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 #include <errno.h>
  18
  19 #include "urcu.h"
  20
  21 pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
  22
  23 /*
  24  * Global grace period counter.
  25  * Contains the current RCU_GP_CTR_BIT.
  26  * Also has a RCU_GP_CTR_BIT of 1, to accelerate the reader fast path.
  27  * Written to only by writer with mutex taken. Read by both writer and readers.
  28  */
  29 long urcu_gp_ctr = RCU_GP_COUNT;
  30
  31 /*
  32  * Written to only by each individual reader. Read by both the reader and the
  33  * writers.
  34  */
  35 long __thread urcu_active_readers;
  36
  37 /* Thread IDs of registered readers */
  38 #define INIT_NUM_THREADS 4
  39
  40 struct reader_registry {
  41         pthread_t tid;
  42         long *urcu_active_readers;
  43         char *need_mb;
  44 };
  45
  46 #ifdef DEBUG_YIELD
  47 unsigned int yield_active;
  48 unsigned int __thread rand_yield;
  49 #endif
  50
  51 static struct reader_registry *registry;
  52 static char __thread need_mb;
  53 static int num_readers, alloc_readers;
  54
  55 void internal_urcu_lock(void)
  56 {
  57         int ret;
  58
  59 #ifndef DISTRUST_SIGNALS_EXTREME
  60         ret = pthread_mutex_lock(&urcu_mutex);
  61         if (ret) {
  62                 perror("Error in pthread mutex lock");
  63                 exit(-1);
  64         }
  65 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
  66         while ((ret = pthread_mutex_trylock(&urcu_mutex)) != 0) {
  67                 if (ret != EBUSY && ret != EINTR) {
  68                         printf("ret = %d, errno = %d\n", ret, errno);
  69                         perror("Error in pthread mutex lock");
  70                         exit(-1);
  71                 }
  72                 if (need_mb) {
  73                         smp_mb();
  74                         need_mb = 0;
  75                         smp_mb();
  76                 }
  77                 poll(NULL,0,10);
  78         }
  79 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
  80 }
  81
  82 void internal_urcu_unlock(void)
  83 {
  84         int ret;
  85
  86         ret = pthread_mutex_unlock(&urcu_mutex);
  87         if (ret) {
  88                 perror("Error in pthread mutex unlock");
  89                 exit(-1);
  90         }
  91 }
  92
  93 /*
  94  * called with urcu_mutex held.
  95  */
  96 static void switch_next_urcu_qparity(void)
  97 {
  98         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_CTR_BIT);
  99 }
 100
 101 #ifdef DEBUG_FULL_MB
 102 static void force_mb_single_thread(struct reader_registry *index)
 103 {
 104         smp_mb();
 105 }
 106
 107 static void force_mb_all_threads(void)
 108 {
 109         smp_mb();
 110 }
 111 #else
 112
 113 static void force_mb_single_thread(struct reader_registry *index)
 114 {
 115         assert(registry);
 116         /*
 117          * pthread_kill has a smp_mb(). But beware, we assume it performs
 118          * a cache flush on architectures with non-coherent cache. Let's play
 119          * safe and don't assume anything : we use smp_mc() to make sure the
 120          * cache flush is enforced.
 121          */
 122         *index->need_mb = 1;
 123         smp_mc();       /* write ->need_mb before sending the signals */
 124         pthread_kill(index->tid, SIGURCU);
 125         smp_mb();
 126         /*
 127          * Wait for sighandler (and thus mb()) to execute on every thread.
 128          * BUSY-LOOP.
 129          */
 130         while (*index->need_mb) {
 131                 poll(NULL, 0, 1);
 132         }
 133         smp_mb();       /* read ->need_mb before ending the barrier */
 134 }
 135
 136 static void force_mb_all_threads(void)
 137 {
 138         struct reader_registry *index;
 139         /*
 140          * Ask for each threads to execute a smp_mb() so we can consider the
 141          * compiler barriers around rcu read lock as real memory barriers.
