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 /* Global grace period counter */
  23 int urcu_gp_ctr;
  24
  25 int __thread urcu_active_readers;
  26
  27 /* Thread IDs of registered readers */
  28 #define INIT_NUM_THREADS 4
  29
  30 struct reader_data {
  31         pthread_t tid;
  32         int *urcu_active_readers;
  33 };
  34
  35 #ifdef DEBUG_YIELD
  36 unsigned int yield_active;
  37 unsigned int __thread rand_yield;
  38 #endif
  39
  40 static struct reader_data *reader_data;
  41 static int num_readers, alloc_readers;
  42 static int sig_done;
  43
  44 void internal_urcu_lock(void)
  45 {
  46         int ret;
  47         ret = pthread_mutex_lock(&urcu_mutex);
  48         if (ret) {
  49                 perror("Error in pthread mutex lock");
  50                 exit(-1);
  51         }
  52 }
  53
  54 void internal_urcu_unlock(void)
  55 {
  56         int ret;
  57
  58         ret = pthread_mutex_unlock(&urcu_mutex);
  59         if (ret) {
  60                 perror("Error in pthread mutex unlock");
  61                 exit(-1);
  62         }
  63 }
  64
  65 /*
  66  * called with urcu_mutex held.
  67  */
  68 static void switch_next_urcu_qparity(void)
  69 {
  70         urcu_gp_ctr ^= RCU_GP_CTR_BIT;
  71 }
  72
  73 static void force_mb_all_threads(void)
  74 {
  75         struct reader_data *index;
  76         /*
  77          * Ask for each threads to execute a mb() so we can consider the
  78          * compiler barriers around rcu read lock as real memory barriers.
  79          */
  80         if (!reader_data)
  81                 return;
  82         debug_yield_write();
  83         sig_done = 0;
  84         debug_yield_write();
  85         mb();   /* write sig_done before sending the signals */
  86         debug_yield_write();
  87         for (index = reader_data; index < reader_data + num_readers; index++) {
  88                 pthread_kill(index->tid, SIGURCU);
  89                 debug_yield_write();
  90         }
  91         /*
  92          * Wait for sighandler (and thus mb()) to execute on every thread.
  93          * BUSY-LOOP.
  94          */
  95         while (sig_done < num_readers)
  96                 barrier();
  97         debug_yield_write();
  98         mb();   /* read sig_done before ending the barrier */
  99         debug_yield_write();
 100 }
 101
 102 void wait_for_quiescent_state(void)
 103 {
 104         struct reader_data *index;
 105
 106         if (!reader_data)
 107                 return;
 108         /* Wait for each thread urcu_active_readers count to become 0.
 109          */
 110         for (index = reader_data; index < reader_data + num_readers; index++) {
 111                 /*
 112                  * BUSY-LOOP.
 113                  */
 114                 while (rcu_old_gp_ongoing(index->urcu_active_readers))
 115                         barrier();
 116         }
 117         /*
 118          * Locally : read *index->urcu_active_readers before freeing old
 119          * pointer.
 120          * Remote (reader threads) : Order urcu_qparity update and other
 121          * thread's quiescent state counter read.
 122          */
 123         force_mb_all_threads();
 124 }
 125
 126 static void switch_qparity(void)
 127 {
 128         /* All threads should read qparity before accessing data structure. */
 129         /* Write ptr before changing the qparity */
 130         force_mb_all_threads();
 131         debug_yield_write();
 132         switch_next_urcu_qparity();
 133         debug_yield_write();
 134
 135         /*
 136          * Wait for previous parity to be empty of readers.
 137          */
 138         wait_for_quiescent_state();
 139 }
 140
 141 void synchronize_rcu(void)
 142 {
 143         debug_yield_write();
 144         internal_urcu_lock();
 145         debug_yield_write();
 146         switch_qparity();
 147         debug_yield_write();
 148         switch_qparity();
 149         debug_yield_write();
 150         internal_urcu_lock();
 151         debug_yield_write();
 152 }
 153
 154 /*
 155  * Return old pointer, OK to free, no more reference exist.
 156  * Called under rcu_write_lock.
