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 #ifndef DEBUG_FULL_MB
  47 static int sig_done;
  48 #endif
  49
  50 void internal_urcu_lock(void)
  51 {
  52         int ret;
  53         ret = pthread_mutex_lock(&urcu_mutex);
  54         if (ret) {
  55                 perror("Error in pthread mutex lock");
  56                 exit(-1);
  57         }
  58 }
  59
  60 void internal_urcu_unlock(void)
  61 {
  62         int ret;
  63
  64         ret = pthread_mutex_unlock(&urcu_mutex);
  65         if (ret) {
  66                 perror("Error in pthread mutex unlock");
  67                 exit(-1);
  68         }
  69 }
  70
  71 /*
  72  * called with urcu_mutex held.
  73  */
  74 static void switch_next_urcu_qparity(void)
  75 {
  76         urcu_gp_ctr ^= RCU_GP_CTR_BIT;
  77 }
  78
  79 #ifdef DEBUG_FULL_MB
  80 static void force_mb_all_threads(void)
  81 {
  82         smp_mb();
  83 }
  84 #else
  85 static void force_mb_all_threads(void)
  86 {
  87         struct reader_data *index;
  88         /*
  89          * Ask for each threads to execute a smp_mb() so we can consider the
  90          * compiler barriers around rcu read lock as real memory barriers.
  91          */
  92         if (!reader_data)
  93                 return;
  94         debug_yield_write();
  95         sig_done = 0;
  96         debug_yield_write();
  97         smp_mb();       /* write sig_done before sending the signals */
  98         debug_yield_write();
  99         for (index = reader_data; index < reader_data + num_readers; index++) {
 100                 pthread_kill(index->tid, SIGURCU);
 101                 debug_yield_write();
 102         }
 103         /*
 104          * Wait for sighandler (and thus mb()) to execute on every thread.
 105          * BUSY-LOOP.
 106          */
 107         while (sig_done < num_readers)
 108                 barrier();
 109         debug_yield_write();
 110         smp_mb();       /* read sig_done before ending the barrier */
 111         debug_yield_write();
 112 }
 113 #endif
 114
 115 void wait_for_quiescent_state(void)
 116 {
 117         struct reader_data *index;
 118
 119         if (!reader_data)
 120                 return;
 121         /* Wait for each thread urcu_active_readers count to become 0.
 122          */
 123         for (index = reader_data; index < reader_data + num_readers; index++) {
 124                 /*
 125                  * BUSY-LOOP.
 126                  */
 127                 while (rcu_old_gp_ongoing(index->urcu_active_readers))
 128                         barrier();
 129         }
 130 }
 131
 132 void synchronize_rcu(void)
 133 {
 134         /* All threads should read qparity before accessing data structure
 135          * where new ptr points to. */
 136         /* Write new ptr before changing the qparity */
 137         force_mb_all_threads();
 138         debug_yield_write();
 139
 140         internal_urcu_lock();
 141         debug_yield_write();
 142
 143         switch_next_urcu_qparity();     /* 0 -> 1 */
 144         debug_yield_write();
 145
 146         /*
 147          * Must commit qparity update to memory before waiting for parity
 148          * 0 quiescent state. Failure to do so could result in the writer
 149          * waiting forever while new readers are always accessing data (no
 150          * progress).
 151          */
 152         smp_mb();
 153
 154         /*
 155          * Wait for previous parity to be empty of readers.
 156          */
 157         wait_for_quiescent_state();     /* Wait readers in parity 0 */
 158         debug_yield_write();
 159
 160         /*
 161          * Must finish waiting for quiescent state for parity 0 before
 162          * committing qparity update to memory. Failure to do so could result in
 163          * the writer waiting forever while new readers are always accessing
 164          * data (no progress).
 165          */
 166         smp_mb();
 167
 168         switch_next_urcu_qparity();     /* 1 -> 0 */
 169         debug_yield_write();
 170
 171         /*
 172          * Must commit qparity update to memory before waiting for parity
 173          * 1 quiescent state. Failure to do so could result in the writer
 174          * waiting forever while new readers are always accessing data (no
 175          * progress).
