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 * 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 static __inline bool
seq_in_modify(seq_t seqp)73 seq_in_modify(seq_t seqp)
74 {
75
76 return (seqp & 1);
77 }
78
79 static __inline void
seq_write_begin(seq_t * seqp)80 seq_write_begin(seq_t *seqp)
81 {
82
83 MPASS(!seq_in_modify(*seqp));
84 *seqp += 1;
85 atomic_thread_fence_rel();
86 }
87
88 static __inline void
seq_write_end(seq_t * seqp)89 seq_write_end(seq_t *seqp)
90 {
91
92 atomic_store_rel_int(seqp, *seqp + 1);
93 MPASS(!seq_in_modify(*seqp));
94 }
95
96 static __inline seq_t
seq_read(const seq_t * seqp)97 seq_read(const seq_t *seqp)
98 {
99 seq_t ret;
100
101 for (;;) {
102 ret = atomic_load_acq_int(__DECONST(seq_t *, seqp));
103 if (seq_in_modify(ret)) {
104 cpu_spinwait();
105 continue;
106 }
107 break;
108 }
109
110 return (ret);
111 }
112
113 static __inline seq_t
seq_consistent_nomb(const seq_t * seqp,seq_t oldseq)114 seq_consistent_nomb(const seq_t *seqp, seq_t oldseq)
115 {
116
117 return (*seqp == oldseq);
118 }
119
120 static __inline seq_t
seq_consistent(const seq_t * seqp,seq_t oldseq)121 seq_consistent(const seq_t *seqp, seq_t oldseq)
122 {
123
124 atomic_thread_fence_acq();
125 return (seq_consistent_nomb(seqp, oldseq));
126 }
127
128 #endif /* _KERNEL */
129 #endif /* _SYS_SEQ_H_ */
130