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>
#include <rte_stdatomic.h>

#ifdef __cplusplus
extern "C" {
#endif

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

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

    /* Init me node */
    rte_atomic_store_explicit(&me->locked, 1, rte_memory_order_relaxed);
    rte_atomic_store_explicit(&me->next, NULL, rte_memory_order_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 = rte_atomic_exchange_explicit(msl, me, rte_memory_order_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.
     */
    rte_atomic_store_explicit(&prev->next, me, rte_memory_order_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.
     */
    rte_atomic_thread_fence(rte_memory_order_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().
     */
    rte_wait_until_equal_32((uint32_t *)(uintptr_t)&me->locked, 0, rte_memory_order_acquire);
}

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

        /* Release the lock by setting it to NULL */
        if (likely(rte_atomic_compare_exchange_strong_explicit(msl, &save_me, NULL,
                rte_memory_order_release, rte_memory_order_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.
         */
        rte_atomic_thread_fence(rte_memory_order_acquire);
        /* More nodes added to the queue by other CPUs.
         * Wait until the next pointer is set.
         */
        RTE_ATOMIC(uintptr_t) *next;
        next = (__rte_atomic uintptr_t *)&me->next;
        RTE_WAIT_UNTIL_MASKED(next, UINTPTR_MAX, !=, 0, rte_memory_order_relaxed);
    }

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

static inline int
rte_mcslock_trylock(RTE_ATOMIC(rte_mcslock_t *) *msl, rte_mcslock_t *me)
{
    /* Init me node */
    rte_atomic_store_explicit(&me->next, NULL, rte_memory_order_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 rte_atomic_compare_exchange_strong_explicit(msl, &expected, me,
            rte_memory_order_acq_rel, rte_memory_order_relaxed);
}

static inline int
rte_mcslock_is_locked(RTE_ATOMIC(rte_mcslock_t *) msl)
{
    return (rte_atomic_load_explicit(&msl, rte_memory_order_relaxed) != NULL);
}

#ifdef __cplusplus
}
#endif

#endif /* _RTE_MCSLOCK_H_ */