rte_mcslock.h

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/* SPDX-License-Identifier: BSD-3-Clause
 * Copyright(c) 2019 Arm Limited
 */

#ifndef _RTE_MCSLOCK_H_
#define _RTE_MCSLOCK_H_

#include <rte_lcore.h>
#include <rte_common.h>
#include <rte_pause.h>
#include <rte_branch_prediction.h>

typedef struct rte_mcslock {
    struct rte_mcslock *next;
    int locked; /* 1 if the queue locked, 0 otherwise */
} rte_mcslock_t;

static inline void
rte_mcslock_lock(rte_mcslock_t **msl, rte_mcslock_t *me)
{
    rte_mcslock_t *prev;

    /* Init me node */
    __atomic_store_n(&me->locked, 1, __ATOMIC_RELAXED);
    __atomic_store_n(&me->next, NULL, __ATOMIC_RELAXED);

    /* If the queue is empty, the exchange operation is enough to acquire
     * the lock. Hence, the exchange operation requires acquire semantics.
     * The store to me->next above should complete before the node is
     * visible to other CPUs/threads. Hence, the exchange operation requires
     * release semantics as well.
     */
    prev = __atomic_exchange_n(msl, me, __ATOMIC_ACQ_REL);
    if (likely(prev == NULL)) {
        /* Queue was empty, no further action required,
         * proceed with lock taken.
         */
        return;
    }
    /* The store to me->next above should also complete before the node is
     * visible to predecessor thread releasing the lock. Hence, the store
     * prev->next also requires release semantics. Note that, for example,
     * on ARM, the release semantics in the exchange operation is not
     * strong as a release fence and is not sufficient to enforce the
     * desired order here.
     */
    __atomic_store_n(&prev->next, me, __ATOMIC_RELEASE);

    /* The while-load of me->locked should not move above the previous
     * store to prev->next. Otherwise it will cause a deadlock. Need a
     * store-load barrier.
     */
    __atomic_thread_fence(__ATOMIC_ACQ_REL);
    /* If the lock has already been acquired, it first atomically
     * places the node at the end of the queue and then proceeds
     * to spin on me->locked until the previous lock holder resets
     * the me->locked using mcslock_unlock().
     */
    while (__atomic_load_n(&me->locked, __ATOMIC_ACQUIRE))
        rte_pause();
}

static inline void
rte_mcslock_unlock(rte_mcslock_t **msl, rte_mcslock_t *me)
{
    /* Check if there are more nodes in the queue. */
    if (likely(__atomic_load_n(&me->next, __ATOMIC_RELAXED) == NULL)) {
        /* No, last member in the queue. */
        rte_mcslock_t *save_me = __atomic_load_n(&me, __ATOMIC_RELAXED);

        /* Release the lock by setting it to NULL */
        if (likely(__atomic_compare_exchange_n(msl, &save_me, NULL, 0,
                __ATOMIC_RELEASE, __ATOMIC_RELAXED)))
            return;

        /* Speculative execution would be allowed to read in the
         * while-loop first. This has the potential to cause a
         * deadlock. Need a load barrier.
         */
        __atomic_thread_fence(__ATOMIC_ACQUIRE);
        /* More nodes added to the queue by other CPUs.
         * Wait until the next pointer is set.
         */
        while (__atomic_load_n(&me->next, __ATOMIC_RELAXED) == NULL)
            rte_pause();
    }

    /* Pass lock to next waiter. */
    __atomic_store_n(&me->next->locked, 0, __ATOMIC_RELEASE);
}

static inline int
rte_mcslock_trylock(rte_mcslock_t **msl, rte_mcslock_t *me)
{
    /* Init me node */
    __atomic_store_n(&me->next, NULL, __ATOMIC_RELAXED);

    /* Try to lock */
    rte_mcslock_t *expected = NULL;

    /* The lock can be taken only when the queue is empty. Hence,
     * the compare-exchange operation requires acquire semantics.
     * The store to me->next above should complete before the node
     * is visible to other CPUs/threads. Hence, the compare-exchange
     * operation requires release semantics as well.
     */
    return __atomic_compare_exchange_n(msl, &expected, me, 0,
            __ATOMIC_ACQ_REL, __ATOMIC_RELAXED);
}

static inline int
rte_mcslock_is_locked(rte_mcslock_t *msl)
{
    return (__atomic_load_n(&msl, __ATOMIC_RELAXED) != NULL);
}

#endif /* _RTE_MCSLOCK_H_ */