8cb32a117bb065405ca3f45e0224dfe8fbebbf5d
[urcu.git] / urcu.c
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
11 #include <stdio.h>
12 #include <pthread.h>
13 #include <signal.h>
14 #include <assert.h>
15 #include <stdlib.h>
16 #include <string.h>
17
18 #include "urcu.h"
19
20 pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
21
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 */
27 long urcu_gp_ctr = RCU_GP_COUNT;
28
29 long __thread urcu_active_readers;
30
31 /* Thread IDs of registered readers */
32 #define INIT_NUM_THREADS 4
33
34 struct reader_data {
35 pthread_t tid;
36 long *urcu_active_readers;
37 };
38
39 #ifdef DEBUG_YIELD
40 unsigned int yield_active;
41 unsigned int __thread rand_yield;
42 #endif
43
44 static struct reader_data *reader_data;
45 static int num_readers, alloc_readers;
46 #ifndef DEBUG_FULL_MB
47 static int sig_done;
48 #endif
49
50 void internal_urcu_lock(void)
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
60 void internal_urcu_unlock(void)
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
71 /*
72 * called with urcu_mutex held.
73 */
74 static void switch_next_urcu_qparity(void)
75 {
76 urcu_gp_ctr ^= RCU_GP_CTR_BIT;
77 }
78
79 #ifdef DEBUG_FULL_MB
80 static void force_mb_single_thread(pthread_t tid)
81 {
82 smp_mb();
83 }
84
85 static void force_mb_all_threads(void)
86 {
87 smp_mb();
88 }
89 #else
90
91 static void force_mb_single_thread(pthread_t tid)
92 {
93 assert(reader_data);
94 sig_done = 0;
95 /*
96 * pthread_kill has a smp_mb(). But beware, we assume it performs
97 * a cache flush on architectures with non-coherent cache. Let's play
98 * safe and don't assume anything : we use smp_mc() to make sure the
99 * cache flush is enforced.
100 * smp_mb(); write sig_done before sending the signals
101 */
102 smp_mc(); /* write sig_done before sending the signals */
103 pthread_kill(tid, SIGURCU);
104 /*
105 * Wait for sighandler (and thus mb()) to execute on every thread.
106 * BUSY-LOOP.
107 */
108 while (LOAD_SHARED(sig_done) < 1)
109 cpu_relax();
110 smp_mb(); /* read sig_done before ending the barrier */
111 }
112
113 static void force_mb_all_threads(void)
114 {
115 struct reader_data *index;
116 /*
117 * Ask for each threads to execute a smp_mb() so we can consider the
118 * compiler barriers around rcu read lock as real memory barriers.
119 */
120 if (!reader_data)
121 return;
122 sig_done = 0;
123 /*
124 * pthread_kill has a smp_mb(). But beware, we assume it performs
125 * a cache flush on architectures with non-coherent cache. Let's play
126 * safe and don't assume anything : we use smp_mc() to make sure the
127 * cache flush is enforced.
128 * smp_mb(); write sig_done before sending the signals
129 */
130 smp_mc(); /* write sig_done before sending the signals */
131 for (index = reader_data; index < reader_data + num_readers; index++)
132 pthread_kill(index->tid, SIGURCU);
133 /*
134 * Wait for sighandler (and thus mb()) to execute on every thread.
135 * BUSY-LOOP.
136 */
137 while (LOAD_SHARED(sig_done) < num_readers)
138 cpu_relax();
139 smp_mb(); /* read sig_done before ending the barrier */
140 }
141 #endif
142
143 void wait_for_quiescent_state(void)
144 {
145 struct reader_data *index;
146
147 if (!reader_data)
148 return;
149 /*
150 * Wait for each thread urcu_active_readers count to become 0.
151 */
152 for (index = reader_data; index < reader_data + num_readers; index++) {
153 int wait_loops = 0;
154 /*
155 * BUSY-LOOP. Force the reader thread to commit its
156 * urcu_active_readers update to memory if we wait for too long.
157 */
158 while (rcu_old_gp_ongoing(index->urcu_active_readers)) {
159 if (wait_loops++ == KICK_READER_LOOPS) {
160 force_mb_single_thread(index->tid);
161 wait_loops = 0;
162 } else {
163 cpu_relax();
164 }
165 }
166 }
167 }
168
169 void synchronize_rcu(void)
170 {
171 internal_urcu_lock();
172
173 /* All threads should read qparity before accessing data structure
174 * where new ptr points to. Must be done within internal_urcu_lock
175 * because it iterates on reader threads.*/
176 /* Write new ptr before changing the qparity */
177 force_mb_all_threads();
178
179 switch_next_urcu_qparity(); /* 0 -> 1 */
180
181 /*
182 * Must commit qparity update to memory before waiting for parity
183 * 0 quiescent state. Failure to do so could result in the writer
184 * waiting forever while new readers are always accessing data (no
185 * progress).
