Is liblfds' Multi-Producer/Multi-Consumer Bounded Queue Truly Lock-Free?

Lock-free Queue Algorithm Analysis

Question: Is the multi-producer/multi-consumer bounded queue algorithm in liblfds lock-free?

Definition of Lock-free:

A lock-free algorithm ensures that at least one thread can make forward progress regardless of any concurrent threads. This means that it cannot have code where a thread relies on another to proceed, such as waiting for a flag to be reset or unset.

Algorithm Analysis:

The algorithm reserves a slot in the queue using a CAS loop to increment the write index. It then copies user data into the reserved slot and updates the sequence number. However, this reservation means that the POP operation depends on the PUSH thread completing the sequence number update.

Lack of Progress Guarantee:

According to the definition of "making progress," the algorithm does not meet the criteria of lock-free. The queue can be observed as full or empty even though PUSH or POP operations are ongoing, preventing other threads from performing those operations.

Partially Blocked Progress:

While the algorithm may allow POP operations to continue up to the in-progress element, this progress is limited. If a thread is context-switched out during the critical region between write index update and sequence number write, all consumer threads will report an empty queue.

Hidden Mutex:

The combination of the write index and slot sequence numbers essentially acts as a per-element mutex. Once a thread successfully increments the write index, all subsequent threads are prevented from writing to the queue until the original thread completes the operation.

Performance Advantages:

Despite not being strictly lock-free, the algorithm offers performance advantages in terms of:

• Uncontended performance: The fast-path consists of a single CompareAndSwap operation.
• Contended performance: The write index variable is contended, but the behavior is reasonable with a well-optimized CAS implementation.

Conclusion:

While the algorithm provides some useful performance properties, it lacks the key correctness property of lock-free operation due to the reservation system and the dependency between PUSH and POP operations.

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