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: stable/10/sys/sys/seq.h 273109 2014-10-14 21:19:23Z mjg $
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 */
39 typedef 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 */
81 static __inline int
atomic_load_acq_rmb_int(volatile u_int * p)82 atomic_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
91 static __inline bool
seq_in_modify(seq_t seqp)92 seq_in_modify(seq_t seqp)
93 {
94
95 return (seqp & 1);
96 }
97
98 static __inline void
seq_write_begin(seq_t * seqp)99 seq_write_begin(seq_t *seqp)
100 {
101
102 MPASS(!seq_in_modify(*seqp));
103 atomic_add_acq_int(seqp, 1);
104 }
105
106 static __inline void
seq_write_end(seq_t * seqp)107 seq_write_end(seq_t *seqp)
108 {
109
110 atomic_add_rel_int(seqp, 1);
111 MPASS(!seq_in_modify(*seqp));
112 }
113
114 static __inline seq_t
seq_read(seq_t * seqp)115 seq_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
131 static __inline seq_t
seq_consistent(seq_t * seqp,seq_t oldseq)132 seq_consistent(seq_t *seqp, seq_t oldseq)
133 {
134
135 return (atomic_load_acq_rmb_int(seqp) == oldseq);
136 }
137
138 static __inline seq_t
seq_consistent_nomb(seq_t * seqp,seq_t oldseq)139 seq_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