rte_ring_rts_elem_pvt.h

Go to the documentation of this file.

/* SPDX-License-Identifier: BSD-3-Clause
 *
 * Copyright (c) 2010-2020 Intel Corporation
 * Copyright (c) 2007-2009 Kip Macy kmacy@freebsd.org
 * All rights reserved.
 * Derived from FreeBSD's bufring.h
 * Used as BSD-3 Licensed with permission from Kip Macy.
 */

#ifndef _RTE_RING_RTS_ELEM_PVT_H_
#define _RTE_RING_RTS_ELEM_PVT_H_

static __rte_always_inline void
__rte_ring_rts_update_tail(struct rte_ring_rts_headtail *ht)
{
    union __rte_ring_rts_poscnt h, ot, nt;

    /*
     * If there are other enqueues/dequeues in progress that
     * might preceded us, then don't update tail with new value.
     */

    /*
     * A0 = {A0.a, A0.b}: Synchronizes with the CAS at R0.
     * The CAS at R0 in same typed thread establishes a happens-before
     * relationship with this load acquire. Ensures that this thread
     * observes the same or later values for h.raw/h.val.cnt
     * observed by the other thread when it updated ht->tail.raw.
     * If not, ht->tail.raw may get updated out of sync (e.g. getting
     * updated to the same value twice). A0.a makes sure this condition
     * holds when CAS succeeds and A0.b when it fails.
     */
    /* A0.a */
    ot.raw = rte_atomic_load_explicit(&ht->tail.raw, rte_memory_order_acquire);

    do {
        /* on 32-bit systems we have to do atomic read here */
        h.raw = rte_atomic_load_explicit(&ht->head.raw, rte_memory_order_relaxed);

        nt.raw = ot.raw;
        if (++nt.val.cnt == h.val.cnt)
            nt.val.pos = h.val.pos;

    /*
     * R0: Synchronizes with A2 of a different thread of the opposite type and A0.b
     * of a different thread of the same type.
     */
    /* A0.b */
    } while (rte_atomic_compare_exchange_strong_explicit(&ht->tail.raw,
            (uint64_t *)(uintptr_t)&ot.raw, nt.raw,
            rte_memory_order_release, rte_memory_order_acquire) == 0);
}

static __rte_always_inline union __rte_ring_rts_poscnt
__rte_ring_rts_head_wait(const struct rte_ring_rts_headtail *ht,
             int memorder)
{
    union __rte_ring_rts_poscnt h;
    uint32_t max = ht->htd_max;


    h.raw = rte_atomic_load_explicit(&ht->head.raw, memorder);

    while (h.val.pos - ht->tail.val.pos > max) {
        rte_pause();
        h.raw = rte_atomic_load_explicit(&ht->head.raw, memorder);
    }

    return h;
}

static __rte_always_inline uint32_t
__rte_ring_rts_move_prod_head(struct rte_ring *r, uint32_t num,
    enum rte_ring_queue_behavior behavior, uint32_t *old_head,
    uint32_t *free_entries)
{
    uint32_t n, cons_tail;
    union __rte_ring_rts_poscnt nh, oh;

    const uint32_t capacity = r->capacity;

    do {
        /* Reset n to the initial burst count */
        n = num;

        /*
         * wait for prod head/tail distance,
         * make sure that we read prod head *before*
         * reading cons tail.
         */
        /*
         * A1 Synchronizes with the CAS at R1.
         * Establishes a happens-before relationship with a thread of the same
         * type that released the ht.raw, ensuring this thread observes all of
         * its memory effects needed to maintain a safe partial order.
         */
        oh = __rte_ring_rts_head_wait(&r->rts_prod, rte_memory_order_acquire);

        /*
         * A2: Establish a synchronizes-with edge using a store-release at R0.
         * This ensures that all memory effects from the preceding opposing
         * thread are observed.
         */
        cons_tail = rte_atomic_load_explicit(&r->cons.tail, rte_memory_order_acquire);

        /*
         *  The subtraction is done between two unsigned 32bits value
         * (the result is always modulo 32 bits even if we have
         * *old_head > cons_tail). So 'free_entries' is always between 0
         * and capacity (which is < size).
         */
        *free_entries = capacity + cons_tail - oh.val.pos;

        /* check that we have enough room in ring */
        if (unlikely(n > *free_entries))
            n = (behavior == RTE_RING_QUEUE_FIXED) ?
                    0 : *free_entries;

        if (n == 0)
            break;

        nh.val.pos = oh.val.pos + n;
        nh.val.cnt = oh.val.cnt + 1;

