Is liblfds' Circular Buffer Queue Truly Lock-Free and Does it Guarantee Progress for All Threads?

Lock-Free Progress Guarantees in a Circular Buffer Queue

The concept of lock-free algorithms ensures the ability of at least one thread to make continuous progress, regardless of the actions of other threads. However, this definition sometimes faces ambiguity, especially in the context of concurrency libraries such as liblfds.

Liblfds employs custom atomics and memory barriers for its bounded queue implementation. Although the algorithm may appear efficient, its lock-free nature remains questionable.

Enforcing Progress:

The PUSH algorithm reserves a slot in the queue for user data. However, until the sequence_number is updated, the slot remains inaccessible for POP operations. This dependence on successful PUSH completion creates a situation where other threads can be blocked or delayed, indicating a possible lack of progress guarantees.

Evaluating the Algorithm:

The algorithm does not strictly meet the definition of lock-free as proposed by the author. The combination of m_write_index and s.sequence_number acts as a per-element mutex, leading to potential failures in the presence of a suspended thread that has reserved a slot.

Assessing Performance and Functional Aspects:

Performance:

Uncontended performance is satisfactory due to minimal atomic operations. Contended performance is also reasonable, although the m_write_index can be a source of contention when multiple readers attempt to access the queue.

Immunity to Context Switch:

Partial immunity is provided, as other threads can still push elements onto the queue even if a thread is context-switched during the critical region. However, popping elements can stall if the in-progress element is affected.

Functional Limitations:

The algorithm is not safe for asynchronous thread termination or for access from interrupt or signal handlers. It may not fully drain all elements if a thread is interrupted during the critical region.

Conclusion:

While the liblfds queue implementation may offer some performance benefits, its lock-free nature is questionable due to the dependence on successful PUSH completion. It does not fully satisfy the strict definition of progress guarantees, and certain edge cases can lead to progress blocking or even failure.

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