How Can We Implement an ABA Counter in C 11 Using Only CAS?

Implementing an ABA Counter with C 11 CAS

In certain concurrent programming scenarios, such as lock-free queues, counters play a crucial role in addressing the ABA problem. This problem arises when a value is updated multiple times, causing the counter to revert to its original value, potentially leading to unforeseen behaviors.

Challenge with C 11 CAS

The C 11 concurrency library provides the std::atomic_compare_exchange_weak function for performing compare-and-swap operations. However, this function lacks the capability to simultaneously update multiple values atomically, which is required for implementing an ABA counter.

Solution: Union Trick

To address this limitation, we can employ a trick involving a union to combine two values into a single atomic object. By storing the counter value in one member and a pointer to the next node in the second member, we can achieve both atomicity and concurrent access to these values.

Atomic Operations with Union

With this union-based approach, we can perform atomic operations on the combined object using std::atomic, which generates efficient assembly instructions like cmpxchg16b on x86-64 architectures. This instruction allows us to update both the next pointer and the count value in a single atomic operation.

Avoiding Contention

A key aspect of our solution is utilizing a separate union member for read-only access to the next pointer. This optimization ensures that we avoid the overhead of a locked cmpxchg16b instruction when we only need to retrieve the pointer without updating it.

Challenges

While this approach offers an efficient way to implement an ABA counter using C 11 CAS, it requires careful consideration of alignment and compiler support. Additionally, it relies on union type-punning, which is guaranteed to work in GNU C , but may not be fully supported by all ISO C compilers.

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