 142          */
 143         if (!registry)
 144                 return;
 145         /*
 146          * pthread_kill has a smp_mb(). But beware, we assume it performs
 147          * a cache flush on architectures with non-coherent cache. Let's play
 148          * safe and don't assume anything : we use smp_mc() to make sure the
 149          * cache flush is enforced.
 150          */
 151         for (index = registry; index < registry + num_readers; index++) {
 152                 *index->need_mb = 1;
 153                 smp_mc();       /* write need_mb before sending the signal */
 154                 pthread_kill(index->tid, SIGURCU);
 155         }
 156         /*
 157          * Wait for sighandler (and thus mb()) to execute on every thread.
 158          *
 159          * Note that the pthread_kill() will never be executed on systems
 160          * that correctly deliver signals in a timely manner.  However, it
 161          * is not uncommon for kernels to have bugs that can result in
 162          * lost or unduly delayed signals.
 163          *
 164          * If you are seeing the below pthread_kill() executing much at
 165          * all, we suggest testing the underlying kernel and filing the
 166          * relevant bug report.  For Linux kernels, we recommend getting
 167          * the Linux Test Project (LTP).
 168          */
 169         for (index = registry; index < registry + num_readers; index++) {
 170                 while (*index->need_mb) {
 171                         pthread_kill(index->tid, SIGURCU);
 172                         poll(NULL, 0, 1);
 173                 }
 174         }
 175         smp_mb();       /* read ->need_mb before ending the barrier */
 176 }
 177 #endif
 178
 179 void wait_for_quiescent_state(void)
 180 {
 181         struct reader_registry *index;
 182
 183         if (!registry)
 184                 return;
 185         /*
 186          * Wait for each thread urcu_active_readers count to become 0.
 187          */
 188         for (index = registry; index < registry + num_readers; index++) {
 189                 int wait_loops = 0;
 190                 /*
 191                  * BUSY-LOOP. Force the reader thread to commit its
 192                  * urcu_active_readers update to memory if we wait for too long.
 193                  */
 194                 while (rcu_old_gp_ongoing(index->urcu_active_readers)) {
 195                         if (wait_loops++ == KICK_READER_LOOPS) {
 196                                 force_mb_single_thread(index);
 197                                 wait_loops = 0;
 198                         } else {
 199                                 cpu_relax();
 200                         }
 201                 }
 202         }
 203 }
 204
 205 void synchronize_rcu(void)
 206 {
 207         internal_urcu_lock();
 208
 209         /* All threads should read qparity before accessing data structure
 210          * where new ptr points to. Must be done within internal_urcu_lock
 211          * because it iterates on reader threads.*/
 212         /* Write new ptr before changing the qparity */
 213         force_mb_all_threads();
 214
 215         switch_next_urcu_qparity();     /* 0 -> 1 */
 216
 217         /*
 218          * Must commit qparity update to memory before waiting for parity
 219          * 0 quiescent state. Failure to do so could result in the writer
 220          * waiting forever while new readers are always accessing data (no
 221          * progress).
 222          * Ensured by STORE_SHARED and LOAD_SHARED.
 223          */
 224
 225         /*
 226          * Wait for previous parity to be empty of readers.
 227          */
 228         wait_for_quiescent_state();     /* Wait readers in parity 0 */
 229
 230         /*
 231          * Must finish waiting for quiescent state for parity 0 before
 232          * committing qparity update to memory. Failure to do so could result in
 233          * the writer waiting forever while new readers are always accessing
 234          * data (no progress).
 235          * Ensured by STORE_SHARED and LOAD_SHARED.
 236          */
 237
 238         switch_next_urcu_qparity();     /* 1 -> 0 */
 239
 240         /*
 241          * Must commit qparity update to memory before waiting for parity
 242          * 1 quiescent state. Failure to do so could result in the writer
 243          * waiting forever while new readers are always accessing data (no
 244          * progress).
 245          * Ensured by STORE_SHARED and LOAD_SHARED.
