FreeBSD/Linux Kernel Cross Reference
sys/sys/seq.h
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 */
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 * }
63 * foo(lobj);
64 */
65
66 /* A hack to get MPASS macro */
67 #include <sys/lock.h>
68
69 #include <machine/cpu.h>
70
71 static __inline bool
72 seq_in_modify(seq_t seqp)
73 {
74
75 return (seqp & 1);
76 }
77
78 static __inline void
79 seq_write_begin(seq_t *seqp)
80 {
81
82 MPASS(!seq_in_modify(*seqp));
83 *seqp += 1;
84 atomic_thread_fence_rel();
85 }
86
87 static __inline void
88 seq_write_end(seq_t *seqp)
89 {
90
91 atomic_store_rel_int(seqp, *seqp + 1);
92 MPASS(!seq_in_modify(*seqp));
93 }
94
95 static __inline seq_t
96 seq_read(const seq_t *seqp)
97 {
98 seq_t ret;
99
100 for (;;) {
101 ret = atomic_load_acq_int(__DECONST(seq_t *, seqp));
102 if (seq_in_modify(ret)) {
103 cpu_spinwait();
104 continue;
105 }
106 break;
107 }
108
109 return (ret);
110 }
111
112 static __inline seq_t
113 seq_consistent_nomb(const seq_t *seqp, seq_t oldseq)
114 {
115
116 return (*seqp == oldseq);
117 }
118
119 static __inline seq_t
120 seq_consistent(const seq_t *seqp, seq_t oldseq)
121 {
122
123 atomic_thread_fence_acq();
124 return (seq_consistent_nomb(seqp, oldseq));
125 }
126
127 #endif /* _KERNEL */
128 #endif /* _SYS_SEQ_H_ */
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