Is the Liblfds Bounded Queue Truly Lock-Free?

Lock-Free Progress Guarantees in a Circular Buffer Queue

Overview:

Many programmers mistakenly believe that "lock-free" simply refers to concurrent programming without mutexes. However, true lock-free algorithms provide progress guarantees, ensuring that at least one thread can make forward progress regardless of other threads' actions.

The Liblfds Bounded Queue:

The implementation of a bounded queue in the liblfds library appears questionable in terms of lock-free progress guarantees. The algorithm reserves slots for pushing, meaning slots cannot be popped until the push thread completes. This dependency raises concerns about whether the queue truly qualifies as lock-free.

Definition of Lock-Free:

A lock-free structure is one where any thread can be indefinitely suspended at any point without blocking other threads from using the structure. By this definition, the liblfds queue is not strictly lock-free. If a pushing thread is suspended, the queue enters an unusable state, violating the contract of a fixed-size queue.

Performance vs. Correctness:

While this implementation may exhibit reasonable performance properties, it lacks some critical correctness properties of a truly lock-free structure. Key functional shortcomings include:

1. Asynchronous thread termination: The queue is not safe for use by threads that may terminate exceptionally in the critical region, leaving the structure in an inconsistent state.
2. Queue access from interrupts or signals: The queue supports basic mutation and access from interrupts, but its behavior is limited compared to true lock-free structures.

Conclusion:

The liblfds bounded queue implementation falls short of the strictest definition of lock-free. It provides some performance advantages but lacks certain functional guarantees that are crucial for ensuring progress and correctness in all scenarios.

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