urcu-bp.c

   1 /*
   2  * urcu-bp.c
   3  *
   4  * Userspace RCU library, "bulletproof" version.
   5  *
   6  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
   7  * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
   8  *
   9  * This library is free software; you can redistribute it and/or
  10  * modify it under the terms of the GNU Lesser General Public
  11  * License as published by the Free Software Foundation; either
  12  * version 2.1 of the License, or (at your option) any later version.
  13  *
  14  * This library is distributed in the hope that it will be useful,
  15  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  16  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  17  * Lesser General Public License for more details.
  18  *
  19  * You should have received a copy of the GNU Lesser General Public
  20  * License along with this library; if not, write to the Free Software
  21  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  22  *
  23  * IBM's contributions to this file may be relicensed under LGPLv2 or later.
  24  */
  25
  26 #include <stdio.h>
  27 #include <pthread.h>
  28 #include <signal.h>
  29 #include <assert.h>
  30 #include <stdlib.h>
  31 #include <string.h>
  32 #include <errno.h>
  33 #include <poll.h>
  34 #include <unistd.h>
  35 #include <sys/mman.h>
  36
  37 #include "urcu-bp-static.h"
  38 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
  39 #include "urcu-bp.h"
  40
  41 /* Sleep delay in us */
  42 #define RCU_SLEEP_DELAY         1000
  43 #define ARENA_INIT_ALLOC        16
  44
  45 void __attribute__((destructor)) urcu_bp_exit(void);
  46
  47 static pthread_mutex_t urcu_mutex = PTHREAD_MUTEX_INITIALIZER;
  48
  49 #ifdef DEBUG_YIELD
  50 unsigned int yield_active;
  51 unsigned int __thread rand_yield;
  52 #endif
  53
  54 /*
  55  * Global grace period counter.
  56  * Contains the current RCU_GP_CTR_BIT.
  57  * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
  58  * Written to only by writer with mutex taken. Read by both writer and readers.
  59  */
  60 long urcu_gp_ctr = RCU_GP_COUNT;
  61
  62 /*
  63  * Pointer to registry elements. Written to only by each individual reader. Read
  64  * by both the reader and the writers.
  65  */
  66 struct urcu_reader __thread *urcu_reader;
  67
  68 static LIST_HEAD(registry);
  69
  70 struct registry_arena {
  71         void *p;
  72         size_t len;
  73         size_t used;
  74 };
  75
  76 static struct registry_arena registry_arena;
  77
  78 static void rcu_gc_registry(void);
  79
  80 static void internal_urcu_lock(void)
  81 {
  82         int ret;
  83
  84 #ifndef DISTRUST_SIGNALS_EXTREME
  85         ret = pthread_mutex_lock(&urcu_mutex);
  86         if (ret) {
  87                 perror("Error in pthread mutex lock");
  88                 exit(-1);
  89         }
  90 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
  91         while ((ret = pthread_mutex_trylock(&urcu_mutex)) != 0) {
  92                 if (ret != EBUSY && ret != EINTR) {
  93                         printf("ret = %d, errno = %d\n", ret, errno);
  94                         perror("Error in pthread mutex lock");
  95                         exit(-1);
  96                 }
  97                 if (urcu_reader.need_mb) {
  98                         smp_mb();
  99                         urcu_reader.need_mb = 0;
 100                         smp_mb();
 101                 }
 102                 poll(NULL,0,10);
 103         }
 104 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 105 }
 106
 107 static void internal_urcu_unlock(void)
 108 {
 109         int ret;
 110
 111         ret = pthread_mutex_unlock(&urcu_mutex);
 112         if (ret) {
 113                 perror("Error in pthread mutex unlock");
 114                 exit(-1);
 115         }
 116 }
 117
 118 /*
 119  * called with urcu_mutex held.