186 */
187 smp_mc();
188
189 /*
190 * Wait for previous parity to be empty of readers.
191 */
192 wait_for_quiescent_state(); /* Wait readers in parity 0 */
193
194 /*
195 * Must finish waiting for quiescent state for parity 0 before
196 * committing qparity update to memory. Failure to do so could result in
197 * the writer waiting forever while new readers are always accessing
198 * data (no progress).
199 */
200 smp_mc();
201
202 switch_next_urcu_qparity(); /* 1 -> 0 */
203
204 /*
205 * Must commit qparity update to memory before waiting for parity
206 * 1 quiescent state. Failure to do so could result in the writer
207 * waiting forever while new readers are always accessing data (no
208 * progress).
209 */
210 smp_mc();
211
212 /*
213 * Wait for previous parity to be empty of readers.
214 */
215 wait_for_quiescent_state(); /* Wait readers in parity 1 */
216
217 /* Finish waiting for reader threads before letting the old ptr being
218 * freed. Must be done within internal_urcu_lock because it iterates on
219 * reader threads. */
220 force_mb_all_threads();
221
222 internal_urcu_unlock();
223 }
224
225 void urcu_add_reader(pthread_t id)
226 {
227 struct reader_data *oldarray;
228
229 if (!reader_data) {
230 alloc_readers = INIT_NUM_THREADS;
231 num_readers = 0;
232 reader_data =
233 malloc(sizeof(struct reader_data) * alloc_readers);
234 }
235 if (alloc_readers < num_readers + 1) {
236 oldarray = reader_data;
237 reader_data = malloc(sizeof(struct reader_data)
238 * (alloc_readers << 1));
239 memcpy(reader_data, oldarray,
240 sizeof(struct reader_data) * alloc_readers);
241 alloc_readers <<= 1;
242 free(oldarray);
243 }
244 reader_data[num_readers].tid = id;
245 /* reference to the TLS of _this_ reader thread. */
246 reader_data[num_readers].urcu_active_readers = &urcu_active_readers;
247 num_readers++;
248 }
249
250 /*
251 * Never shrink (implementation limitation).
252 * This is O(nb threads). Eventually use a hash table.
253 */
254 void urcu_remove_reader(pthread_t id)
255 {
256 struct reader_data *index;
257
258 assert(reader_data != NULL);
259 for (index = reader_data; index < reader_data + num_readers; index++) {
260 if (pthread_equal(index->tid, id)) {
261 memcpy(index, &reader_data[num_readers - 1],
262 sizeof(struct reader_data));
263 reader_data[num_readers - 1].tid = 0;
264 reader_data[num_readers - 1].urcu_active_readers = NULL;
265 num_readers--;
266 return;
267 }
268 }
269 /* Hrm not found, forgot to register ? */
270 assert(0);
271 }
272
273 void urcu_register_thread(void)
274 {
275 internal_urcu_lock();
276 urcu_add_reader(pthread_self());
277 internal_urcu_unlock();
278 }
279
280 void urcu_unregister_thread(void)
281 {
282 internal_urcu_lock();
283 urcu_remove_reader(pthread_self());
284 internal_urcu_unlock();
285 }
286
287 #ifndef DEBUG_FULL_MB
288 void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
289 {
290 /*
291 * Executing this smp_mb() is the only purpose of this signal handler.
292 * It punctually promotes barrier() into smp_mb() on every thread it is
293 * executed on.
294 */
295 smp_mb();
296 atomic_inc(&sig_done);
297 }
298
299 void __attribute__((constructor)) urcu_init(void)
300 {
301 struct sigaction act;
302 int ret;
303
304 act.sa_sigaction = sigurcu_handler;
305 ret = sigaction(SIGURCU, &act, NULL);
306 if (ret) {
307 perror("Error in sigaction");
308 exit(-1);
309 }
310 }
311
312 void __attribute__((destructor)) urcu_exit(void)
313 {
314 struct sigaction act;
315 int ret;
316
317 ret = sigaction(SIGURCU, NULL, &act);
318 if (ret) {
319 perror("Error in sigaction");
320 exit(-1);
321 }
322 assert(act.sa_sigaction == sigurcu_handler);
323 free(reader_data);
324 }
325 #endif
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