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
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
|
/* $Id$ */
/***
This file is part of PulseAudio.
Copyright 2006 Lennart Poettering
PulseAudio is free software; you can redistribute it and/or modify
it under the terms of the GNU Lesser General Public License as
published by the Free Software Foundation; either version 2.1 of the
License, or (at your option) any later version.
PulseAudio is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with PulseAudio; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307
USA.
***/
#ifdef HAVE_CONFIG_H
#include <config.h>
#endif
#include <pulse/xmalloc.h>
#include <pulsecore/atomic.h>
#include <pulsecore/log.h>
#include <pulsecore/thread.h>
#include <pulsecore/macro.h>
#include <pulsecore/core-util.h>
#include <pulsecore/macro.h>
#include "flist.h"
/* Algorithm is not perfect, in a few corner cases it will fail to pop
* from the flist although it isn't empty, and fail to push into the
* flist, although it isn't full.
*
* We keep a fixed size array of entries, each item is either marked
* UNUSED, USED or BUSY and contains a user data pointer. When pushing
* into the queue we look for an UNUSED cell and mark it BUSY with a
* CAS operation. If successful we use it and mark it USED, otherwise
* we go on and look for the next UNUSED cell. The algorithm for
* popping an item from the queue is practically inverse: look for a
* USED cell and and mark it BUSY with a CAS operation, after reading
* from it mark it UNUSED again.
*
* To accelerate finding of used/unused cells we maintain a read and a
* write index which is used like a ring buffer. After each push we
* increase the write index and after each pop we increase the read
* index.
*
* The indexes are incremented atomically and are never truncated to
* the buffer size. Instead we assume that the buffer size is a power
* of two and that the truncation can thus be done by applying a
* simple AND on read.
*
* To make sure that we do not look for empty cells indefinitely we
* maintain a length value which stores the number of used cells. From
* this value the number of unused cells is easily calculated. Please
* note that the length value is not updated atomically with the read
* and write index and might thus be a few cells off the real
* value. To deal with this we always look for N_EXTRA_SCAN extra
* cells when pushing/popping entries.
*
* It might make sense to replace this implementation with a link list
* stack or queue, which however requires DCAS to be simple. Patches
* welcome.
*
* Please note that this algorithm is home grown.*/
#define FLIST_SIZE 128
#define N_EXTRA_SCAN 2
/* For debugging purposes we can define _Y to put and extra thread
* yield between each operation. */
#ifdef PROFILE
#define _Y pa_thread_yield()
#else
#define _Y do { } while(0)
#endif
enum {
STATE_UNUSED,
STATE_USED,
STATE_BUSY
};
struct cell {
pa_atomic_t state;
void *data;
};
struct pa_flist {
unsigned size;
pa_atomic_t length;
pa_atomic_t read_idx;
pa_atomic_t write_idx;
};
#define PA_FLIST_CELLS(x) ((struct cell*) ((uint8_t*) (x) + PA_ALIGN(sizeof(struct pa_flist))))
pa_flist *pa_flist_new(unsigned size) {
pa_flist *l;
if (!size)
size = FLIST_SIZE;
pa_assert(pa_is_power_of_two(size));
l = pa_xmalloc0(PA_ALIGN(sizeof(pa_flist)) + (sizeof(struct cell) * size));
l->size = size;
pa_atomic_store(&l->read_idx, 0);
pa_atomic_store(&l->write_idx, 0);
pa_atomic_store(&l->length, 0);
return l;
}
static int reduce(pa_flist *l, int value) {
return value & (unsigned) (l->size - 1);
}
void pa_flist_free(pa_flist *l, pa_free_cb_t free_cb) {
pa_assert(l);
if (free_cb) {
struct cell *cells;
int len, idx;
cells = PA_FLIST_CELLS(l);
idx = reduce(l, pa_atomic_load(&l->read_idx));
len = pa_atomic_load(&l->length);
for (; len > 0; len--) {
if (pa_atomic_load(&cells[idx].state) == STATE_USED)
free_cb(cells[idx].data);
idx = reduce(l, idx + 1);
}
}
pa_xfree(l);
}
int pa_flist_push(pa_flist*l, void *p) {
int idx, len, n;
struct cell *cells;
pa_assert(l);
pa_assert(p);
cells = PA_FLIST_CELLS(l);
n = len = (int) l->size - pa_atomic_load(&l->length) + N_EXTRA_SCAN;
_Y;
idx = reduce(l, pa_atomic_load(&l->write_idx));
for (; n > 0 ; n--) {
_Y;
if (pa_atomic_cmpxchg(&cells[idx].state, STATE_UNUSED, STATE_BUSY)) {
_Y;
pa_atomic_inc(&l->write_idx);
_Y;
cells[idx].data = p;
_Y;
pa_atomic_store(&cells[idx].state, STATE_USED);
_Y;
pa_atomic_inc(&l->length);
return 0;
}
_Y;
idx = reduce(l, idx + 1);
}
#ifdef PROFILE
if (len > N_EXTRA_SCAN)
pa_log_warn("Didn't find free cell after %u iterations.", len);
#endif
return -1;
}
void* pa_flist_pop(pa_flist*l) {
int idx, len, n;
struct cell *cells;
pa_assert(l);
cells = PA_FLIST_CELLS(l);
n = len = pa_atomic_load(&l->length) + N_EXTRA_SCAN;
_Y;
idx = reduce(l, pa_atomic_load(&l->read_idx));
for (; n > 0 ; n--) {
_Y;
if (pa_atomic_cmpxchg(&cells[idx].state, STATE_USED, STATE_BUSY)) {
void *p;
_Y;
pa_atomic_inc(&l->read_idx);
_Y;
p = cells[idx].data;
_Y;
pa_atomic_store(&cells[idx].state, STATE_UNUSED);
_Y;
pa_atomic_dec(&l->length);
return p;
}
_Y;
idx = reduce(l, idx+1);
}
#ifdef PROFILE
if (len > N_EXTRA_SCAN)
pa_log_warn("Didn't find used cell after %u iterations.", len);
#endif
return NULL;
}
|