c++ - Lock-free Progress Guarantees

Question

Anecdotally, I've found that a lot of programmers mistakenly believe that "lock-free" simply means "concurrent programming without mutexes". Usually, there's also a correlated misunderstanding that the purpose of writing lock-free code is for better concurrent performance. Of course, the correct definition of lock-free is actually about progress guarantees. A lock-free algorithm guarantees that at least one thread is able to make forward progress regardless of what any other threads are doing.

This means a lock-free algorithm can never have code where one thread is depending on another thread in order to proceed. E.g., lock-free code can not have a situation where Thread A sets a flag, and then Thread B keeps looping while waiting for Thread A to unset the flag. Code like that is basically implementing a lock (or what I would call a mutex in disguise).

However, other cases are more subtle and there are some cases where I honestly can't really tell if an algorithm qualifies as lock-free or not, because the notion of "making progress" sometimes appears subjective to me.

One such case is in the (well-regarded, afaik) concurrency library, liblfds. I was studying the implementation of a multi-producer/multi-consumer bounded queue in liblfds - the implementation is very straightforward, but I cannot really tell if it should qualify as lock-free.

The relevant algorithm is in lfds711_queue_bmm_enqueue.c. Liblfds uses custom atomics and memory barriers, but the algorithm is simple enough for me to describe in a paragraph or so.

The queue itself is a bounded contiguous array (ringbuffer). There is a shared read_index and write_index. Each slot in the queue contains a field for user-data, and a sequence_number value, which is basically like an epoch counter. (This avoids ABA issues).

The PUSH algorithm is as follows:

1. Atomically LOAD the write_index
2. Attempt to reserve a slot in the queue at write_index % queue_size using a CompareAndSwap loop that attempts to set write_index to write_index + 1.
3. If the CompareAndSwap is successful, copy the user data into the reserved slot.
4. Finally, update the sequence_index on the slot by making it equal to write_index + 1.

The actual source code uses custom atomics and memory barriers, so for further clarity about this algorithm I've briefly translated it into (untested) standard C++ atomics for better readability, as follows:

bool mcmp_queue::enqueue(void* data)
{
    int write_index = m_write_index.load(std::memory_order_relaxed);

    for (;;)
    {
        slot& s = m_slots[write_index % m_num_slots];
        int sequence_number = s.sequence_number.load(std::memory_order_acquire);
        int difference = sequence_number - write_index;

        if (difference == 0)
        {
            if (m_write_index.compare_exchange_weak(
                write_index,
                write_index + 1,
                std::memory_order_acq_rel
            ))
            {
                break;
            }
        }

        if (difference < 0) return false; // queue is full
    }

    // Copy user-data and update sequence number
    //
    s.user_data = data;
    s.sequence_number.store(write_index + 1, std::memory_order_release);
    return true;
}

Now, a thread that wants to POP an element from the slot at read_index will not be able to do so until it observes that the slot's sequence_number is equal to read_index + 1.

Okay, so there are no mutexes here, and the algorithm likely performs well (it's only a single CAS for PUSH and POP), but is this lock-free? The reason it's unclear to me is because the definition of "making progress" seems murky when there is the possibility that a PUSH or POP can always just fail if the queue is observed to be full or empty.

But what's questionable to me is that the PUSH algorithm essentially reserves a slot, meaning that the slot can never be POP'd until the push thread gets around to updating the sequence number. This means that a POP thread that wants to pop a value depends on the PUSH thread having completed the operation. Otherwise, the POP thread will always return false because it thinks the queue is EMPTY. It seems debatable to me whether this actually falls within the definition of "making progress".

Generally, truly lock-free algorithms involve a phase where a pre-empted thread actually tries to ASSIST the other thread in completing an operation. So, in order to be truly lock-free, I would think that a POP thread that observes an in-progress PUSH would actually need to try and complete the PUSH, and then only after that, perform the original POP operation. If the POP thread simply returns that the queue is EMPTY when a PUSH is in progress, the POP thread is basically blocked until the PUSH thread completes the operation. If the PUSH thread dies, or goes to sleep for 1,000 years, or otherwise gets scheduled into oblivion, the POP thread can do nothing except continuously report that the queue is EMPTY.

So does this fit the defintion of lock-free? From one perspective, you can argue that the POP thread can always make progress, because it can always report that the queue is EMPTY (which is at least some form of progress I guess.) But to me, this isn't really making progress, since the only reason the queue is observed as empty is because we are blocked by a concurrent PUSH operation.

So, my question is: is this algorithm truly lock-free? Or is the index reservation system basically a mutex in disguise?

Answer 1

This queue data structure is not strictly lock-free by what I consider the most reasonable definition. That definition is something like:

A structure is lock-free if only if any thread can be indefinitely suspended at any point while still leaving the structure usable by the remaining threads.

Of course this implies a suitable definition of usable, but for most structures this is fairly simple: the structure should continue to obey its contracts and allow elements to be inserted and removed as expected.

