/*-
* 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 <sys/lock.h>
#include <machine/cpu.h>
/*
* Stuff below is going away when we gain suitable memory barriers.
*
* atomic_load_acq_int at least on amd64 provides a full memory barrier,
* in a way which affects performance.
*
* Hack below covers all architectures and avoids most of the penalty at least
* on amd64 but still has unnecessary cost.
*/
static __inline int
atomic_load_rmb_int(volatile u_int *p)
{
volatile u_int v;
v = *p;
atomic_load_acq_int(&v);
return (v);
}
static __inline int
atomic_rmb_load_int(volatile u_int *p)
{
volatile u_int v = 0;
atomic_load_acq_int(&v);
v = *p;
return (v);
}
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));
atomic_add_acq_int(seqp, 1);
}
static __inline void
seq_write_end(seq_t *seqp)
{
atomic_add_rel_int(seqp, 1);
MPASS(!seq_in_modify(*seqp));
}
static __inline seq_t
seq_read(seq_t *seqp)
{
seq_t ret;
for (;;) {
ret = atomic_load_rmb_int(seqp);
if (seq_in_modify(ret)) {
cpu_spinwait();
continue;
}
break;
}
return (ret);
}
static __inline seq_t
seq_consistent(seq_t *seqp, seq_t oldseq)
{
return (atomic_rmb_load_int(seqp) == oldseq);
}
static __inline seq_t
seq_consistent_nomb(seq_t *seqp, seq_t oldseq)
{
return (*seqp == oldseq);
}
#endif /* _KERNEL */
#endif /* _SYS_SEQ_H_ */