 246          */
 247
 248         /*
 249          * Wait for previous parity to be empty of readers.
 250          */
 251         wait_for_quiescent_state();     /* Wait readers in parity 1 */
 252
 253         /* Finish waiting for reader threads before letting the old ptr being
 254          * freed. Must be done within internal_urcu_lock because it iterates on
 255          * reader threads. */
 256         force_mb_all_threads();
 257
 258         internal_urcu_unlock();
 259 }
 260
 261 void urcu_add_reader(pthread_t id)
 262 {
 263         struct reader_registry *oldarray;
 264
 265         if (!registry) {
 266                 alloc_readers = INIT_NUM_THREADS;
 267                 num_readers = 0;
 268                 registry =
 269                         malloc(sizeof(struct reader_registry) * alloc_readers);
 270         }
 271         if (alloc_readers < num_readers + 1) {
 272                 oldarray = registry;
 273                 registry = malloc(sizeof(struct reader_registry)
 274                                 * (alloc_readers << 1));
 275                 memcpy(registry, oldarray,
 276                         sizeof(struct reader_registry) * alloc_readers);
 277                 alloc_readers <<= 1;
 278                 free(oldarray);
 279         }
 280         registry[num_readers].tid = id;
 281         /* reference to the TLS of _this_ reader thread. */
 282         registry[num_readers].urcu_active_readers = &urcu_active_readers;
 283         registry[num_readers].need_mb = &need_mb;
 284         num_readers++;
 285 }
 286
 287 /*
 288  * Never shrink (implementation limitation).
 289  * This is O(nb threads). Eventually use a hash table.
 290  */
 291 void urcu_remove_reader(pthread_t id)
 292 {
 293         struct reader_registry *index;
 294
 295         assert(registry != NULL);
 296         for (index = registry; index < registry + num_readers; index++) {
 297                 if (pthread_equal(index->tid, id)) {
 298                         memcpy(index, &registry[num_readers - 1],
 299                                 sizeof(struct reader_registry));
 300                         registry[num_readers - 1].tid = 0;
 301                         registry[num_readers - 1].urcu_active_readers = NULL;
 302                         num_readers--;
 303                         return;
 304                 }
 305         }
 306         /* Hrm not found, forgot to register ? */
 307         assert(0);
 308 }
 309
 310 void urcu_register_thread(void)
 311 {
 312         internal_urcu_lock();
 313         urcu_add_reader(pthread_self());
 314         internal_urcu_unlock();
 315 }
 316
 317 void urcu_unregister_thread(void)
 318 {
 319         internal_urcu_lock();
 320         urcu_remove_reader(pthread_self());
 321         internal_urcu_unlock();
 322 }
 323
 324 #ifndef DEBUG_FULL_MB
 325 void sigurcu_handler(int signo, siginfo_t *siginfo, void *context)
 326 {
 327         /*
 328          * Executing this smp_mb() is the only purpose of this signal handler.
 329          * It punctually promotes barrier() into smp_mb() on every thread it is
 330          * executed on.
 331          */
 332         smp_mb();
 333         need_mb = 0;
 334         smp_mb();
 335 }
 336
 337 void __attribute__((constructor)) urcu_init(void)
 338 {
 339         struct sigaction act;
 340         int ret;
 341
 342         act.sa_sigaction = sigurcu_handler;
 343         ret = sigaction(SIGURCU, &act, NULL);
 344         if (ret) {
 345                 perror("Error in sigaction");
 346                 exit(-1);
 347         }
 348 }
 349
 350 void __attribute__((destructor)) urcu_exit(void)
 351 {
 352         struct sigaction act;
 353         int ret;
 354
 355         ret = sigaction(SIGURCU, NULL, &act);
 356         if (ret) {
 357                 perror("Error in sigaction");
 358                 exit(-1);
 359         }
 360         assert(act.sa_sigaction == sigurcu_handler);
 361         free(registry);
 362 }
 363 #endif