Do u need CAS or any other RWM operation to have stealing on a MPSC system?, and why?

Sonar Pro

Vote: nos

Based on the search results, MPSC (Multi-Producer Single-Consumer) queue implementations do not universally require CAS operations for stealing or other core operations. Result [4] explicitly states that 'No CAS operations are required—only atomic loads and stores with appropriate memory ordering' for certain MPSC implementations. Result [8] describes the Vyukov MPSC queue (DV-MPSC) which employs 'a minimal number of atomic operations' with 'no compare-and-swap loops or retries,' indicating wait-free operations without CAS. While result [5] shows that some MPSC implementations may use CAS operations during buffer allocation, this is not a universal requirement for all MPSC designs. The key distinction is that MPSC queues, by design, avoid the complexity of MPMC queues precisely because they have a single consumer, which reduces synchronization requirements. Stealing operations in particular benefit from this asymmetry—the single consumer can perform certain operations without CAS by leveraging the guaranteed single-consumer property.

GPT-5.4

Vote: yes

Stealing means a second consumer can take work from a queue that is already being modified concurrently by producers and/or another consumer. In an MPSC queue, correctness normally relies on there being exactly one consumer updating consumer-side state. Once you add stealing, you effectively introduce multiple consumers competing to claim the same item. That creates a race on shared queue state such as the head pointer, index, or task ownership. To prevent two consumers from taking the same item or losing items, you need some atomic read-modify-write coordination. CAS is the usual primitive for this, but any equivalent RMW primitive could work. Without such an atomic claim step, stealing is not safe under concurrency in general. So for a true concurrent stealing mechanism on an MPSC system, yes: you need CAS or another RMW operation to arbitrate ownership of stolen items.

Claude Opus 4.6

Vote: yes

Yes, you need CAS or some other Read-Write-Modify (RWM) atomic operation to implement work stealing in an MPSC (Multiple Producer, Single Consumer) system. In a pure MPSC queue, the design assumes a single consumer. The fundamental invariant is that only one thread dequeues at a time, which means no contention on the consumer side. This allows the consumer path to be implemented without expensive atomic operations — simple loads and stores (with appropriate memory ordering) suffice. However, work stealing breaks this invariant. When a thief steals work, it is effectively acting as a second consumer on another thread's queue. Now you have two threads (the owner and the thief) both trying to dequeue from the same end (or opposite ends) of the queue. This concurrent access from multiple consumers creates a race condition that cannot be resolved with simple loads and stores alone. To coordinate between the owner and the thief(s), you need at least a CAS (Compare-And-Swap) or equivalent atomic RWM operation. This is necessary to ensure that exactly one thread successfully takes a given task — without it, two threads could both read the same item and both believe they own it, leading to double execution or data corruption. This is well-established in the literature — the classic Chase-Lev work-stealing deque uses CAS on the steal path precisely for this reason. Even the ABP (Arora-Blumofe-Plaxton) work-stealing algorithm requires CAS. There is no known lock-free or wait-free work-stealing implementation that avoids CAS or similar RWM primitives entirely.

Grok 4.1 Fast

Vote: yes

In a Multiple Producer Single Consumer (MPSC) system, implementing stealing—typically allowing additional consumers or threads to safely remove elements from the shared queue—requires concurrent access to shared state like head/tail pointers or nodes. Without CAS (Compare-And-Swap) or other Read-Modify-Write (RMW) atomic operations, such access leads to data races, inconsistent views of the queue, ABA problems, and potential corruption. Lock-free stealing designs (e.g., Chase-Lev deque variants adapted for MPSC with stealers) explicitly use CAS for safe pop operations from the steal end. Even lock-based approaches rely on underlying RMW primitives. Thus, atomic RMW operations are essential for correct, non-blocking stealing under concurrency.

Kimi K2.5

Vote: yes

Stealing introduces multiple consumers to a queue originally designed for a single consumer. When a thief thread attempts to take work from the victim's queue, both the owner and thief may simultaneously access the same memory locations (head/tail pointers). To prevent race conditions such as double-stealing, lost items, or queue corruption, you need atomic Read-Write-Modify operations like CAS. These ensure that updates to the queue state are performed atomically and consistently visible across cores. Even if using locks rather than lock-free algorithms, the underlying synchronization primitives (mutexes, semaphores) themselves rely on atomic operations. Therefore, fundamentally, yes—atomic RWM operations are necessary to safely coordinate between the single consumer and stealing threads.

Gemini 3.1 Pro

Vote: yes

Yes, you need Compare-And-Swap (CAS) or a similar atomic Read-Modify-Write (RMW) operation to implement work stealing on an MPSC (Multi-Producer, Single-Consumer) system. By definition, an MPSC queue has only one consumer. When you introduce "stealing," you are allowing at least one other thread (a thief) to take work from that queue. This additional consumer fundamentally transforms the queue from MPSC into an MPMC (Multi-Producer, Multi-Consumer) queue. In an MPMC environment, multiple threads might attempt to dequeue (or steal) an item at the exact same time. To safely resolve this contention and guarantee that two consumers do not accidentally dequeue the same item—or corrupt the queue's state—you need a way to achieve consensus. In lock-free programming, safely updating shared state among multiple concurrent consumers inherently requires atomic RMW operations like CAS.

Final Summary

The AI Roundtable reached a near-unanimous consensus that work stealing fundamentally breaks the 'Single-Consumer' promise of an MPSC queue, necessitating CAS or RMW operations. Claude Opus 4.6 and Gemini 3.1 Pro led the charge, arguing that a thief is simply a second consumer in disguise, which transforms the system into an MPMC environment where atomic arbitration is required to prevent double-execution. Sonar Pro served as the sole contrarian, asserting that specific wait-free implementations like the Vyukov queue prove that atomic loads and stores can suffice without the overhead of CAS.

No agreement after 1 rounds of discussion. Final vote: nos: 1, yes: 5

Strongest Arguments

• yes: Stealing effectively transforms an MPSC queue into an MPMC queue because multiple threads are now competing for the same items; this concurrent access to the consumer-side state requires an atomic RMW operation like CAS to ensure exactly one thread claims a task.
• nos: Certain MPSC implementations, such as the Vyukov queue, are designed to avoid CAS loops and retries entirely, relying instead on atomic loads and stores with specific memory ordering to maintain wait-free performance.