urcu.c

   1 #include <stdio.h>
   2 #include <pthread.h>
   3 #include <signal.h>
   4 #include <assert.h>
   5 #include <stdlib.h>
   6 #include <string.h>
   7
   8 #include "urcu.h"
   9
  10 pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
  11
  12 /* Global quiescent period parity */
  13 int urcu_qparity;
  14
  15 int __thread urcu_active_readers[2];
  16
  17 /* Thread IDs of registered readers */
  18 #define INIT_NUM_THREADS 4
  19
  20 struct reader_data {
  21         pthread_t tid;
  22         int *urcu_active_readers;
  23 };
  24
  25 static struct reader_data *reader_data;
  26 static int num_readers, alloc_readers;
  27 static int sig_done;
  28
  29 void rcu_write_lock(void)
  30 {
  31         int ret;
  32         ret = pthread_mutex_lock(&urcu_mutex);
  33         if (ret) {
  34                 perror("Error in pthread mutex lock");
  35                 exit(-1);
  36         }
  37 }
  38
  39 void rcu_write_unlock(void)
  40 {
  41         int ret;
  42
  43         ret = pthread_mutex_unlock(&urcu_mutex);
  44         if (ret) {
  45                 perror("Error in pthread mutex unlock");
  46                 exit(-1);
  47         }
  48 }
  49
  50 /*
  51  * called with urcu_mutex held.
  52  */
  53 static int switch_next_urcu_qparity(void)
  54 {
  55         int old_parity = urcu_qparity;
  56         urcu_qparity = 1 - old_parity;
  57         return old_parity;
  58 }
  59
  60 static void force_mb_all_threads(void)
  61 {
  62         struct reader_data *index;
  63         /*
  64          * Ask for each threads to execute a mb() so we can consider the
  65          * compiler barriers around rcu read lock as real memory barriers.
  66          */
  67         if (!reader_data)
  68                 return;
  69         sig_done = 0;
  70         mb();   /* write sig_done before sending the signals */
  71         for (index = reader_data; index < reader_data + num_readers; index++)
  72                 pthread_kill(index->tid, SIGURCU);
  73         /*
  74          * Wait for sighandler (and thus mb()) to execute on every thread.
  75          * BUSY-LOOP.
  76          */
  77         while (sig_done < num_readers)
  78                 barrier();
  79         mb();   /* read sig_done before ending the barrier */
  80 }
  81
  82 void wait_for_quiescent_state(int parity)
  83 {
  84         struct reader_data *index;
  85
  86         if (!reader_data)
  87                 return;
  88         /* Wait for each thread urcu_active_readers count to become 0.
  89          */
  90         for (index = reader_data; index < reader_data + num_readers; index++) {
  91                 /*
  92                  * BUSY-LOOP.
  93                  */
  94                 while (*index->urcu_active_readers != 0)
  95                         barrier();
  96         }
  97         /*
  98          * Locally : read *index->urcu_active_readers before freeing old
  99          * pointer.
 100          * Remote (reader threads) : Order urcu_qparity update and other
 101          * thread's quiescent state counter read.
 102          */
 103         force_mb_all_threads();
 104 }
 105
 106 /*
 107  * Return old pointer, OK to free, no more reference exist.
 108  * Called under rcu_write_lock.
 109  */
 110 void *urcu_publish_content(void **ptr, void *new)
 111 {
 112         int ret, prev_parity;
 113         void *oldptr;
 114
 115         /*
 116          * We can publish the new pointer before we change the current qparity.
 117          * Readers seeing the new pointer while being in the previous qparity
 118          * window will make us wait until the end of the quiescent state before
 119          * we release the unrelated memory area. However, given we hold the
 120          * urcu_mutex, we are making sure that no further garbage collection can
 121          * occur until we release the mutex, therefore we guarantee that this
 122          * given reader will have completed its execution using the new pointer
 123          * when the next quiescent state window will be over.
 124          */
 125         oldptr = *ptr;
 126         *ptr = new;
 127         wmb();          /* Write ptr before changing the qparity */
 128         /* All threads should read qparity before ptr */
 129         force_mb_all_threads();
 130         prev_parity = switch_next_urcu_qparity();
 131
 132         /*
 133          * Wait for previous parity to be empty of readers.
 134          */
 135         wait_for_quiescent_state(prev_parity);
 136         /*
 137          * Deleting old data is ok !
 138          */
 139
 140         return oldptr;
 141 }
 142
 143 void urcu_add_reader(pthread_t id)
 144 {
 145         struct reader_data *oldarray;
 146
 147         if (!reader_data) {
 148                 alloc_readers = INIT_NUM_THREADS;
 149                 num_readers = 0;
 150                 reader_data =
 151                         malloc(sizeof(struct reader_data) * alloc_readers);
 152         }
 153         if (alloc_readers < num_readers + 1) {
 154                 oldarray = reader_data;
 155                 reader_data = malloc(sizeof(struct reader_data)
 156                                 * (alloc_readers << 1));
 157                 memcpy(reader_data, oldarray,
 158                         sizeof(struct reader_data) * alloc_readers);
 159                 alloc_readers <<= 1;
 160                 free(oldarray);
 161         }
 162         reader_data[num_readers].tid = id;
 163         /* reference to the TLS of _this_ reader thread. */
 164         reader_data[num_readers].urcu_active_readers = urcu_active_readers;
 165         num_readers++;
 166 }
 167
 168 /*
 169  * Never shrink (implementation limitation).
 170  * This is O(nb threads). Eventually use a hash table.
 171  */
 172 void urcu_remove_reader(pthread_t id)
 173 {
 174         struct reader_data *index;
 175
 176         assert(reader_data != NULL);
 177         for (index = reader_data; index < reader_data + num_readers; index++) {
 178                 if (index->tid == id) {
 179                         memcpy(index, &reader_data[num_readers - 1],
 180                                 sizeof(struct reader_data));
 181                         reader_data[num_readers - 1].tid = 0;
 182                         reader_data[num_readers - 1].urcu_active_readers = NULL;
 183                         num_readers--;
 184                         return;
 185                 }
 186         }
 187         /* Hrm not found, forgot to register ? */
 188         assert(0);
 189 }
 190
 191 void urcu_register_thread(void)
 192 {
 193         rcu_write_lock();
 194         urcu_add_reader(pthread_self());
 195         rcu_write_unlock();
 196 }
 197
 198 void urcu_unregister_thread(void)
 199 {
 200         pthread_t self = pthread_self();
 201         rcu_write_lock();
 202         urcu_remove_reader(pthread_self());
 203         rcu_write_unlock();
 204 }
 205
 206 void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
 207 {
 208         mb();
 209         atomic_inc(&sig_done);
 210 }
 211
 212 void __attribute__((constructor)) urcu_init(void)
 213 {
 214         struct sigaction act;
 215         int ret;
 216
 217         act.sa_sigaction = sigurcu_handler;
 218         ret = sigaction(SIGURCU, &act, NULL);
 219         if (ret) {
 220                 perror("Error in sigaction");
 221                 exit(-1);
 222         }
 223 }
 224
 225 void __attribute__((destructor)) urcu_exit(void)
 226 {
 227         struct sigaction act;
 228         int ret;
 229
 230         ret = sigaction(SIGURCU, NULL, &act);
 231         if (ret) {
 232                 perror("Error in sigaction");
 233                 exit(-1);
 234         }
 235         assert(act.sa_sigaction == sigurcu_handler);
 236         free(reader_data);
 237 }