# 2022q1 Homework5 (quiz8)
contributed by < `yaohwang99` >

## Problem 2

typedef uintptr_t ringidx_t;

struct element {
    void *ptr;
    uintptr_t idx;
};

Aligned to the size of cache line for cache friendly code.

struct lfring {
    ringidx_t head;
    ringidx_t tail ALIGNED(CACHE_LINE);
    uint32_t mask;
    uint32_t flags;
    struct element ring[] ALIGNED(CACHE_LINE);
} ALIGNED(CACHE_LINE);

typedef struct lfring lfring_t;

---

1. Allocate memory for `ifring`
2. `ringsz` is rounded up to power of 2, therefore we can use mask to calculate modulo.
3. The indices of `ring[i]` is `i - ringsz`, for example, if ringsz is 8, then the indices are -8, -7, -6,...,-1.

lfring_t *lfring_alloc(uint32_t n_elems, uint32_t flags)
{
    unsigned long ringsz = ROUNDUP_POW2(n_elems);
    if (n_elems == 0 || ringsz == 0 || ringsz > 0x80000000) {
        assert(0 && "invalid number of elements");
        return NULL;
    }
    if ((flags & ~SUPPORTED_FLAGS) != 0) {
        assert(0 && "invalid flags");
        return NULL;
    }

    size_t nbytes = sizeof(lfring_t) + ringsz * sizeof(struct element);
    lfring_t *lfr = osal_alloc(nbytes, CACHE_LINE);
    if (!lfr)
        return NULL;

    lfr->head = 0, lfr->tail = 0;
    lfr->mask = ringsz - 1;
    lfr->flags = flags;
    for (ringidx_t i = 0; i < ringsz; i++) {
        lfr->ring[i].ptr = NULL;
        lfr->ring[i].idx = i - ringsz;
    }
    return lfr;
}

digraph{
compound = true
rankdir = LR
    subgraph cluster0{
    style = filled
    color = gray90
        node0[shape = record, label = "head|tail|mask|flag|<ring>ring"]
    }
    subgraph cluster1{
    style = filled
    color = gray90
        node1[shape = record, label = "{<ptr0>ptr|ptr|ptr|ptr|ptr|ptr|ptr|ptr}|{idx|idx|idx|idx|idx|idx|idx|idx}"]
    }
    NULL[shape = plaintext]
    node0->node1[lhead = cluster1]
    node1:ptr0->NULL
}

---

The enqueue steps can be summarized as the following:
1. Check if the slot is available.
2. Prepare the `neu` data.
3. If the slot is still available, write `neu` to the ring, otherwise, move to next slot and repeat from step 1.
4. Update tail if the tail is the newest.

uint32_t lfring_enqueue(lfring_t *lfr,
                        void *const *restrict elems,
                        uint32_t n_elems)
{
    intptr_t actual = 0;
    ringidx_t mask = lfr->mask;
    ringidx_t size = mask + 1;
    ringidx_t tail = __atomic_load_n(&lfr->tail, __ATOMIC_RELAXED);

    if (lfr->flags & LFRING_FLAG_SP) { /* single-producer */
        ringidx_t head = __atomic_load_n(&lfr->head, __ATOMIC_ACQUIRE);
        actual = MIN((intptr_t)(head + size - tail), (intptr_t) n_elems);
        if (actual <= 0)
            return 0;

        for (uint32_t i = 0; i < (uint32_t) actual; i++) {
            assert(lfr->ring[tail & mask].idx == tail - size);
            lfr->ring[tail & mask].ptr = *elems++;
            lfr->ring[tail & mask].idx = tail;
            tail++;
        }
        __atomic_store_n(&lfr->tail, tail, __ATOMIC_RELEASE);
        return (uint32_t) actual;
    }

    /* else: lock-free multi-producer */

At line 29, `before(tail, __atomic_load_n(&lfr->head, __ATOMIC_ACQUIRE) + size)` checks if there are empty slots in the ring.
At line 39, `(old.e.idx == tail - size)` if `tail` is available, if not, then `old.e.idx` should be equal to `tail` if it is used by other producer recently.
`cond_reload(tail, &lfr->tail)` returns the newest tail, either `tail + 1` or `lfr->tail`.

restart:
    while ((uint32_t) actual < n_elems &&
           before(tail, __atomic_load_n(&lfr->head, __ATOMIC_ACQUIRE) + size)) {
        union {
            struct element e;
            ptrpair_t pp;
        } old, neu;
        void *elem = elems[actual];
        struct element *slot = &lfr->ring[tail & mask];
        old.e.ptr = __atomic_load_n(&slot->ptr, __ATOMIC_RELAXED);
        old.e.idx = __atomic_load_n(&slot->idx, __ATOMIC_RELAXED);
        do {
            if (UNLIKELY(old.e.idx != tail - size)) {
                if (old.e.idx != tail) {
                    /* We are far behind. Restart with fresh index */
                    tail = cond_reload(tail, &lfr->tail);
                    goto restart;
                }
                /* slot already enqueued */
                tail++; /* Try next slot */
                goto restart;
            }

            /* Found slot that was used one lap back.
             * Try to enqueue next element.
             */
            neu.e.ptr = elem;
            neu.e.idx = tail; /* Set idx on enqueue */
        } while (!lf_compare_exchange((ptrpair_t *) slot, &old.pp, neu.pp));

        /* Enqueue succeeded */
        actual++;
        tail++; /* Continue with next slot */
    }
    (void) cond_update(&lfr->tail, tail);
    return (uint32_t) actual;
}

The dequeue steps can be summarized as the following:
1. Check if the slots contain item.
2. Copy to `elems[]`.
3. If the slots are not dequeued by other consumer, update `lfr->head`, otherwise, update `head` to the new `lfr->head` and repeat from step 1.

uint32_t lfring_dequeue(lfring_t *lfr,
                        void **restrict elems,
                        uint32_t n_elems,
                        uint32_t *index)
{
    ringidx_t mask = lfr->mask;
    intptr_t actual;
    ringidx_t head = __atomic_load_n(&lfr->head, __ATOMIC_RELAXED);
    ringidx_t tail = __atomic_load_n(&lfr->tail, __ATOMIC_ACQUIRE);
    do {
        actual = MIN((intptr_t)(tail - head), (intptr_t) n_elems);
        if (UNLIKELY(actual <= 0)) {
            /* Ring buffer is empty, scan for new but unreleased elements */
            tail = find_tail(lfr, head, tail);
            actual = MIN((intptr_t)(tail - head), (intptr_t) n_elems);
            if (actual <= 0)
                return 0;
        }
        for (uint32_t i = 0; i < (uint32_t) actual; i++)
            elems[i] = lfr->ring[(head + i) & mask].ptr;
        smp_fence(LoadStore);                        // Order loads only
        if (UNLIKELY(lfr->flags & LFRING_FLAG_SC)) { /* Single-consumer */
            __atomic_store_n(&lfr->head, head + actual, __ATOMIC_RELAXED);
            break;
        }

        /* else: lock-free multi-consumer */
    } while (!__atomic_compare_exchange_n(
        &lfr->head, &head, /* Updated on failure */
        /* XXXXX */ head + actual,
        /* weak */ false, __ATOMIC_RELAXED, __ATOMIC_RELAXED));
    *index = (uint32_t) head;
    return (uint32_t) actual;
}