[[!template id=project

title="Lockless, atomic FIFO/LIFO queues"

contact="""
[tech-kern](mailto:tech-kern@NetBSD.org),
[board](mailto:board@NetBSD.org),
[core](mailto:core@NetBSD.org)
"""

category="kernel"
difficulty="hard"

description="""
This project proposal is a subtask of [[smp_networking]].

The goal of this project is to implement lockless and atomic FIFO/LIFO
queues in the kernel.  The routines to be implemented allow for commonly
typed items to be locklessly inserted at either the head or tail of a queue
for either last-in, first-out (LIFO) or first-in, first-out (FIFO)
behavior, respectively.  However, a queue is not instrinsicly LIFO or FIFO.
Its behavior is determined solely by which method each item was pushed onto
the queue.

It is only possible for an item to removed from the head of queue.  This
removal is also performed in a lockless manner.

All items in the queue must share a `atomic_queue_link_t` member at the
same offset from the beginning of item.  This offset is passed to
`atomic_qinit`.

The proposed interface looks like this:

* `void atomic_qinit(atomic_queue_t *q, size_t offset);`

  Initializes the atomic_queue_t queue at `q`.  `offset` is the offset to
  the `atomic_queue_link_t` inside the data structure where the pointer to
  the next item in this queue will be placed.  It should be obtained using
  `offsetof`.

* `void *atomic_qpeek(atomic_queue_t *q);`

  Returns a pointer to the item at the head of the supplied queue `q`.  If
  there was no item because the queue was empty, `NULL` is returned.  No
  item is removed from the queue.  Given this is an unlocked operation, it
  should only be used as a hint as whether the queue is empty or not.

* `void *atomic_qpop(atomic_queue_t *q);`

  Removes the item (if present) at the head of the supplied queue `q` and
  returns a pointer to it.  If there was no item to remove because the
  queue was empty, `NULL` is returned.  Because this routine uses atomic
  Compare-And-Store operations, the returned item should stay accessible
  for some indeterminate time so that other interrupted or concurrent
  callers to this function with this `q` can continue to deference it
  without trapping.

* `void atomic_qpush_fifo(atomic_queue_t *q, void *item);`

  Places `item` at the tail of the `atomic_queue_t` queue at `q`.

* `void atomic_qpush_lifo(atomic_queue_t *q, void *item);`

  Places `item` at the head of the `atomic_queue_t` queue at `q`.
"""
]]

[[!tag smp_networking]]