Non-intrusive MPSC node-based queue


+ Waitfree and fast producers. One XCHG is maximum what one can get with multi-producer non-distributed queue.

+ Extremely fast consumer. On fast-path it's atomic-free, XCHG executed per node batch, in order to grab 'last item'.

+ No need for node order reversion. So pop operation is always O(1).

+ ABA-free.

+ No need for PDR. That is, one can use this algorithm out-of-the-box. No need for thread registration/deregistration, periodic activity, deferred garbage etc.


- Push function is blocking wrt consumer. I.e. if producer blocked in (*), then consumer is blocked too. Fortunately 'window of inconsistency' is extremely small - producer must be blocked exactly in (*). Actually it's disadvantage only as compared with totally lockfree algorithm. It's still much better lock-based algorithm.

- The algorithm is not linearizable.

struct mpscq_node_t


mpscq_node_t* volatile next;

void* state;


struct mpscq_t


mpscq_node_t* volatile head;

mpscq_node_t* tail;


void mpscq_create(mpscq_t* self, mpscq_node_t* stub)


stub->next = 0;

self->head = stub;

self->tail = stub;


void mpscq_push(mpscq_t* self, mpscq_node_t* n)


n->next = 0;

mpscq_node_t* prev = XCHG(&self->head, n); // serialization-point wrt producers, acquire-release

prev->next = n; // serialization-point wrt consumer, release


mpscq_node_t* mpscq_pop(mpscq_t* self)


mpscq_node_t* tail = self->tail;

mpscq_node_t* next = tail->next; // serialization-point wrt producers, acquire

if (next)


self->tail = next;

tail->state = next->state;

return tail;


return 0;