more information, please follow other related articles on the PHP Chinese website!