// SPDX-License-Identifier: GPL-2.0-only
/*
 * Copyright 2023 Red Hat
 */

#include "funnel-queue.h"

#include "cpu.h"
#include "memory-alloc.h"
#include "permassert.h"

int vdo_make_funnel_queue(struct funnel_queue **queue_ptr)
{
        int result;
        struct funnel_queue *queue;

        result = vdo_allocate(1, struct funnel_queue, "funnel queue", &queue);
        if (result != VDO_SUCCESS)
                return result;

        /*
         * Initialize the stub entry and put it in the queue, establishing the invariant that
         * queue->newest and queue->oldest are never null.
         */
        queue->stub.next = NULL;
        queue->newest = &queue->stub;
        queue->oldest = &queue->stub;

        *queue_ptr = queue;
        return VDO_SUCCESS;
}

void vdo_free_funnel_queue(struct funnel_queue *queue)
{
        vdo_free(queue);
}

static struct funnel_queue_entry *get_oldest(struct funnel_queue *queue)
{
        /*
         * Barrier requirements: We need a read barrier between reading a "next" field pointer
         * value and reading anything it points to. There's an accompanying barrier in
         * vdo_funnel_queue_put() between its caller setting up the entry and making it visible.
         */
        struct funnel_queue_entry *oldest = queue->oldest;
        struct funnel_queue_entry *next = READ_ONCE(oldest->next);

        if (oldest == &queue->stub) {
                /*
                 * When the oldest entry is the stub and it has no successor, the queue is
                 * logically empty.
                 */
                if (next == NULL)
                        return NULL;
                /*
                 * The stub entry has a successor, so the stub can be dequeued and ignored without
                 * breaking the queue invariants.
                 */
                oldest = next;
                queue->oldest = oldest;
                next = READ_ONCE(oldest->next);
        }

        /*
         * We have a non-stub candidate to dequeue. If it lacks a successor, we'll need to put the
         * stub entry back on the queue first.
         */
        if (next == NULL) {
                struct funnel_queue_entry *newest = READ_ONCE(queue->newest);

                if (oldest != newest) {
                        /*
                         * Another thread has already swung queue->newest atomically, but not yet
                         * assigned previous->next. The queue is really still empty.
                         */
                        return NULL;
                }

                /*
                 * Put the stub entry back on the queue, ensuring a successor will eventually be
                 * seen.
                 */
                vdo_funnel_queue_put(queue, &queue->stub);

                /* Check again for a successor. */
                next = READ_ONCE(oldest->next);
                if (next == NULL) {
                        /*
                         * We lost a race with a producer who swapped queue->newest before we did,
                         * but who hasn't yet updated previous->next. Try again later.
                         */
                        return NULL;
                }
        }

        return oldest;
}

/*
 * Poll a queue, removing the oldest entry if the queue is not empty. This function must only be
 * called from a single consumer thread.
 */
struct funnel_queue_entry *vdo_funnel_queue_poll(struct funnel_queue *queue)
{
        struct funnel_queue_entry *oldest = get_oldest(queue);

        if (oldest == NULL)
                return oldest;

        /*
         * Dequeue the oldest entry and return it. Only one consumer thread may call this function,
         * so no locking, atomic operations, or fences are needed; queue->oldest is owned by the
         * consumer and oldest->next is never used by a producer thread after it is swung from NULL
         * to non-NULL.
         */
        queue->oldest = READ_ONCE(oldest->next);
        /*
         * Make sure the caller sees the proper stored data for this entry. Since we've already
         * fetched the entry pointer we stored in "queue->oldest", this also ensures that on entry
         * to the next call we'll properly see the dependent data.
         */
        smp_rmb();
        /*
         * If "oldest" is a very light-weight work item, we'll be looking for the next one very
         * soon, so prefetch it now.
         */
        uds_prefetch_address(queue->oldest, true);
        WRITE_ONCE(oldest->next, NULL);
        return oldest;
}

/*
 * Check whether the funnel queue is empty or not. If the queue is in a transition state with one
 * or more entries being added such that the list view is incomplete, this function will report the
 * queue as empty.
 */
bool vdo_is_funnel_queue_empty(struct funnel_queue *queue)
{
        return get_oldest(queue) == NULL;
}

/*
 * Check whether the funnel queue is idle or not. If the queue has entries available to be
 * retrieved, it is not idle. If the queue is in a transition state with one or more entries being
 * added such that the list view is incomplete, it may not be possible to retrieve an entry with
 * the vdo_funnel_queue_poll() function, but the queue will not be considered idle.
 */
bool vdo_is_funnel_queue_idle(struct funnel_queue *queue)
{
        /*
         * Oldest is not the stub, so there's another entry, though if next is NULL we can't
         * retrieve it yet.
         */
        if (queue->oldest != &queue->stub)
                return false;

        /*
         * Oldest is the stub, but newest has been updated by _put(); either there's another,
         * retrievable entry in the list, or the list is officially empty but in the intermediate
         * state of having an entry added.
         *
         * Whether anything is retrievable depends on whether stub.next has been updated and become
         * visible to us, but for idleness we don't care. And due to memory ordering in _put(), the
         * update to newest would be visible to us at the same time or sooner.
         */
        if (READ_ONCE(queue->newest) != &queue->stub)
                return false;

        return true;
}