    /*
     * R1: Establishes a synchronizes-with edge with the load-acquire
     * of ht.raw at A1. Ensures that the store-release to the tail by
     * this thread, if it was of the opposite type, becomes
     * visible to another thread of the current type. That thread will
     * then observe the updates in the same order, keeping a safe
     * partial order.
     */
    } while (rte_atomic_compare_exchange_strong_explicit(&r->rts_prod.head.raw,
            (uint64_t *)(uintptr_t)&oh.raw, nh.raw,
            rte_memory_order_release, rte_memory_order_relaxed) == 0);

    *old_head = oh.val.pos;
    return n;
}

static __rte_always_inline unsigned int
__rte_ring_rts_move_cons_head(struct rte_ring *r, uint32_t num,
    enum rte_ring_queue_behavior behavior, uint32_t *old_head,
    uint32_t *entries)
{
    uint32_t n, prod_tail;
    union __rte_ring_rts_poscnt nh, oh;

    /* move cons.head atomically */
    do {
        /* Restore n as it may change every loop */
        n = num;

        /*
         * wait for cons head/tail distance,
         * make sure that we read cons head *before*
         * reading prod tail.
         */
        /*
         * A3: Synchronizes with the CAS at R2.
         * Establishes a happens-before relationship with a thread of the same
         * type that released the ht.raw, ensuring this thread observes all of
         * its memory effects needed to maintain a safe partial order.
         */
        oh = __rte_ring_rts_head_wait(&r->rts_cons, rte_memory_order_acquire);

        /*
         * A4: Establish a synchronizes-with edge using a store-release at R0.
         * This ensures that all memory effects from the preceding opposing
         * thread are observed.
         */
        prod_tail = rte_atomic_load_explicit(&r->prod.tail, rte_memory_order_acquire);

        /* The subtraction is done between two unsigned 32bits value
         * (the result is always modulo 32 bits even if we have
         * cons_head > prod_tail). So 'entries' is always between 0
         * and size(ring)-1.
         */
        *entries = prod_tail - oh.val.pos;

        /* Set the actual entries for dequeue */
        if (n > *entries)
            n = (behavior == RTE_RING_QUEUE_FIXED) ? 0 : *entries;

        if (unlikely(n == 0))
            break;

        nh.val.pos = oh.val.pos + n;
        nh.val.cnt = oh.val.cnt + 1;

    /*
     * R2: Establishes a synchronizes-with edge with the load-acquire
     * of ht.raw at A3. Ensures that the store-release to the tail by
     * this thread, if it was of the opposite type, becomes
     * visible to another thread of the current type. That thread will
     * then observe the updates in the same order, keeping a safe
     * partial order.
     */
    } while (rte_atomic_compare_exchange_strong_explicit(&r->rts_cons.head.raw,
            (uint64_t *)(uintptr_t)&oh.raw, nh.raw,
            rte_memory_order_release, rte_memory_order_relaxed) == 0);

    *old_head = oh.val.pos;
    return n;
}

static __rte_always_inline unsigned int
__rte_ring_do_rts_enqueue_elem(struct rte_ring *r, const void *obj_table,
    uint32_t esize, uint32_t n, enum rte_ring_queue_behavior behavior,
    uint32_t *free_space)
{
    uint32_t free, head;

    n =  __rte_ring_rts_move_prod_head(r, n, behavior, &head, &free);

    if (n != 0) {
        __rte_ring_enqueue_elems(r, head, obj_table, esize, n);
        __rte_ring_rts_update_tail(&r->rts_prod);
    }

    if (free_space != NULL)
        *free_space = free - n;
    return n;
}

static __rte_always_inline unsigned int
__rte_ring_do_rts_dequeue_elem(struct rte_ring *r, void *obj_table,
    uint32_t esize, uint32_t n, enum rte_ring_queue_behavior behavior,
    uint32_t *available)
{
    uint32_t entries, head;

    n = __rte_ring_rts_move_cons_head(r, n, behavior, &head, &entries);

    if (n != 0) {
        __rte_ring_dequeue_elems(r, head, obj_table, esize, n);
        __rte_ring_rts_update_tail(&r->rts_cons);
    }

    if (available != NULL)
        *available = entries - n;
    return n;
}

#endif /* _RTE_RING_RTS_ELEM_PVT_H_ */