 157  */
 158 void *urcu_publish_content(void **ptr, void *new)
 159 {
 160         void *oldptr;
 161
 162         debug_yield_write();
 163         internal_urcu_lock();
 164         debug_yield_write();
 165         /*
 166          * We can publish the new pointer before we change the current qparity.
 167          * Readers seeing the new pointer while being in the previous qparity
 168          * window will make us wait until the end of the quiescent state before
 169          * we release the unrelated memory area. However, given we hold the
 170          * urcu_mutex, we are making sure that no further garbage collection can
 171          * occur until we release the mutex, therefore we guarantee that this
 172          * given reader will have completed its execution using the new pointer
 173          * when the next quiescent state window will be over.
 174          */
 175         oldptr = *ptr;
 176         debug_yield_write();
 177         *ptr = new;
 178
 179         debug_yield_write();
 180         switch_qparity();
 181         debug_yield_write();
 182         switch_qparity();
 183         debug_yield_write();
 184         internal_urcu_unlock();
 185         debug_yield_write();
 186
 187         return oldptr;
 188 }
 189
 190 void urcu_add_reader(pthread_t id)
 191 {
 192         struct reader_data *oldarray;
 193
 194         if (!reader_data) {
 195                 alloc_readers = INIT_NUM_THREADS;
 196                 num_readers = 0;
 197                 reader_data =
 198                         malloc(sizeof(struct reader_data) * alloc_readers);
 199         }
 200         if (alloc_readers < num_readers + 1) {
 201                 oldarray = reader_data;
 202                 reader_data = malloc(sizeof(struct reader_data)
 203                                 * (alloc_readers << 1));
 204                 memcpy(reader_data, oldarray,
 205                         sizeof(struct reader_data) * alloc_readers);
 206                 alloc_readers <<= 1;
 207                 free(oldarray);
 208         }
 209         reader_data[num_readers].tid = id;
 210         /* reference to the TLS of _this_ reader thread. */
 211         reader_data[num_readers].urcu_active_readers = &urcu_active_readers;
 212         num_readers++;
 213 }
 214
 215 /*
 216  * Never shrink (implementation limitation).
 217  * This is O(nb threads). Eventually use a hash table.
 218  */
 219 void urcu_remove_reader(pthread_t id)
 220 {
 221         struct reader_data *index;
 222
 223         assert(reader_data != NULL);
 224         for (index = reader_data; index < reader_data + num_readers; index++) {
 225                 if (pthread_equal(index->tid, id)) {
 226                         memcpy(index, &reader_data[num_readers - 1],
 227                                 sizeof(struct reader_data));
 228                         reader_data[num_readers - 1].tid = 0;
 229                         reader_data[num_readers - 1].urcu_active_readers = NULL;
 230                         num_readers--;
 231                         return;
 232                 }
 233         }
 234         /* Hrm not found, forgot to register ? */
 235         assert(0);
 236 }
 237
 238 void urcu_register_thread(void)
 239 {
 240         internal_urcu_lock();
 241         urcu_add_reader(pthread_self());
 242         internal_urcu_unlock();
 243 }
 244
 245 void urcu_unregister_thread(void)
 246 {
 247         internal_urcu_lock();
 248         urcu_remove_reader(pthread_self());
 249         internal_urcu_unlock();
 250 }
 251
 252 void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
 253 {
 254         mb();
 255         atomic_inc(&sig_done);
 256 }
 257
 258 void __attribute__((constructor)) urcu_init(void)
 259 {
 260         struct sigaction act;
 261         int ret;
 262
 263         act.sa_sigaction = sigurcu_handler;
 264         ret = sigaction(SIGURCU, &act, NULL);
 265         if (ret) {
 266                 perror("Error in sigaction");
 267                 exit(-1);
 268         }
 269 }
 270
 271 void __attribute__((destructor)) urcu_exit(void)
 272 {
 273         struct sigaction act;
 274         int ret;
 275
 276         ret = sigaction(SIGURCU, NULL, &act);
 277         if (ret) {
 278                 perror("Error in sigaction");
 279                 exit(-1);
 280         }
 281         assert(act.sa_sigaction == sigurcu_handler);
 282         free(reader_data);
 283 }