Is a Circular Buffer Queue Truly Lock-Free If PUSH Operations Can Block POP Operations?

Can a Queue be Lock-free if PUSH Operations Can Block POP Operations?

Anecdotally, "lock-free" is often mistakenly used to mean "concurrent programming without mutexes." Lock-free algorithms actually provide guarantees of progress regardless of other threads' actions. This means there should be no code where one thread depends on another to proceed.

Consider a circular buffer queue in liblfds, which aims for concurrency without explicit mutexes. The PUSH algorithm involves reserving a slot by comparing the write index and updating the sequence number. While efficient with a single CAS, it raises questions about lock-freeness.

On one hand, threads can always enqueue if slots are available. But on the other hand, if a PUSH operation is interrupted before updating the sequence number, subsequent POP operations will fail, making the queue appear empty.

Under the definition of lock-freeness as "a structure is usable if any thread is indefinitely suspended," this queue is not strictly lock-free. It has a hidden mutex mechanism (the write index and sequence number), where writers can fail to insert elements due to a suspended writer in the critical region.

However, the queue may still exhibit some useful properties. It has reasonable uncontended performance due to its low overhead, handles contended performance reasonably, and is partially context-switch immune. Additionally, it supports queue access from interrupts or signals, though it has limitations in handling asynchronous thread termination.

While the liblfds queue may not fully meet the strict definition of lock-freeness, it may still be beneficial for certain applications. It provides partial progress guarantees and decent performance characteristics, without the complexities of mutex-based solutions.

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