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: releng/12.0/sys/sys/seq.h 331660 2018-03-28 04:38:45Z 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 * seq allows readers and writers to work with a consistent snapshot. Modifying
45 * operations must be enclosed within a transaction delineated by
46 * seq_write_beg/seq_write_end. The trick works by having the writer increment
47 * the sequence number twice, at the beginning and end of the transaction.
48 * The reader detects that the sequence number has not changed between its start
49 * and end, and that the sequence number is even, to validate consistency.
50 *
51 * Some fencing (both hard fencing and compiler barriers) may be needed,
52 * depending on the cpu. Modern AMD cpus provide strong enough guarantees to not
53 * require any fencing by the reader or writer.
54 *
55 * Example usage:
56 *
57 * writers:
58 * lock_exclusive(&obj->lock);
59 * seq_write_begin(&obj->seq);
60 * obj->var1 = ...;
61 * obj->var2 = ...;
62 * seq_write_end(&obj->seq);
63 * unlock_exclusive(&obj->lock);
64 *
65 * readers:
66 * int var1, var2;
67 * seq_t seq;
68 *
69 * for (;;) {
70 * seq = seq_read(&obj->seq);
71 * var1 = obj->var1;
72 * var2 = obj->var2;
73 * if (seq_consistent(&obj->seq, seq))
74 * break;
75 * }
76 * .....
77 *
78 * Writers may not block or sleep in any way.
79 *
80 * There are 2 minor caveats in this implementation:
81 *
82 * 1. There is no guarantee of progress. That is, a large number of writers can
83 * interfere with the execution of the readers and cause the code to live-lock
84 * in a loop trying to acquire a consistent snapshot.
85 *
86 * 2. If the reader loops long enough, the counter may overflow and eventually
87 * wrap back to its initial value, fooling the reader into accepting the
88 * snapshot. Given that this needs 4 billion transactional writes across a
89 * single contended reader, it is unlikely to ever happen.
90 */
91
92 /* A hack to get MPASS macro */
93 #include <sys/lock.h>
94
95 #include <machine/cpu.h>
96
97 static __inline bool
98 seq_in_modify(seq_t seqp)
99 {
100
101 return (seqp & 1);
102 }
103
104 static __inline void
105 seq_write_begin(seq_t *seqp)
106 {
107
108 critical_enter();
109 MPASS(!seq_in_modify(*seqp));
110 *seqp += 1;
111 atomic_thread_fence_rel();
112 }
113
114 static __inline void
115 seq_write_end(seq_t *seqp)
116 {
117
118 atomic_store_rel_int(seqp, *seqp + 1);
119 MPASS(!seq_in_modify(*seqp));
120 critical_exit();
121 }
122
123 static __inline seq_t
124 seq_read(const seq_t *seqp)
125 {
126 seq_t ret;
127
128 for (;;) {
129 ret = atomic_load_acq_int(__DECONST(seq_t *, seqp));
130 if (seq_in_modify(ret)) {
131 cpu_spinwait();
132 continue;
133 }
134 break;
135 }
136
137 return (ret);
138 }
139
140 static __inline seq_t
141 seq_consistent_nomb(const seq_t *seqp, seq_t oldseq)
142 {
143
144 return (*seqp == oldseq);
145 }
146
147 static __inline seq_t
148 seq_consistent(const seq_t *seqp, seq_t oldseq)
149 {
150
151 atomic_thread_fence_acq();
152 return (seq_consistent_nomb(seqp, oldseq));
153 }
154
155 #endif /* _KERNEL */
156 #endif /* _SYS_SEQ_H_ */
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