Add barriers
[urcu.git] / urcu.c
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1/*
2 * urcu.c
3 *
4 * Userspace RCU library
5 *
6 * Copyright February 2009 - Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
7 *
8 * Distributed under GPLv2
9 */
10
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11#include <stdio.h>
12#include <pthread.h>
13#include <signal.h>
14#include <assert.h>
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15#include <stdlib.h>
16#include <string.h>
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17
18#include "urcu.h"
19
20pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
21
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22/*
23 * Global grace period counter.
24 * Contains the current RCU_GP_CTR_BIT.
25 * Also has a RCU_GP_CTR_BIT of 1, to accelerate the reader fast path.
26 */
27long urcu_gp_ctr = RCU_GP_COUNT;
27b012e2 28
6e8b8429 29long __thread urcu_active_readers;
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30
31/* Thread IDs of registered readers */
32#define INIT_NUM_THREADS 4
33
34struct reader_data {
35 pthread_t tid;
128166c9 36 long *urcu_active_readers;
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37};
38
cf380c2f 39#ifdef DEBUG_YIELD
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40unsigned int yield_active;
41unsigned int __thread rand_yield;
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42#endif
43
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44static struct reader_data *reader_data;
45static int num_readers, alloc_readers;
b715b99e 46#ifndef DEBUG_FULL_MB
27b012e2 47static int sig_done;
b715b99e 48#endif
27b012e2 49
c265818b 50void internal_urcu_lock(void)
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51{
52 int ret;
53 ret = pthread_mutex_lock(&urcu_mutex);
54 if (ret) {
55 perror("Error in pthread mutex lock");
56 exit(-1);
57 }
58}
59
c265818b 60void internal_urcu_unlock(void)
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61{
62 int ret;
63
64 ret = pthread_mutex_unlock(&urcu_mutex);
65 if (ret) {
66 perror("Error in pthread mutex unlock");
67 exit(-1);
68 }
69}
70
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71/*
72 * called with urcu_mutex held.
73 */
1430ee0b 74static void switch_next_urcu_qparity(void)
27b012e2 75{
1430ee0b 76 urcu_gp_ctr ^= RCU_GP_CTR_BIT;
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77}
78
bb488185 79#ifdef DEBUG_FULL_MB
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80static void force_mb_single_thread(pthread_t tid)
81{
82 smp_mb();
83}
84
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85static void force_mb_all_threads(void)
86{
b715b99e 87 smp_mb();
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88}
89#else
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90
91static void force_mb_single_thread(pthread_t tid)
92{
93 assert(reader_data);
94 sig_done = 0;
95 smp_mb(); /* write sig_done before sending the signals */
96 pthread_kill(tid, SIGURCU);
97 /*
98 * Wait for sighandler (and thus mb()) to execute on every thread.
99 * BUSY-LOOP.
100 */
101 while (sig_done < 1)
102 smp_rmb(); /* ensure we re-read sig-done */
103 smp_mb(); /* read sig_done before ending the barrier */
104}
105
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106static void force_mb_all_threads(void)
107{
f69f195a 108 struct reader_data *index;
27b012e2 109 /*
b715b99e 110 * Ask for each threads to execute a smp_mb() so we can consider the
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111 * compiler barriers around rcu read lock as real memory barriers.
112 */
113 if (!reader_data)
114 return;
27b012e2 115 sig_done = 0;
b715b99e 116 smp_mb(); /* write sig_done before sending the signals */
ae878d0d 117 for (index = reader_data; index < reader_data + num_readers; index++)
f69f195a 118 pthread_kill(index->tid, SIGURCU);
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119 /*
120 * Wait for sighandler (and thus mb()) to execute on every thread.
121 * BUSY-LOOP.
122 */
123 while (sig_done < num_readers)
40e140c9 124 smp_rmb(); /* ensure we re-read sig-done */
b715b99e 125 smp_mb(); /* read sig_done before ending the barrier */
27b012e2 126}
bb488185 127#endif
27b012e2 128
1430ee0b 129void wait_for_quiescent_state(void)
27b012e2 130{
f69f195a 131 struct reader_data *index;
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132
133 if (!reader_data)
134 return;
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135 /*
136 * Wait for each thread urcu_active_readers count to become 0.
27b012e2 137 */
f69f195a 138 for (index = reader_data; index < reader_data + num_readers; index++) {
40e140c9 139 int wait_loops = 0;
27b012e2 140 /*
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141 * BUSY-LOOP. Force the reader thread to commit its
142 * urcu_active_readers update to memory if we wait for too long.
27b012e2 143 */
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144 while (rcu_old_gp_ongoing(index->urcu_active_readers)) {
145 if (wait_loops++ == KICK_READER_LOOPS) {
146 force_mb_single_thread(index->tid);
147 wait_loops = 0;
148 }
149 }
27b012e2 150 }
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151}
152
9598a481 153void synchronize_rcu(void)
2bc59bd7 154{
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155 /* All threads should read qparity before accessing data structure
156 * where new ptr points to. */
157 /* Write new ptr before changing the qparity */
2bc59bd7 158 force_mb_all_threads();
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159
160 internal_urcu_lock();
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161
162 switch_next_urcu_qparity(); /* 0 -> 1 */
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163
164 /*
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165 * Must commit qparity update to memory before waiting for parity
166 * 0 quiescent state. Failure to do so could result in the writer
167 * waiting forever while new readers are always accessing data (no
168 * progress).