 120  */
 121 static void switch_next_urcu_qparity(void)
 122 {
 123         STORE_SHARED(urcu_gp_ctr, urcu_gp_ctr ^ RCU_GP_CTR_BIT);
 124 }
 125
 126 void wait_for_quiescent_state(void)
 127 {
 128         LIST_HEAD(qsreaders);
 129         int wait_loops = 0;
 130         struct urcu_reader *index, *tmp;
 131
 132         if (list_empty(&registry))
 133                 return;
 134         /*
 135          * Wait for each thread urcu_reader.ctr count to become 0.
 136          */
 137         for (;;) {
 138                 wait_loops++;
 139                 list_for_each_entry_safe(index, tmp, &registry, head) {
 140                         if (!rcu_old_gp_ongoing(&index->ctr))
 141                                 list_move(&index->head, &qsreaders);
 142                 }
 143
 144                 if (list_empty(&registry)) {
 145                         break;
 146                 } else {
 147                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
 148                                 usleep(RCU_SLEEP_DELAY);
 149                         else
 150                                 cpu_relax();
 151                 }
 152         }
 153         /* put back the reader list in the registry */
 154         list_splice(&qsreaders, &registry);
 155 }
 156
 157 void synchronize_rcu(void)
 158 {
 159         sigset_t newmask, oldmask;
 160         int ret;
 161
 162         ret = sigemptyset(&newmask);
 163         assert(!ret);
 164         ret = pthread_sigmask(SIG_SETMASK, &newmask, &oldmask);
 165         assert(!ret);
 166
 167         internal_urcu_lock();
 168
 169         /* Remove old registry elements */
 170         rcu_gc_registry();
 171
 172         /* All threads should read qparity before accessing data structure
 173          * where new ptr points to. Must be done within internal_urcu_lock
 174          * because it iterates on reader threads.*/
 175         /* Write new ptr before changing the qparity */
 176         smp_mb();
 177
 178         switch_next_urcu_qparity();     /* 0 -> 1 */
 179
 180         /*
 181          * Must commit qparity update to memory before waiting for parity
 182          * 0 quiescent state. Failure to do so could result in the writer
 183          * waiting forever while new readers are always accessing data (no
 184          * progress).
 185          * Ensured by STORE_SHARED and LOAD_SHARED.
 186          */
 187
 188         /*
 189          * Adding a smp_mb() which is _not_ formally required, but makes the
 190          * model easier to understand. It does not have a big performance impact
 191          * anyway, given this is the write-side.
 192          */
 193         smp_mb();
 194
 195         /*
 196          * Wait for previous parity to be empty of readers.
 197          */
 198         wait_for_quiescent_state();     /* Wait readers in parity 0 */
 199
 200         /*
 201          * Must finish waiting for quiescent state for parity 0 before
 202          * committing qparity update to memory. Failure to do so could result in
 203          * the writer waiting forever while new readers are always accessing
 204          * data (no progress).
 205          * Ensured by STORE_SHARED and LOAD_SHARED.
 206          */
 207
 208         /*
 209          * Adding a smp_mb() which is _not_ formally required, but makes the
 210          * model easier to understand. It does not have a big performance impact
 211          * anyway, given this is the write-side.
 212          */
 213         smp_mb();
 214
 215         switch_next_urcu_qparity();     /* 1 -> 0 */
 216
 217         /*
 218          * Must commit qparity update to memory before waiting for parity
 219          * 1 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          * Adding a smp_mb() which is _not_ formally required, but makes the
 227          * model easier to understand. It does not have a big performance impact
 228          * anyway, given this is the write-side.
 229          */
 230         smp_mb();
 231
 232         /*
 233          * Wait for previous parity to be empty of readers.
 234          */
 235         wait_for_quiescent_state();     /* Wait readers in parity 1 */
 236
 237         /* Finish waiting for reader threads before letting the old ptr being
 238          * freed. Must be done within internal_urcu_lock because it iterates on
 239          * reader threads. */
 240         smp_mb();
 241
 242         internal_urcu_unlock();
 243         ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
 244         assert(!ret);
 245 }
 246
 247 /*
 248  * library wrappers to be used by non-LGPL compatible source code.