In this case a thread that has succeeded in incrementing m_write_increment, but hasn't yet written s.sequence_number leaves the container in what will soon be an unusable state. If such a thread is killed, the container will eventually report both "full" and "empty" to push and pop respectively, violating the contract of a fixed size queue.

There is a hidden mutex here (the combination of m_write_index and the associated s.sequence_number) - but it basically works like a per-element mutex. So the failure only becomes apparent to writers once you've looped around and a new writer tries to get the mutex, but in fact all subsequent writers have effectively failed to insert their element into the queue since no reader will ever see it.

Now this doesn't mean this is a bad implementation of a concurrent queue. For some uses it may behave mostly as if it was lock free. For example, this structure may have most of the useful performance properties of a truly lock-free structure, but at the same time it lacks some of the useful correctness properties. Basically the term lock-free usually implies a whole bunch of properties, only a subset of which will usually be important for any particular use. Let's look at them one by one and see how this structure does. We'll broadly categorize them into performance and functional categories.

Performance

Uncontended Performance

The uncontended or "best case" performance is important for many structures. While you need a concurrent structure for correctness, you'll usually still try to design your application so that contention is kept to a minimum, so the uncontended cost is often important. Some lock-free structures help here, by reducing the number of expensive atomic operations in the uncontended fast-path, or avoiding a syscall.

This queue implementation does a reasonable job here: there is only a single "definitely expensive" operation: the compare_exchange_weak, and a couple of possibly expensive operations (the memory_order_acquire load and memory_order_release store)¹, and little other overhead.

This compares to something like std::mutex which would imply something like one atomic operation for lock and another for unlock, and in practice on Linux the pthread calls have non-negligible overhead as well.

So I expect this queue to perform reasonably well in the uncontended fast-path.

Contended Performance

One advantage of lock-free structures is that they often allow better scaling when a structure is heavily contended. This isn't necessarily an inherent advantage: some lock-based structures with multiple locks or read-write locks may exhibit scaling that matches or exceeds some lock-free approaches, but it is usually that case that lock-free structures exhibit better scaling that a simple one-lock-to-rule-them-all alternative.

This queue performs reasonably in this respect. The m_write_index variable is atomically updated by all readers and will be a point of contention, but the behavior should be reasonable as long as the underlying hardware CAS implementation is reasonable.

Note that a queue is generally a fairly poor concurrent structure since inserts and removals all happen at the same places (the head and the tail), so contention is inherent in the definition of the structure. Compare this to a concurrent map, where different elements have no particular ordered relationship: such a structure can offer efficient contention-free simultaneous mutation if different elements are being accessed.

Context-switch Immunity

One performance advantage of lock-free structures that is related to the core definition above (and also to the functional guarantees) is that a context switch of a thread which is mutating the structure doesn't delay all the other mutators. In a heavily loaded system (especially when runnable threads >> available cores), a thread may be switched out for hundreds of milliseconds or seconds. During this time, any concurrent mutators will block and incur additional scheduling costs (or they will spin which may also produce poor behavior). Even though such "unluckly scheduling" may be rare, when it does occur the entire system may incur a serious latency spike.

Lock-free structures avoid this since there is no "critical region" where a thread can be context switched out and subsequently block forward progress by other threads.

This structure offers partial protection in this area — the specifics of which depend on the queue size and application behavior. Even if a thread is switched out in the critical region between the m_write_index update and the sequence number write, other threads can continue to push elements to the queue as long as they don't wrap all the way around to the in-progress element from the stalled thread. Threads can also pop elements, but only up to the in-progress element.

While the push behavior may not be a problem for high-capacity queues, the pop behavior can be a problem: if the queue has a high throughput compared to the average time a thread is context switched out, and the average fullness, the queue will quickly appear empty to all consumer threads, even if there are many elements added beyond the in-progress element. This isn't affected by the queue capacity, but simply the application behavior. It means that the consumer side may completely stall when this occurs. In this respect, the queue doesn't look very lock-free at all!

Functional Aspects

Async Thread Termination

On advantage of lock-free structures it they are safe for use by threads that may be asynchronously canceled or may otherwise terminate exceptionally in the critical region. Cancelling a thread at any point leaves the structure is a consistent state.

This is not the case for this queue, as described above.

Queue Access from Interrupt or Signal

A related advantage is that lock-free structures can usually be examined or mutated from an interrupt or signal. This is useful in many cases where an interrupt or signal shares a structure with regular process threads.

This queue mostly supports this use case. Even if the signal or interrupt occurs when another thread is in the critical region, the asynchronous code can still push an element onto the queue (which will only be seen later by consuming threads) and can still pop an element off of the queue.

The behavior isn't as complete as a true lock-free structure: imagine a signal handler with a way to tell the remaining application threads (other than the interrupted one) to quiesce and which then drains all the remaining elements of the queue. With a true lock-free structure, this would allow the signal handler to full drain all the elements, but this queue might fail to do that in the case a thread was interrupted or switched out in the critical region.

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¹ In particular, on x86, this will only use an atomic operation for the CAS as the memory model is strong enough to avoid the need for atomics or fencing for the other operations. Recent ARM can do acquire and release fairly efficiently as well.