rte_seqcount.h

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/* SPDX-License-Identifier: BSD-3-Clause
 * Copyright(c) 2022 Ericsson AB
 */

#ifndef _RTE_SEQCOUNT_H_
#define _RTE_SEQCOUNT_H_

#include <stdbool.h>
#include <stdint.h>

#include <rte_atomic.h>
#include <rte_branch_prediction.h>
#include <rte_stdatomic.h>

#ifdef __cplusplus
extern "C" {
#endif

typedef struct {
    RTE_ATOMIC(uint32_t) sn; 
} rte_seqcount_t;

#define RTE_SEQCOUNT_INITIALIZER { .sn = 0 }

static inline void
rte_seqcount_init(rte_seqcount_t *seqcount)
{
    seqcount->sn = 0;
}

static inline uint32_t
rte_seqcount_read_begin(const rte_seqcount_t *seqcount)
{
    /* rte_memory_order_acquire to prevent loads after (in program order)
     * from happening before the sn load. Synchronizes-with the
     * store release in rte_seqcount_write_end().
     */
    return rte_atomic_load_explicit(&seqcount->sn, rte_memory_order_acquire);
}

static inline bool
rte_seqcount_read_retry(const rte_seqcount_t *seqcount, uint32_t begin_sn)
{
    uint32_t end_sn;

    /* An odd sequence number means the protected data was being
     * modified already at the point of the rte_seqcount_read_begin()
     * call.
     */
    if (unlikely(begin_sn & 1))
        return true;

    /* make sure the data loads happens before the sn load */
    rte_atomic_thread_fence(rte_memory_order_acquire);

    end_sn = rte_atomic_load_explicit(&seqcount->sn, rte_memory_order_relaxed);

    /* A writer incremented the sequence number during this read
     * critical section.
     */
    return begin_sn != end_sn;
}

static inline void
rte_seqcount_write_begin(rte_seqcount_t *seqcount)
{
    uint32_t sn;

    sn = seqcount->sn + 1;

    rte_atomic_store_explicit(&seqcount->sn, sn, rte_memory_order_relaxed);

    /* rte_memory_order_release to prevent stores after (in program order)
     * from happening before the sn store.
     */
    rte_atomic_thread_fence(rte_memory_order_release);
}

static inline void
rte_seqcount_write_end(rte_seqcount_t *seqcount)
{
    uint32_t sn;

    sn = seqcount->sn + 1;

    /* Synchronizes-with the load acquire in rte_seqcount_read_begin(). */
    rte_atomic_store_explicit(&seqcount->sn, sn, rte_memory_order_release);
}

#ifdef __cplusplus
}
#endif

#endif /* _RTE_SEQCOUNT_H_ */