1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
|
/*-
* Copyright (c) 2014 Mateusz Guzik <mjg@FreeBSD.org>
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*
* $FreeBSD$
*/
#ifndef _SYS_SEQ_H_
#define _SYS_SEQ_H_
#ifdef _KERNEL
#include <sys/systm.h>
#endif
#include <sys/types.h>
/*
* seq_t may be included in structs visible to userspace
*/
typedef uint32_t seq_t;
#ifdef _KERNEL
/*
* seq allows readers and writers to work with a consistent snapshot. Modifying
* operations must be enclosed within a transaction delineated by
* seq_write_beg/seq_write_end. The trick works by having the writer increment
* the sequence number twice, at the beginning and end of the transaction.
* The reader detects that the sequence number has not changed between its start
* and end, and that the sequence number is even, to validate consistency.
*
* Some fencing (both hard fencing and compiler barriers) may be needed,
* depending on the cpu. Modern AMD cpus provide strong enough guarantees to not
* require any fencing by the reader or writer.
*
* Example usage:
*
* writers:
* lock_exclusive(&obj->lock);
* seq_write_begin(&obj->seq);
* obj->var1 = ...;
* obj->var2 = ...;
* seq_write_end(&obj->seq);
* unlock_exclusive(&obj->lock);
*
* readers:
* int var1, var2;
* seq_t seq;
*
* for (;;) {
* seq = seq_read(&obj->seq);
* var1 = obj->var1;
* var2 = obj->var2;
* if (seq_consistent(&obj->seq, seq))
* break;
* }
* .....
*
* Writers may not block or sleep in any way.
*
* There are 2 minor caveats in this implementation:
*
* 1. There is no guarantee of progress. That is, a large number of writers can
* interfere with the execution of the readers and cause the code to live-lock
* in a loop trying to acquire a consistent snapshot.
*
* 2. If the reader loops long enough, the counter may overflow and eventually
* wrap back to its initial value, fooling the reader into accepting the
* snapshot. Given that this needs 4 billion transactional writes across a
* single contended reader, it is unlikely to ever happen.
*/
/* A hack to get MPASS macro */
#include <sys/lock.h>
#include <machine/cpu.h>
static __inline bool
seq_in_modify(seq_t seqp)
{
return (seqp & 1);
}
static __inline void
seq_write_begin(seq_t *seqp)
{
critical_enter();
MPASS(!seq_in_modify(*seqp));
*seqp += 1;
atomic_thread_fence_rel();
}
static __inline void
seq_write_end(seq_t *seqp)
{
atomic_store_rel_32(seqp, *seqp + 1);
MPASS(!seq_in_modify(*seqp));
critical_exit();
}
static __inline seq_t
seq_read(const seq_t *seqp)
{
seq_t ret;
for (;;) {
ret = atomic_load_acq_32(__DECONST(seq_t *, seqp));
if (seq_in_modify(ret)) {
cpu_spinwait();
continue;
}
break;
}
return (ret);
}
static __inline seq_t
seq_consistent_nomb(const seq_t *seqp, seq_t oldseq)
{
return (*seqp == oldseq);
}
static __inline seq_t
seq_consistent(const seq_t *seqp, seq_t oldseq)
{
atomic_thread_fence_acq();
return (seq_consistent_nomb(seqp, oldseq));
}
#endif /* _KERNEL */
#endif /* _SYS_SEQ_H_ */
|
