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 static int sig_done;
  47
  48 void internal_urcu_lock(void)
  49 {
  50         int ret;
  51         ret = pthread_mutex_lock(&urcu_mutex);
  52         if (ret) {
  53                 perror("Error in pthread mutex lock");
  54                 exit(-1);
  55         }
  56 }
  57
  58 void internal_urcu_unlock(void)
  59 {
  60         int ret;
  61
  62         ret = pthread_mutex_unlock(&urcu_mutex);
  63         if (ret) {
  64                 perror("Error in pthread mutex unlock");
  65                 exit(-1);
  66         }
  67 }
  68
  69 /*
  70  * called with urcu_mutex held.
  71  */
  72 static void switch_next_urcu_qparity(void)
  73 {
  74         urcu_gp_ctr ^= RCU_GP_CTR_BIT;
  75 }
  76
  77 static void force_mb_all_threads(void)
  78 {
  79         struct reader_data *index;
  80         /*
  81          * Ask for each threads to execute a mb() so we can consider the
  82          * compiler barriers around rcu read lock as real memory barriers.
  83          */
  84         if (!reader_data)
  85                 return;
  86         debug_yield_write();
  87         sig_done = 0;
  88         debug_yield_write();
  89         mb();   /* write sig_done before sending the signals */
  90         debug_yield_write();
  91         for (index = reader_data; index < reader_data + num_readers; index++) {
  92                 pthread_kill(index->tid, SIGURCU);
  93                 debug_yield_write();
  94         }
  95         /*
  96          * Wait for sighandler (and thus mb()) to execute on every thread.
  97          * BUSY-LOOP.
  98          */
  99         while (sig_done < num_readers)
 100                 barrier();
 101         debug_yield_write();
 102         mb();   /* read sig_done before ending the barrier */
 103         debug_yield_write();
 104 }
 105
 106 void wait_for_quiescent_state(void)
 107 {
 108         struct reader_data *index;
 109
 110         if (!reader_data)
 111                 return;
 112         /* Wait for each thread urcu_active_readers count to become 0.
 113          */
 114         for (index = reader_data; index < reader_data + num_readers; index++) {
 115                 /*
 116                  * BUSY-LOOP.
 117                  */
 118                 while (rcu_old_gp_ongoing(index->urcu_active_readers))
 119                         barrier();
 120         }
 121         /*
 122          * Locally : read *index->urcu_active_readers before freeing old
 123          * pointer.
 124          * Remote (reader threads) : Order urcu_qparity update and other
 125          * thread's quiescent state counter read.
 126          */
 127         force_mb_all_threads();
 128 }
 129
 130 static void switch_qparity(void)
 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         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();
 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_unlock();
 155         debug_yield_write();
 156 }
 157
 158 void urcu_add_reader(pthread_t id)
 159 {
 160         struct reader_data *oldarray;
 161
 162         if (!reader_data) {
 163                 alloc_readers = INIT_NUM_THREADS;
 164                 num_readers = 0;
 165                 reader_data =
 166                         malloc(sizeof(struct reader_data) * alloc_readers);
 167         }
 168         if (alloc_readers < num_readers + 1) {
 169                 oldarray = reader_data;
 170                 reader_data = malloc(sizeof(struct reader_data)
 171                                 * (alloc_readers << 1));
 172                 memcpy(reader_data, oldarray,
 173                         sizeof(struct reader_data) * alloc_readers);
 174                 alloc_readers <<= 1;
 175                 free(oldarray);
 176         }
 177         reader_data[num_readers].tid = id;
 178         /* reference to the TLS of _this_ reader thread. */
 179         reader_data[num_readers].urcu_active_readers = &urcu_active_readers;
 180         num_readers++;
 181 }
 182
 183 /*
 184  * Never shrink (implementation limitation).
 185  * This is O(nb threads). Eventually use a hash table.
 186  */
 187 void urcu_remove_reader(pthread_t id)
 188 {
 189         struct reader_data *index;
 190
 191         assert(reader_data != NULL);
 192         for (index = reader_data; index < reader_data + num_readers; index++) {
 193                 if (pthread_equal(index->tid, id)) {
 194                         memcpy(index, &reader_data[num_readers - 1],
 195                                 sizeof(struct reader_data));
 196                         reader_data[num_readers - 1].tid = 0;
 197                         reader_data[num_readers - 1].urcu_active_readers = NULL;
 198                         num_readers--;
 199                         return;
 200                 }
 201         }
 202         /* Hrm not found, forgot to register ? */
 203         assert(0);
 204 }
 205
 206 void urcu_register_thread(void)
 207 {
 208         internal_urcu_lock();
 209         urcu_add_reader(pthread_self());
 210         internal_urcu_unlock();
 211 }
 212
 213 void urcu_unregister_thread(void)
 214 {
 215         internal_urcu_lock();
 216         urcu_remove_reader(pthread_self());
 217         internal_urcu_unlock();
 218 }
 219
 220 void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
 221 {
 222         mb();
 223         atomic_inc(&sig_done);
 224 }
 225
 226 void __attribute__((constructor)) urcu_init(void)
 227 {
 228         struct sigaction act;
 229         int ret;
 230
 231         act.sa_sigaction = sigurcu_handler;
 232         ret = sigaction(SIGURCU, &act, NULL);
 233         if (ret) {
 234                 perror("Error in sigaction");
 235                 exit(-1);
 236         }
 237 }
 238
 239 void __attribute__((destructor)) urcu_exit(void)
 240 {
 241         struct sigaction act;
 242         int ret;
 243
 244         ret = sigaction(SIGURCU, NULL, &act);
 245         if (ret) {
 246                 perror("Error in sigaction");
 247                 exit(-1);
 248         }
 249         assert(act.sa_sigaction == sigurcu_handler);
 250         free(reader_data);
 251 }