 176          */
 177         smp_mb();
 178
 179         /*
 180          * Wait for previous parity to be empty of readers.
 181          */
 182         wait_for_quiescent_state();     /* Wait readers in parity 1 */
 183         debug_yield_write();
 184
 185         internal_urcu_unlock();
 186         debug_yield_write();
 187
 188         /* All threads should finish using the data referred to by old ptr
 189          * before decrementing their urcu_active_readers count */
 190         /* Finish waiting for reader threads before letting the old ptr being
 191          * freed. */
 192         force_mb_all_threads();
 193         debug_yield_write();
 194 }
 195
 196 void urcu_add_reader(pthread_t id)
 197 {
 198         struct reader_data *oldarray;
 199
 200         if (!reader_data) {
 201                 alloc_readers = INIT_NUM_THREADS;
 202                 num_readers = 0;
 203                 reader_data =
 204                         malloc(sizeof(struct reader_data) * alloc_readers);
 205         }
 206         if (alloc_readers < num_readers + 1) {
 207                 oldarray = reader_data;
 208                 reader_data = malloc(sizeof(struct reader_data)
 209                                 * (alloc_readers << 1));
 210                 memcpy(reader_data, oldarray,
 211                         sizeof(struct reader_data) * alloc_readers);
 212                 alloc_readers <<= 1;
 213                 free(oldarray);
 214         }
 215         reader_data[num_readers].tid = id;
 216         /* reference to the TLS of _this_ reader thread. */
 217         reader_data[num_readers].urcu_active_readers = &urcu_active_readers;
 218         num_readers++;
 219 }
 220
 221 /*
 222  * Never shrink (implementation limitation).
 223  * This is O(nb threads). Eventually use a hash table.
 224  */
 225 void urcu_remove_reader(pthread_t id)
 226 {
 227         struct reader_data *index;
 228
 229         assert(reader_data != NULL);
 230         for (index = reader_data; index < reader_data + num_readers; index++) {
 231                 if (pthread_equal(index->tid, id)) {
 232                         memcpy(index, &reader_data[num_readers - 1],
 233                                 sizeof(struct reader_data));
 234                         reader_data[num_readers - 1].tid = 0;
 235                         reader_data[num_readers - 1].urcu_active_readers = NULL;
 236                         num_readers--;
 237                         return;
 238                 }
 239         }
 240         /* Hrm not found, forgot to register ? */
 241         assert(0);
 242 }
 243
 244 void urcu_register_thread(void)
 245 {
 246         internal_urcu_lock();
 247         urcu_add_reader(pthread_self());
 248         internal_urcu_unlock();
 249 }
 250
 251 void urcu_unregister_thread(void)
 252 {
 253         internal_urcu_lock();
 254         urcu_remove_reader(pthread_self());
 255         internal_urcu_unlock();
 256 }
 257
 258 #ifndef DEBUG_FULL_MB
 259 void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
 260 {
 261         smp_mb();
 262         atomic_inc(&sig_done);
 263 }
 264
 265 void __attribute__((constructor)) urcu_init(void)
 266 {
 267         struct sigaction act;
 268         int ret;
 269
 270         act.sa_sigaction = sigurcu_handler;
 271         ret = sigaction(SIGURCU, &act, NULL);
 272         if (ret) {
 273                 perror("Error in sigaction");
 274                 exit(-1);
 275         }
 276 }
 277
 278 void __attribute__((destructor)) urcu_exit(void)
 279 {
 280         struct sigaction act;
 281         int ret;
 282
 283         ret = sigaction(SIGURCU, NULL, &act);
 284         if (ret) {
 285                 perror("Error in sigaction");
 286                 exit(-1);
 287         }
 288         assert(act.sa_sigaction == sigurcu_handler);
 289         free(reader_data);
 290 }
 291 #endif