source: rtems-libbsd/freebsd/sys/sys/seq.h @ b86b097

/*-
 * Copyright (c) 2014 Mateusz Guzik <mjg@FreeBSD.org>
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * $FreeBSD$
 */

#ifndef _SYS_SEQ_H_
#define _SYS_SEQ_H_

#ifdef _KERNEL
#include <sys/systm.h>
#endif
#include <sys/types.h>

/*
 * seq_t may be included in structs visible to userspace
 */
typedef uint32_t seq_t;

#ifdef _KERNEL

/*
 * Typical usage:
 *
 * writers:
 *      lock_exclusive(&obj->lock);
 *      seq_write_begin(&obj->seq);
 *      .....
 *      seq_write_end(&obj->seq);
 *      unlock_exclusive(&obj->unlock);
 *
 * readers:
 *      obj_t lobj;
 *      seq_t seq;
 *
 *      for (;;) {
 *              seq = seq_read(&gobj->seq);
 *              lobj = gobj;
 *              if (seq_consistent(&gobj->seq, seq))
 *                      break;
 *              cpu_spinwait();
 *      }
 *      foo(lobj);
 */             

/* A hack to get MPASS macro */
#include <rtems/bsd/sys/lock.h>

#include <machine/cpu.h>

static __inline bool
seq_in_modify(seq_t seqp)
{

        return (seqp & 1);
}

static __inline void
seq_write_begin(seq_t *seqp)
{

        MPASS(!seq_in_modify(*seqp));
        *seqp += 1;
        atomic_thread_fence_rel();
}

static __inline void
seq_write_end(seq_t *seqp)
{

        atomic_store_rel_32(seqp, *seqp + 1);
        MPASS(!seq_in_modify(*seqp));
}

static __inline seq_t
seq_read(const seq_t *seqp)
{
        seq_t ret;

        for (;;) {
                ret = atomic_load_acq_32(__DECONST(seq_t *, seqp));
                if (seq_in_modify(ret)) {
                        cpu_spinwait();
                        continue;
                }
                break;
        }

        return (ret);
}

static __inline seq_t
seq_consistent_nomb(const seq_t *seqp, seq_t oldseq)
{

        return (*seqp == oldseq);
}

static __inline seq_t
seq_consistent(const seq_t *seqp, seq_t oldseq)
{

        atomic_thread_fence_acq();
        return (seq_consistent_nomb(seqp, oldseq));
}

#endif  /* _KERNEL */
#endif  /* _SYS_SEQ_H_ */