Element ordering handling in a ring buffer

A data processing apparatus comprising:

• ring buffer storage circuitry to store components of a ring buffer comprising multiple slots to hold queued data items;
• and
• data processing circuitry to perform an enqueuing operation to add one or more processed data item indications to the ring buffer and to perform a dequeuing operation to remove one or more processed data item indications from the ring buffer,
• wherein the enqueuing operation comprises performing an atomic compare-and-swap operation to store a first processed data item indication to an enqueuing-target slot in the ring buffer contingent on an in-order marker not being present in the enqueuing-target slot and, when the in-order marker is present in the enqueuing-target slot, determining a ready-to-dequeue condition to be true for the first processed data item indication, and
• wherein the dequeuing operation comprises, when the ready-to-dequeue condition for a dequeuing-target slot is true:
• writing a null data item to the dequeuing-target slot;
• and
• when removing the one or more processed data item indications from the ring buffer in-order, the dequeuing operation further comprises:
• a) dependent on whether a next contiguous slot has null content, determining a retirement condition and, when the retirement condition is true, performing a retirement process on the next contiguous slot comprising making the next contiguous slot available to a subsequent enqueuing operation;
• b) repeating step a) through subsequent slots until for a reached slot the retirement condition is not true;
• c) performing an atomic compare-and-swap operation to store the in-order marker to the reached slot contingent on content of the reached slot;
• and
• d) when the atomic compare-and-swap operation to store the in-order marker to the reached slot fails, performing the retirement process on the reached slot and returning to step a), wherein the next contiguous slot is now treated as a slot which contiguously follows the reached slot.