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
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
|
// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Semaphore implementation exposed to Go.
// Intended use is provide a sleep and wakeup
// primitive that can be used in the contended case
// of other synchronization primitives.
// Thus it targets the same goal as Linux's futex,
// but it has much simpler semantics.
//
// That is, don't think of these as semaphores.
// Think of them as a way to implement sleep and wakeup
// such that every sleep is paired with a single wakeup,
// even if, due to races, the wakeup happens before the sleep.
//
// See Mullender and Cox, ``Semaphores in Plan 9,''
// http://swtch.com/semaphore.pdf
package runtime
#include "runtime.h"
typedef struct Sema Sema;
struct Sema
{
uint32 *addr;
G *g;
Sema *prev;
Sema *next;
};
// TODO: For now, a linked list; maybe a hash table of linked lists later.
static Sema *semfirst, *semlast;
static Lock semlock;
static void
semqueue(uint32 *addr, Sema *s)
{
s->addr = addr;
s->g = nil;
runtime·lock(&semlock);
s->prev = semlast;
s->next = nil;
if(semlast)
semlast->next = s;
else
semfirst = s;
semlast = s;
runtime·unlock(&semlock);
}
static void
semdequeue(Sema *s)
{
runtime·lock(&semlock);
if(s->next)
s->next->prev = s->prev;
else
semlast = s->prev;
if(s->prev)
s->prev->next = s->next;
else
semfirst = s->next;
s->prev = nil;
s->next = nil;
runtime·unlock(&semlock);
}
static void
semwakeup(uint32 *addr)
{
Sema *s;
runtime·lock(&semlock);
for(s=semfirst; s; s=s->next) {
if(s->addr == addr && s->g) {
runtime·ready(s->g);
s->g = nil;
break;
}
}
runtime·unlock(&semlock);
}
// Step 1 of sleep: make ourselves available for wakeup.
// TODO(rsc): Maybe we can write a version without
// locks by using cas on s->g. Maybe not: I need to
// think more about whether it would be correct.
static void
semsleep1(Sema *s)
{
runtime·lock(&semlock);
s->g = g;
runtime·unlock(&semlock);
}
// Decided not to go through with it: undo step 1.
static void
semsleepundo1(Sema *s)
{
runtime·lock(&semlock);
if(s->g != nil) {
s->g = nil; // back ourselves out
} else {
// If s->g == nil already, semwakeup
// already readied us. Since we never stopped
// running, readying us just set g->readyonstop.
// Clear it.
if(g->readyonstop == 0)
*(int32*)0x555 = 555;
g->readyonstop = 0;
}
runtime·unlock(&semlock);
}
// Step 2: wait for the wakeup.
static void
semsleep2(Sema *s)
{
USED(s);
g->status = Gwaiting;
runtime·gosched();
}
static int32
cansemacquire(uint32 *addr)
{
uint32 v;
while((v = *addr) > 0)
if(runtime·cas(addr, v, v-1))
return 1;
return 0;
}
// For now has no return value.
// Might return an ok (not interrupted) bool in the future?
void
runtime·semacquire(uint32 *addr)
{
Sema s;
// Easy case.
if(cansemacquire(addr))
return;
// Harder case:
// queue
// try semacquire one more time, sleep if failed
// dequeue
// wake up one more guy to avoid races (TODO(rsc): maybe unnecessary?)
semqueue(addr, &s);
for(;;) {
semsleep1(&s);
if(cansemacquire(addr)) {
semsleepundo1(&s);
break;
}
semsleep2(&s);
}
semdequeue(&s);
semwakeup(addr);
}
void
runtime·semrelease(uint32 *addr)
{
uint32 v;
for(;;) {
v = *addr;
if(runtime·cas(addr, v, v+1))
break;
}
semwakeup(addr);
}
func Semacquire(addr *uint32) {
runtime·semacquire(addr);
}
func Semrelease(addr *uint32) {
runtime·semrelease(addr);
}
|
