rte_pflock.h

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/* SPDX-License-Identifier: BSD-3-Clause
 * Copyright(c) 2021 Microsoft Corp.
 * All rights reserved.
 *
 * Derived from Concurrency Kit
 * Copyright 2011-2015 Samy Al Bahra.
 */

#ifndef _RTE_PFLOCK_H_
#define _RTE_PFLOCK_H_

#include <rte_common.h>
#include <rte_pause.h>
#include <rte_stdatomic.h>

#ifdef __cplusplus
extern "C" {
#endif

struct rte_pflock {
    struct {
        RTE_ATOMIC(uint16_t) in;
        RTE_ATOMIC(uint16_t) out;
    } rd, wr;
};
typedef struct rte_pflock rte_pflock_t;

/*
 * Allocation of bits to reader
 *
 * 15                 4 3 2 1 0
 * +-------------------+---+-+-+
 * | rin: reads issued |x|x| | |
 * +-------------------+---+-+-+
 *                          ^ ^
 *                          | |
 * PRES: writer present ----/ |
 * PHID: writer phase id -----/
 *
 * 15                4 3 2 1 0
 * +------------------+------+
 * |rout:read complete|unused|
 * +------------------+------+
 *
 * The maximum number of readers is 4095
 */

/* Constants used to map the bits in reader counter */
#define RTE_PFLOCK_WBITS 0x3    /* Writer bits in reader. */
#define RTE_PFLOCK_PRES  0x2    /* Writer present bit. */
#define RTE_PFLOCK_PHID  0x1    /* Phase ID bit. */
#define RTE_PFLOCK_LSB   0xFFF0 /* reader bits. */
#define RTE_PFLOCK_RINC  0x10   /* Reader increment. */

#define RTE_PFLOCK_INITIALIZER {  }

static inline void
rte_pflock_init(struct rte_pflock *pf)
{
    pf->rd.in = 0;
    pf->rd.out = 0;
    pf->wr.in = 0;
    pf->wr.out = 0;
}

static inline void
rte_pflock_read_lock(rte_pflock_t *pf)
{
    uint16_t w;

    /*
     * If no writer is present, then the operation has completed
     * successfully.
     */
    w = rte_atomic_fetch_add_explicit(&pf->rd.in, RTE_PFLOCK_RINC, rte_memory_order_acquire)
        & RTE_PFLOCK_WBITS;
    if (w == 0)
        return;

    /* Wait for current write phase to complete. */
    RTE_WAIT_UNTIL_MASKED(&pf->rd.in, RTE_PFLOCK_WBITS, !=, w, rte_memory_order_acquire);
}

static inline void
rte_pflock_read_unlock(rte_pflock_t *pf)
{
    rte_atomic_fetch_add_explicit(&pf->rd.out, RTE_PFLOCK_RINC, rte_memory_order_release);
}

static inline void
rte_pflock_write_lock(rte_pflock_t *pf)
{
    uint16_t ticket, w;

    /* Acquire ownership of write-phase.
     * This is same as rte_ticketlock_lock().
     */
    ticket = rte_atomic_fetch_add_explicit(&pf->wr.in, 1, rte_memory_order_relaxed);
    rte_wait_until_equal_16((uint16_t *)(uintptr_t)&pf->wr.out, ticket,
        rte_memory_order_acquire);

    /*
     * Acquire ticket on read-side in order to allow them
     * to flush. Indicates to any incoming reader that a
     * write-phase is pending.
     *
     * The load of rd.out in wait loop could be executed
     * speculatively.
     */
    w = RTE_PFLOCK_PRES | (ticket & RTE_PFLOCK_PHID);
    ticket = rte_atomic_fetch_add_explicit(&pf->rd.in, w, rte_memory_order_relaxed);

    /* Wait for any pending readers to flush. */
    rte_wait_until_equal_16((uint16_t *)(uintptr_t)&pf->rd.out, ticket,
        rte_memory_order_acquire);
}

static inline void
rte_pflock_write_unlock(rte_pflock_t *pf)
{
    /* Migrate from write phase to read phase. */
    rte_atomic_fetch_and_explicit(&pf->rd.in, RTE_PFLOCK_LSB, rte_memory_order_release);

    /* Allow other writers to continue. */
    rte_atomic_fetch_add_explicit(&pf->wr.out, 1, rte_memory_order_release);
}

#ifdef __cplusplus
}
#endif

#endif /* RTE_PFLOCK_H */