2bc59bd7 169 */
b715b99e 170 smp_mb();
2bc59bd7 171
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172 /*
173 * Wait for previous parity to be empty of readers.
174 */
175 wait_for_quiescent_state(); /* Wait readers in parity 0 */
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176
177 /*
178 * Must finish waiting for quiescent state for parity 0 before
179 * committing qparity update to memory. Failure to do so could result in
180 * the writer waiting forever while new readers are always accessing
181 * data (no progress).
182 */
b715b99e 183 smp_mb();
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184
185 switch_next_urcu_qparity(); /* 1 -> 0 */
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186
187 /*
188 * Must commit qparity update to memory before waiting for parity
189 * 1 quiescent state. Failure to do so could result in the writer
190 * waiting forever while new readers are always accessing data (no
191 * progress).
192 */
b715b99e 193 smp_mb();
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194
195 /*
196 * Wait for previous parity to be empty of readers.
197 */
198 wait_for_quiescent_state(); /* Wait readers in parity 1 */
9598a481 199
e7b43771 200 internal_urcu_unlock();
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201
202 /* All threads should finish using the data referred to by old ptr
203 * before decrementing their urcu_active_readers count */
204 /* Finish waiting for reader threads before letting the old ptr being
205 * freed. */
206 force_mb_all_threads();
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207}
208
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209void urcu_add_reader(pthread_t id)
210{
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211 struct reader_data *oldarray;
212
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213 if (!reader_data) {
214 alloc_readers = INIT_NUM_THREADS;
f69f195a 215 num_readers = 0;
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216 reader_data =
217 malloc(sizeof(struct reader_data) * alloc_readers);
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218 }
219 if (alloc_readers < num_readers + 1) {
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220 oldarray = reader_data;
221 reader_data = malloc(sizeof(struct reader_data)
222 * (alloc_readers << 1));
223 memcpy(reader_data, oldarray,
224 sizeof(struct reader_data) * alloc_readers);
225 alloc_readers <<= 1;
226 free(oldarray);
227 }
228 reader_data[num_readers].tid = id;
229 /* reference to the TLS of _this_ reader thread. */
1430ee0b 230 reader_data[num_readers].urcu_active_readers = &urcu_active_readers;
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231 num_readers++;
232}
233
234/*
235 * Never shrink (implementation limitation).
236 * This is O(nb threads). Eventually use a hash table.
237 */
238void urcu_remove_reader(pthread_t id)
239{
240 struct reader_data *index;
241
242 assert(reader_data != NULL);
243 for (index = reader_data; index < reader_data + num_readers; index++) {
e6d6e2dc 244 if (pthread_equal(index->tid, id)) {
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245 memcpy(index, &reader_data[num_readers - 1],
246 sizeof(struct reader_data));
247 reader_data[num_readers - 1].tid = 0;
248 reader_data[num_readers - 1].urcu_active_readers = NULL;
249 num_readers--;
250 return;
251 }
252 }
253 /* Hrm not found, forgot to register ? */
254 assert(0);
255}
256
257void urcu_register_thread(void)
258{
c265818b 259 internal_urcu_lock();
41718ff9 260 urcu_add_reader(pthread_self());
c265818b 261 internal_urcu_unlock();
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262}
263
f69f195a 264void urcu_unregister_thread(void)
27b012e2 265{
c265818b 266 internal_urcu_lock();
41718ff9 267 urcu_remove_reader(pthread_self());
c265818b 268 internal_urcu_unlock();
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269}
270
bb488185 271#ifndef DEBUG_FULL_MB
f69f195a 272void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
27b012e2 273{
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274 /*
275 * Executing this smp_mb() is the only purpose of this signal handler.
276 * It punctually promotes barrier() into smp_mb() on every thread it is
277 * executed on.
278 */
b715b99e 279 smp_mb();
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280 atomic_inc(&sig_done);
281}
282
283void __attribute__((constructor)) urcu_init(void)
284{
285 struct sigaction act;
286 int ret;
287
288 act.sa_sigaction = sigurcu_handler;
289 ret = sigaction(SIGURCU, &act, NULL);
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290 if (ret) {
291 perror("Error in sigaction");
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292 exit(-1);
293 }
294}
295
296void __attribute__((destructor)) urcu_exit(void)
297{
298 struct sigaction act;
299 int ret;
300
301 ret = sigaction(SIGURCU, NULL, &act);
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302 if (ret) {
303 perror("Error in sigaction");
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304 exit(-1);
305 }
306 assert(act.sa_sigaction == sigurcu_handler);
307 free(reader_data);
308}
bb488185 309#endif
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