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

/*-
 * 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

/*
 * seq allows readers and writers to work with a consistent snapshot. Modifying
 * operations must be enclosed within a transaction delineated by
 * seq_write_beg/seq_write_end. The trick works by having the writer increment
 * the sequence number twice, at the beginning and end of the transaction.
 * The reader detects that the sequence number has not changed between its start
 * and end, and that the sequence number is even, to validate consistency.
 *
 * Some fencing (both hard fencing and compiler barriers) may be needed,
 * depending on the cpu. Modern AMD cpus provide strong enough guarantees to not
 * require any fencing by the reader or writer.
 *
 * Example usage:
 *
 * writers:
 *     lock_exclusive(&obj->lock);
 *     seq_write_begin(&obj->seq);
 *     obj->var1 = ...;
 *     obj->var2 = ...;
 *     seq_write_end(&obj->seq);
 *     unlock_exclusive(&obj->lock);
 *
 * readers:
 *    int var1, var2;
 *    seq_t seq;
 *
 *    for (;;) {
 *            seq = seq_read(&obj->seq);
 *            var1 = obj->var1;
 *            var2 = obj->var2;
 *            if (seq_consistent(&obj->seq, seq))
 *                   break;
 *    }
 *    .....
 *
 * Writers may not block or sleep in any way.
 *
 * There are 2 minor caveats in this implementation:
 *
 * 1. There is no guarantee of progress. That is, a large number of writers can
 * interfere with the execution of the readers and cause the code to live-lock
 * in a loop trying to acquire a consistent snapshot.
 *
 * 2. If the reader loops long enough, the counter may overflow and eventually
 * wrap back to its initial value, fooling the reader into accepting the
 * snapshot.  Given that this needs 4 billion transactional writes across a
 * single contended reader, it is unlikely to ever happen.
 */             

/* A hack to get MPASS macro */
#include <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)
{

        critical_enter();
        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));
        critical_exit();
}

static __inline seq_t
seq_load(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_ */