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_single_thread(pthread_t tid)
  81 {
  82         smp_mb();
  83 }
  84
  85 static void force_mb_all_threads(void)
  86 {
  87         smp_mb();
  88 }
  89 #else
  90
  91 static void force_mb_single_thread(pthread_t tid)
  92 {
  93         assert(reader_data);
  94         sig_done = 0;
  95         /*
  96          * pthread_kill has a smp_mb(). But beware, we assume it performs
  97          * a cache flush on architectures with non-coherent cache. Let's play
  98          * safe and don't assume anything : we use smp_mc() to make sure the
  99          * cache flush is enforced.
 100          * smp_mb();    write sig_done before sending the signals
 101          */
 102         smp_mc();       /* write sig_done before sending the signals */
 103         pthread_kill(tid, SIGURCU);
 104         /*
 105          * Wait for sighandler (and thus mb()) to execute on every thread.
 106          * BUSY-LOOP.
 107          */
 108         while (LOAD_REMOTE(sig_done) < 1)
 109                 cpu_relax();
 110         smp_mb();       /* read sig_done before ending the barrier */
 111 }
 112
 113 static void force_mb_all_threads(void)
 114 {
 115         struct reader_data *index;
 116         /*
 117          * Ask for each threads to execute a smp_mb() so we can consider the
 118          * compiler barriers around rcu read lock as real memory barriers.
 119          */
 120         if (!reader_data)
 121                 return;
 122         sig_done = 0;
 123         /*
 124          * pthread_kill has a smp_mb(). But beware, we assume it performs
 125          * a cache flush on architectures with non-coherent cache. Let's play
 126          * safe and don't assume anything : we use smp_mc() to make sure the
 127          * cache flush is enforced.
 128          * smp_mb();    write sig_done before sending the signals
 129          */
 130         smp_mc();       /* write sig_done before sending the signals */
 131         for (index = reader_data; index < reader_data + num_readers; index++)
 132                 pthread_kill(index->tid, SIGURCU);
 133         /*
 134          * Wait for sighandler (and thus mb()) to execute on every thread.
 135          * BUSY-LOOP.
 136          */
 137         while (LOAD_REMOTE(sig_done) < num_readers)
 138                 cpu_relax();
 139         smp_mb();       /* read sig_done before ending the barrier */
 140 }
 141 #endif
 142
 143 void wait_for_quiescent_state(void)
 144 {
 145         struct reader_data *index;
 146
 147         if (!reader_data)
 148                 return;
 149         /*
 150          * Wait for each thread urcu_active_readers count to become 0.
 151          */
 152         for (index = reader_data; index < reader_data + num_readers; index++) {
 153                 int wait_loops = 0;
 154                 /*
 155                  * BUSY-LOOP. Force the reader thread to commit its
 156                  * urcu_active_readers update to memory if we wait for too long.
 157                  */
 158                 while (rcu_old_gp_ongoing(index->urcu_active_readers)) {
 159                         if (wait_loops++ == KICK_READER_LOOPS) {
 160                                 force_mb_single_thread(index->tid);
 161                                 wait_loops = 0;
 162                         }
 163                 }
 164         }
 165 }
 166
 167 void synchronize_rcu(void)
 168 {
 169         internal_urcu_lock();
 170
 171         /* All threads should read qparity before accessing data structure
 172          * where new ptr points to. Must be done within internal_urcu_lock
 173          * because it iterates on reader threads.*/
 174         /* Write new ptr before changing the qparity */
 175         force_mb_all_threads();
 176
 177         switch_next_urcu_qparity();     /* 0 -> 1 */
 178
 179         /*
 180          * Must commit qparity update to memory before waiting for parity
 181          * 0 quiescent state. Failure to do so could result in the writer
 182          * waiting forever while new readers are always accessing data (no
 183          * progress).
 184          */
 185         smp_mc();
 186
 187         /*
 188          * Wait for previous parity to be empty of readers.
 189          */
 190         wait_for_quiescent_state();     /* Wait readers in parity 0 */
 191
 192         /*
 193          * Must finish waiting for quiescent state for parity 0 before
 194          * committing qparity update to memory. Failure to do so could result in
 195          * the writer waiting forever while new readers are always accessing
 196          * data (no progress).
