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 #ifdef DEBUG_FULL_MB
  78 static void force_mb_all_threads(void)
  79 {
  80         mb();
  81 }
  82 #else
  83 static void force_mb_all_threads(void)
  84 {
  85         struct reader_data *index;
  86         /*
  87          * Ask for each threads to execute a mb() so we can consider the
  88          * compiler barriers around rcu read lock as real memory barriers.
  89          */
  90         if (!reader_data)
  91                 return;
  92         debug_yield_write();
  93         sig_done = 0;
  94         debug_yield_write();
  95         mb();   /* write sig_done before sending the signals */
  96         debug_yield_write();
  97         for (index = reader_data; index < reader_data + num_readers; index++) {
  98                 pthread_kill(index->tid, SIGURCU);
  99                 debug_yield_write();
 100         }
 101         /*
 102          * Wait for sighandler (and thus mb()) to execute on every thread.
 103          * BUSY-LOOP.
 104          */
 105         while (sig_done < num_readers)
 106                 barrier();
 107         debug_yield_write();
 108         mb();   /* read sig_done before ending the barrier */
 109         debug_yield_write();
 110 }
 111 #endif
 112
 113 void wait_for_quiescent_state(void)
 114 {
 115         struct reader_data *index;
 116
 117         if (!reader_data)
 118                 return;
 119         /* Wait for each thread urcu_active_readers count to become 0.
 120          */
 121         for (index = reader_data; index < reader_data + num_readers; index++) {
 122                 /*
 123                  * BUSY-LOOP.
 124                  */
 125                 while (rcu_old_gp_ongoing(index->urcu_active_readers))
 126                         barrier();
 127         }
 128         /*
 129          * Locally : read *index->urcu_active_readers before freeing old
 130          * pointer.
 131          * Remote (reader threads) : Order urcu_qparity update and other
 132          * thread's quiescent state counter read.
 133          */
 134         force_mb_all_threads();
 135 }
 136
 137 static void switch_qparity(void)
 138 {
 139         /* All threads should read qparity before accessing data structure. */
 140         /* Write ptr before changing the qparity */
 141         force_mb_all_threads();
 142         debug_yield_write();
 143         switch_next_urcu_qparity();
 144         debug_yield_write();
 145
 146         /*
 147          * Wait for previous parity to be empty of readers.
 148          */
 149         wait_for_quiescent_state();
 150 }
 151
 152 void synchronize_rcu(void)
 153 {
 154         debug_yield_write();
 155         internal_urcu_lock();
 156         debug_yield_write();
 157         switch_qparity();
 158         debug_yield_write();
 159         switch_qparity();
 160         debug_yield_write();
 161         internal_urcu_unlock();
 162         debug_yield_write();
 163 }
 164
 165 void urcu_add_reader(pthread_t id)
 166 {
 167         struct reader_data *oldarray;
 168
 169         if (!reader_data) {
 170                 alloc_readers = INIT_NUM_THREADS;
 171                 num_readers = 0;
 172                 reader_data =
 173                         malloc(sizeof(struct reader_data) * alloc_readers);
 174         }
 175         if (alloc_readers < num_readers + 1) {
 176                 oldarray = reader_data;
 177                 reader_data = malloc(sizeof(struct reader_data)
 178                                 * (alloc_readers << 1));
 179                 memcpy(reader_data, oldarray,
 180                         sizeof(struct reader_data) * alloc_readers);
 181                 alloc_readers <<= 1;
 182                 free(oldarray);
 183         }
 184         reader_data[num_readers].tid = id;
 185         /* reference to the TLS of _this_ reader thread. */
 186         reader_data[num_readers].urcu_active_readers = &urcu_active_readers;
 187         num_readers++;
 188 }
 189
 190 /*
 191  * Never shrink (implementation limitation).
 192  * This is O(nb threads). Eventually use a hash table.
 193  */
 194 void urcu_remove_reader(pthread_t id)
 195 {
 196         struct reader_data *index;
 197
 198         assert(reader_data != NULL);
 199         for (index = reader_data; index < reader_data + num_readers; index++) {
 200                 if (pthread_equal(index->tid, id)) {
 201                         memcpy(index, &reader_data[num_readers - 1],
 202                                 sizeof(struct reader_data));
 203                         reader_data[num_readers - 1].tid = 0;
 204                         reader_data[num_readers - 1].urcu_active_readers = NULL;
 205                         num_readers--;
 206                         return;
 207                 }
 208         }
 209         /* Hrm not found, forgot to register ? */
 210         assert(0);
 211 }
 212
 213 void urcu_register_thread(void)
 214 {
 215         internal_urcu_lock();
 216         urcu_add_reader(pthread_self());
 217         internal_urcu_unlock();
 218 }
 219
 220 void urcu_unregister_thread(void)
 221 {
 222         internal_urcu_lock();
 223         urcu_remove_reader(pthread_self());
 224         internal_urcu_unlock();
 225 }
 226
 227 #ifndef DEBUG_FULL_MB
 228 void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
 229 {
 230         mb();
 231         atomic_inc(&sig_done);
 232 }
 233
 234 void __attribute__((constructor)) urcu_init(void)
 235 {
 236         struct sigaction act;
 237         int ret;
 238
 239         act.sa_sigaction = sigurcu_handler;
 240         ret = sigaction(SIGURCU, &act, NULL);
 241         if (ret) {
 242                 perror("Error in sigaction");
 243                 exit(-1);
 244         }
 245 }
 246
 247 void __attribute__((destructor)) urcu_exit(void)
 248 {
 249         struct sigaction act;
 250         int ret;
 251
 252         ret = sigaction(SIGURCU, NULL, &act);
 253         if (ret) {
 254                 perror("Error in sigaction");
 255                 exit(-1);
 256         }
 257         assert(act.sa_sigaction == sigurcu_handler);
 258         free(reader_data);
 259 }
 260 #endif