1/*-
2 * Copyright (c) 2014 Mateusz Guzik <mjg@FreeBSD.org>
3 *
4 * Redistribution and use in source and binary forms, with or without
5 * modification, are permitted provided that the following conditions
6 * are met:
7 * 1. Redistributions of source code must retain the above copyright
8 *    notice, this list of conditions and the following disclaimer.
9 * 2. Redistributions in binary form must reproduce the above copyright
10 *    notice, this list of conditions and the following disclaimer in the
11 *    documentation and/or other materials provided with the distribution.
12 *
13 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
14 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
15 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
16 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
17 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
18 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
19 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
20 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
21 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
22 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
23 * SUCH DAMAGE.
24 *
25 * $FreeBSD$
26 */
27
28#ifndef _SYS_SEQ_H_
29#define _SYS_SEQ_H_
30
31#ifdef _KERNEL
32#include <sys/systm.h>
33#endif
34#include <sys/types.h>
35
36/*
37 * seq_t may be included in structs visible to userspace
38 */
39typedef uint32_t seq_t;
40
41#ifdef _KERNEL
42
43/*
44 * Typical usage:
45 *
46 * writers:
47 * 	lock_exclusive(&obj->lock);
48 * 	seq_write_begin(&obj->seq);
49 * 	.....
50 * 	seq_write_end(&obj->seq);
51 * 	unlock_exclusive(&obj->unlock);
52 *
53 * readers:
54 * 	obj_t lobj;
55 * 	seq_t seq;
56 *
57 * 	for (;;) {
58 * 		seq = seq_read(&gobj->seq);
59 * 		lobj = gobj;
60 * 		if (seq_consistent(&gobj->seq, seq))
61 * 			break;
62 * 		cpu_spinwait();
63 * 	}
64 * 	foo(lobj);
65 */
66
67/* A hack to get MPASS macro */
68#include <sys/lock.h>
69
70#include <machine/cpu.h>
71
72/*
73 * This is a temporary hack until memory barriers are cleaned up.
74 *
75 * atomic_load_acq_int at least on amd64 provides a full memory barrier,
76 * in a way which affects perforance.
77 *
78 * Hack below covers all architectures and avoids most of the penalty at least
79 * on amd64.
80 */
81static __inline int
82atomic_load_acq_rmb_int(volatile u_int *p)
83{
84	volatile u_int v;
85
86	v = *p;
87	atomic_load_acq_int(&v);
88	return (v);
89}
90
91static __inline bool
92seq_in_modify(seq_t seqp)
93{
94
95	return (seqp & 1);
96}
97
98static __inline void
99seq_write_begin(seq_t *seqp)
100{
101
102	MPASS(!seq_in_modify(*seqp));
103	atomic_add_acq_int(seqp, 1);
104}
105
106static __inline void
107seq_write_end(seq_t *seqp)
108{
109
110	atomic_add_rel_int(seqp, 1);
111	MPASS(!seq_in_modify(*seqp));
112}
113
114static __inline seq_t
115seq_read(seq_t *seqp)
116{
117	seq_t ret;
118
119	for (;;) {
120		ret = atomic_load_acq_rmb_int(seqp);
121		if (seq_in_modify(ret)) {
122			cpu_spinwait();
123			continue;
124		}
125		break;
126	}
127
128	return (ret);
129}
130
131static __inline seq_t
132seq_consistent(seq_t *seqp, seq_t oldseq)
133{
134
135	return (atomic_load_acq_rmb_int(seqp) == oldseq);
136}
137
138static __inline seq_t
139seq_consistent_nomb(seq_t *seqp, seq_t oldseq)
140{
141
142	return (*seqp == oldseq);
143}
144
145#endif	/* _KERNEL */
146#endif	/* _SYS_SEQ_H_ */
147