 197          */
 198         smp_mc();
 199
 200         switch_next_urcu_qparity();     /* 1 -> 0 */
 201
 202         /*
 203          * Must commit qparity update to memory before waiting for parity
 204          * 1 quiescent state. Failure to do so could result in the writer
 205          * waiting forever while new readers are always accessing data (no
 206          * progress).
 207          */
 208         smp_mc();
 209
 210         /*
 211          * Wait for previous parity to be empty of readers.
 212          */
 213         wait_for_quiescent_state();     /* Wait readers in parity 1 */
 214
 215         /* Finish waiting for reader threads before letting the old ptr being
 216          * freed. Must be done within internal_urcu_lock because it iterates on
 217          * reader threads. */
 218         force_mb_all_threads();
 219
 220         internal_urcu_unlock();
 221 }
 222
 223 void urcu_add_reader(pthread_t id)
 224 {
 225         struct reader_data *oldarray;
 226
 227         if (!reader_data) {
 228                 alloc_readers = INIT_NUM_THREADS;
 229                 num_readers = 0;
 230                 reader_data =
 231                         malloc(sizeof(struct reader_data) * alloc_readers);
 232         }
 233         if (alloc_readers < num_readers + 1) {
 234                 oldarray = reader_data;
 235                 reader_data = malloc(sizeof(struct reader_data)
 236                                 * (alloc_readers << 1));
 237                 memcpy(reader_data, oldarray,
 238                         sizeof(struct reader_data) * alloc_readers);
 239                 alloc_readers <<= 1;
 240                 free(oldarray);
 241         }
 242         reader_data[num_readers].tid = id;
 243         /* reference to the TLS of _this_ reader thread. */
 244         reader_data[num_readers].urcu_active_readers = &urcu_active_readers;
 245         num_readers++;
 246 }
 247
 248 /*
 249  * Never shrink (implementation limitation).
 250  * This is O(nb threads). Eventually use a hash table.
 251  */
 252 void urcu_remove_reader(pthread_t id)
 253 {
 254         struct reader_data *index;
 255
 256         assert(reader_data != NULL);
 257         for (index = reader_data; index < reader_data + num_readers; index++) {
 258                 if (pthread_equal(index->tid, id)) {
 259                         memcpy(index, &reader_data[num_readers - 1],
 260                                 sizeof(struct reader_data));
 261                         reader_data[num_readers - 1].tid = 0;
 262                         reader_data[num_readers - 1].urcu_active_readers = NULL;
 263                         num_readers--;
 264                         return;
 265                 }
 266         }
 267         /* Hrm not found, forgot to register ? */
 268         assert(0);
 269 }
 270
 271 void urcu_register_thread(void)
 272 {
 273         internal_urcu_lock();
 274         urcu_add_reader(pthread_self());
 275         internal_urcu_unlock();
 276 }
 277
 278 void urcu_unregister_thread(void)
 279 {
 280         internal_urcu_lock();
 281         urcu_remove_reader(pthread_self());
 282         internal_urcu_unlock();
 283 }
 284
 285 #ifndef DEBUG_FULL_MB
 286 void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
 287 {
 288         /*
 289          * Executing this smp_mb() is the only purpose of this signal handler.
 290          * It punctually promotes barrier() into smp_mb() on every thread it is
 291          * executed on.
 292          */
 293         smp_mb();
 294         atomic_inc(&sig_done);
 295 }
 296
 297 void __attribute__((constructor)) urcu_init(void)
 298 {
 299         struct sigaction act;
 300         int ret;
 301
 302         act.sa_sigaction = sigurcu_handler;
 303         ret = sigaction(SIGURCU, &act, NULL);
 304         if (ret) {
 305                 perror("Error in sigaction");
 306                 exit(-1);
 307         }
 308 }
 309
 310 void __attribute__((destructor)) urcu_exit(void)
 311 {
 312         struct sigaction act;
 313         int ret;
 314
 315         ret = sigaction(SIGURCU, NULL, &act);
 316         if (ret) {
 317                 perror("Error in sigaction");
 318                 exit(-1);
 319         }
 320         assert(act.sa_sigaction == sigurcu_handler);
 321         free(reader_data);
 322 }
 323 #endif