 249  */
 250
 251 void rcu_read_lock(void)
 252 {
 253         _rcu_read_lock();
 254 }
 255
 256 void rcu_read_unlock(void)
 257 {
 258         _rcu_read_unlock();
 259 }
 260
 261 /*
 262  * only grow for now.
 263  */
 264 static void resize_arena(struct registry_arena *arena, size_t len)
 265 {
 266         void *new_arena;
 267
 268         new_arena = mmap(arena->p, len,
 269                          PROT_READ | PROT_WRITE,
 270                          MAP_ANONYMOUS | MAP_PRIVATE,
 271                          -1, 0);
 272         /*
 273          * re-used the same region ?
 274          */
 275         if (new_arena == arena->p)
 276                 return;
 277
 278         memcpy(new_arena, arena->p, arena->len);
 279         bzero(new_arena + arena->len, len - arena->len);
 280         arena->p = new_arena;
 281 }
 282
 283 /* Called with signals off and mutex locked */
 284 static void add_thread(void)
 285 {
 286         struct urcu_reader *urcu_reader_reg;
 287
 288         if (registry_arena.len
 289             < registry_arena.used + sizeof(struct urcu_reader))
 290                 resize_arena(&registry_arena,
 291                 max(registry_arena.len << 1, ARENA_INIT_ALLOC));
 292         /*
 293          * Find a free spot.
 294          */
 295         for (urcu_reader_reg = registry_arena.p;
 296              (void *)urcu_reader_reg < registry_arena.p + registry_arena.len;
 297              urcu_reader_reg++) {
 298                 if (!urcu_reader_reg->alloc)
 299                         break;
 300         }
 301         urcu_reader_reg->alloc = 1;
 302         registry_arena.used += sizeof(struct urcu_reader);
 303
 304         /* Add to registry */
 305         urcu_reader_reg->tid = pthread_self();
 306         assert(urcu_reader_reg->ctr == 0);
 307         list_add(&urcu_reader_reg->head, &registry);
 308         urcu_reader = urcu_reader_reg;
 309 }
 310
 311 /* Called with signals off and mutex locked */
 312 static void rcu_gc_registry(void)
 313 {
 314         struct urcu_reader *urcu_reader_reg;
 315         pthread_t tid;
 316         int ret;
 317
 318         for (urcu_reader_reg = registry_arena.p;
 319              (void *)urcu_reader_reg < registry_arena.p + registry_arena.len;
 320              urcu_reader_reg++) {
 321                 if (!urcu_reader_reg->alloc)
 322                         continue;
 323                 tid = urcu_reader_reg->tid;
 324                 ret = pthread_kill(tid, 0);
 325                 assert(ret != EINVAL);
 326                 if (ret == ESRCH) {
 327                         list_del(&urcu_reader_reg->head);
 328                         urcu_reader_reg->alloc = 0;
 329                         registry_arena.used -= sizeof(struct urcu_reader);
 330                 }
 331         }
 332 }
 333
 334 /* Disable signals, take mutex, add to registry */
 335 void rcu_bp_register(void)
 336 {
 337         sigset_t newmask, oldmask;
 338         int ret;
 339
 340         ret = sigemptyset(&newmask);
 341         assert(!ret);
 342         ret = pthread_sigmask(SIG_SETMASK, &newmask, &oldmask);
 343         assert(!ret);
 344
 345         /*
 346          * Check if a signal concurrently registered our thread since
 347          * the check in rcu_read_lock(). */
 348         if (urcu_reader)
 349                 goto end;
 350
 351         internal_urcu_lock();
 352         add_thread();
 353         internal_urcu_unlock();
 354 end:
 355         ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
 356         assert(!ret);
 357 }
 358
 359 void urcu_bp_exit()
 360 {
 361         munmap(registry_arena.p, registry_arena.len);
 362 }