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 quiescent period parity */
  23 int urcu_qparity;
  24
  25 int __thread urcu_active_readers[2];
  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 int switch_next_urcu_qparity(void)
  69 {
  70         int old_parity = urcu_qparity;
  71         urcu_qparity = 1 - old_parity;
  72         return old_parity;
  73 }
  74
  75 static void force_mb_all_threads(void)
  76 {
  77         struct reader_data *index;
  78         /*
  79          * Ask for each threads to execute a mb() so we can consider the
  80          * compiler barriers around rcu read lock as real memory barriers.
  81          */
  82         if (!reader_data)
  83                 return;
  84         debug_yield_write();
  85         sig_done = 0;
  86         debug_yield_write();
  87         mb();   /* write sig_done before sending the signals */
  88         debug_yield_write();
  89         for (index = reader_data; index < reader_data + num_readers; index++) {
  90                 pthread_kill(index->tid, SIGURCU);
  91                 debug_yield_write();
  92         }
  93         /*
  94          * Wait for sighandler (and thus mb()) to execute on every thread.
  95          * BUSY-LOOP.
  96          */
  97         while (sig_done < num_readers)
  98                 barrier();
  99         debug_yield_write();
 100         mb();   /* read sig_done before ending the barrier */
 101         debug_yield_write();
 102 }
 103
 104 void wait_for_quiescent_state(int parity)
 105 {
 106         struct reader_data *index;
 107
 108         if (!reader_data)
 109                 return;
 110         /* Wait for each thread urcu_active_readers count to become 0.
 111          */
 112         for (index = reader_data; index < reader_data + num_readers; index++) {
 113                 /*
 114                  * BUSY-LOOP.
 115                  */
 116                 while (index->urcu_active_readers[parity] != 0)
 117                         barrier();
 118         }
 119         /*
 120          * Locally : read *index->urcu_active_readers before freeing old
 121          * pointer.
 122          * Remote (reader threads) : Order urcu_qparity update and other
 123          * thread's quiescent state counter read.
 124          */
 125         force_mb_all_threads();
 126 }
 127
 128 static void switch_qparity(void)
 129 {
 130         int prev_parity;
 131
 132         /* All threads should read qparity before accessing data structure. */
 133         /* Write ptr before changing the qparity */
 134         force_mb_all_threads();
 135         debug_yield_write();
 136         prev_parity = switch_next_urcu_qparity();
 137         debug_yield_write();
 138
 139         /*
 140          * Wait for previous parity to be empty of readers.
 141          */
 142         wait_for_quiescent_state(prev_parity);
 143 }
 144
 145 void synchronize_rcu(void)
 146 {
 147         debug_yield_write();
 148         internal_urcu_lock();
 149         debug_yield_write();
 150         switch_qparity();
 151         debug_yield_write();
 152         switch_qparity();
 153         debug_yield_write();
 154         internal_urcu_lock();
 155         debug_yield_write();
 156 }
 157
 158 /*
 159  * Return old pointer, OK to free, no more reference exist.
 160  * Called under rcu_write_lock.
 161  */
 162 void *urcu_publish_content(void **ptr, void *new)
 163 {
 164         void *oldptr;
 165
 166         debug_yield_write();
 167         internal_urcu_lock();
 168         debug_yield_write();
 169         /*
 170          * We can publish the new pointer before we change the current qparity.
 171          * Readers seeing the new pointer while being in the previous qparity
 172          * window will make us wait until the end of the quiescent state before
 173          * we release the unrelated memory area. However, given we hold the
 174          * urcu_mutex, we are making sure that no further garbage collection can
 175          * occur until we release the mutex, therefore we guarantee that this
 176          * given reader will have completed its execution using the new pointer
 177          * when the next quiescent state window will be over.
 178          */
 179         oldptr = *ptr;
 180         debug_yield_write();
 181         *ptr = new;
 182
 183         debug_yield_write();
 184         switch_qparity();
 185         debug_yield_write();
 186         switch_qparity();
 187         debug_yield_write();
 188         internal_urcu_unlock();
 189         debug_yield_write();
 190
 191         return oldptr;
 192 }
 193
 194 void urcu_add_reader(pthread_t id)
 195 {
 196         struct reader_data *oldarray;
 197
 198         if (!reader_data) {
 199                 alloc_readers = INIT_NUM_THREADS;
 200                 num_readers = 0;
 201                 reader_data =
 202                         malloc(sizeof(struct reader_data) * alloc_readers);
 203         }
 204         if (alloc_readers < num_readers + 1) {
 205                 oldarray = reader_data;
 206                 reader_data = malloc(sizeof(struct reader_data)
 207                                 * (alloc_readers << 1));
 208                 memcpy(reader_data, oldarray,
 209                         sizeof(struct reader_data) * alloc_readers);
 210                 alloc_readers <<= 1;
 211                 free(oldarray);
 212         }
 213         reader_data[num_readers].tid = id;
 214         /* reference to the TLS of _this_ reader thread. */
 215         reader_data[num_readers].urcu_active_readers = urcu_active_readers;
 216         num_readers++;
 217 }
 218
 219 /*
 220  * Never shrink (implementation limitation).
 221  * This is O(nb threads). Eventually use a hash table.
 222  */
 223 void urcu_remove_reader(pthread_t id)
 224 {
 225         struct reader_data *index;
 226
 227         assert(reader_data != NULL);
 228         for (index = reader_data; index < reader_data + num_readers; index++) {
 229                 if (pthread_equal(index->tid, id)) {
 230                         memcpy(index, &reader_data[num_readers - 1],
 231                                 sizeof(struct reader_data));
 232                         reader_data[num_readers - 1].tid = 0;
 233                         reader_data[num_readers - 1].urcu_active_readers = NULL;
 234                         num_readers--;
 235                         return;
 236                 }
 237         }
 238         /* Hrm not found, forgot to register ? */
 239         assert(0);
 240 }
 241
 242 void urcu_register_thread(void)
 243 {
 244         internal_urcu_lock();
 245         urcu_add_reader(pthread_self());
 246         internal_urcu_unlock();
 247 }
 248
 249 void urcu_unregister_thread(void)
 250 {
 251         internal_urcu_lock();
 252         urcu_remove_reader(pthread_self());
 253         internal_urcu_unlock();
 254 }
 255
 256 void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
 257 {
 258         mb();
 259         atomic_inc(&sig_done);
 260 }
 261
 262 void __attribute__((constructor)) urcu_init(void)
 263 {
 264         struct sigaction act;
 265         int ret;
 266
 267         act.sa_sigaction = sigurcu_handler;
 268         ret = sigaction(SIGURCU, &act, NULL);
 269         if (ret) {
 270                 perror("Error in sigaction");
 271                 exit(-1);
 272         }
 273 }
 274
 275 void __attribute__((destructor)) urcu_exit(void)
 276 {
 277         struct sigaction act;
 278         int ret;
 279
 280         ret = sigaction(SIGURCU, NULL, &act);
 281         if (ret) {
 282                 perror("Error in sigaction");
 283                 exit(-1);
 284         }
 285         assert(act.sa_sigaction == sigurcu_handler);
 286         free(reader_data);
 287 }