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 #define _GNU_SOURCE
  27 #include <stdio.h>
  28 #include <pthread.h>
  29 #include <signal.h>
  30 #include <assert.h>
  31 #include <stdlib.h>
  32 #include <string.h>
  33 #include <errno.h>
  34 #include <poll.h>
  35 #include <unistd.h>
  36 #include <sys/mman.h>
  37
  38 #include "urcu-bp-static.h"
  39 /* Do not #define _LGPL_SOURCE to ensure we can emit the wrapper symbols */
  40 #include "urcu-bp.h"
  41
  42 /* Sleep delay in us */
  43 #define RCU_SLEEP_DELAY         1000
  44 #define ARENA_INIT_ALLOC        16
  45
  46 void __attribute__((destructor)) rcu_bp_exit(void);
  47
  48 static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
  49
  50 #ifdef DEBUG_YIELD
  51 unsigned int yield_active;
  52 unsigned int __thread rand_yield;
  53 #endif
  54
  55 /*
  56  * Global grace period counter.
  57  * Contains the current RCU_GP_CTR_PHASE.
  58  * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
  59  * Written to only by writer with mutex taken. Read by both writer and readers.
  60  */
  61 long rcu_gp_ctr = RCU_GP_COUNT;
  62
  63 /*
  64  * Pointer to registry elements. Written to only by each individual reader. Read
  65  * by both the reader and the writers.
  66  */
  67 struct rcu_reader __thread *rcu_reader;
  68
  69 static LIST_HEAD(registry);
  70
  71 struct registry_arena {
  72         void *p;
  73         size_t len;
  74         size_t used;
  75 };
  76
  77 static struct registry_arena registry_arena;
  78
  79 static void rcu_gc_registry(void);
  80
  81 static void mutex_lock(pthread_mutex_t *mutex)
  82 {
  83         int ret;
  84
  85 #ifndef DISTRUST_SIGNALS_EXTREME
  86         ret = pthread_mutex_lock(mutex);
  87         if (ret) {
  88                 perror("Error in pthread mutex lock");
  89                 exit(-1);
  90         }
  91 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
  92         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
  93                 if (ret != EBUSY && ret != EINTR) {
  94                         printf("ret = %d, errno = %d\n", ret, errno);
  95                         perror("Error in pthread mutex lock");
  96                         exit(-1);
  97                 }
  98                 if (rcu_reader.need_mb) {
  99                         smp_mb();
 100                         rcu_reader.need_mb = 0;
 101                         smp_mb();
 102                 }
 103                 poll(NULL,0,10);
 104         }
 105 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 106 }
 107
 108 static void mutex_unlock(pthread_mutex_t *mutex)
 109 {
 110         int ret;
 111
 112         ret = pthread_mutex_unlock(mutex);
 113         if (ret) {
 114                 perror("Error in pthread mutex unlock");
 115                 exit(-1);
 116         }
 117 }
 118
 119 void update_counter_and_wait(void)
 120 {
 121         LIST_HEAD(qsreaders);
 122         int wait_loops = 0;
 123         struct rcu_reader *index, *tmp;
 124
 125         /* Switch parity: 0 -> 1, 1 -> 0 */
 126         STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE);
 127
 128         /*
 129          * Must commit qparity update to memory before waiting for other parity
 130          * quiescent state. Failure to do so could result in the writer waiting
 131          * forever while new readers are always accessing data (no progress).
 132          * Ensured by STORE_SHARED and LOAD_SHARED.
 133          */
 134
 135         /*
 136          * Adding a smp_mb() which is _not_ formally required, but makes the
 137          * model easier to understand. It does not have a big performance impact
 138          * anyway, given this is the write-side.
 139          */
 140         smp_mb();
 141
 142         /*
 143          * Wait for each thread rcu_reader.ctr count to become 0.
 144          */
 145         for (;;) {
 146                 wait_loops++;
 147                 list_for_each_entry_safe(index, tmp, &registry, head) {
 148                         if (!rcu_old_gp_ongoing(&index->ctr))
 149                                 list_move(&index->head, &qsreaders);
 150                 }
 151
 152                 if (list_empty(&registry)) {
 153                         break;
 154                 } else {
 155                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
 156                                 usleep(RCU_SLEEP_DELAY);
 157                         else
 158                                 cpu_relax();
 159                 }
 160         }
 161         /* put back the reader list in the registry */
 162         list_splice(&qsreaders, &registry);
 163 }
 164
 165 void synchronize_rcu(void)
 166 {
 167         sigset_t newmask, oldmask;
 168         int ret;
 169
 170         ret = sigemptyset(&newmask);
 171         assert(!ret);
 172         ret = pthread_sigmask(SIG_SETMASK, &newmask, &oldmask);
 173         assert(!ret);
 174
 175         mutex_lock(&rcu_gp_lock);
 176
 177         if (list_empty(&registry))
 178                 goto out;
 179
 180         /* All threads should read qparity before accessing data structure
 181          * where new ptr points to. */
 182         /* Write new ptr before changing the qparity */
 183         smp_mb();
 184
 185         /* Remove old registry elements */
 186         rcu_gc_registry();
 187
 188         /*
 189          * Wait for previous parity to be empty of readers.
 190          */
 191         update_counter_and_wait();      /* 0 -> 1, wait readers in parity 0 */
 192
 193         /*
 194          * Adding a smp_mb() which is _not_ formally required, but makes the
 195          * model easier to understand. It does not have a big performance impact
 196          * anyway, given this is the write-side.
 197          */
 198         smp_mb();
 199
 200         /*
 201          * Wait for previous parity to be empty of readers.
 202          */
 203         update_counter_and_wait();      /* 1 -> 0, wait readers in parity 1 */
 204
 205         /*
 206          * Finish waiting for reader threads before letting the old ptr being
 207          * freed.
 208          */
 209         smp_mb();
 210 out:
 211         mutex_unlock(&rcu_gp_lock);
 212         ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
 213         assert(!ret);
 214 }
 215
 216 /*
 217  * library wrappers to be used by non-LGPL compatible source code.
 218  */
 219
 220 void rcu_read_lock(void)
 221 {
 222         _rcu_read_lock();
 223 }
 224
 225 void rcu_read_unlock(void)
 226 {
 227         _rcu_read_unlock();
 228 }
 229
 230 /*
 231  * only grow for now.
 232  */
 233 static void resize_arena(struct registry_arena *arena, size_t len)
 234 {
 235         void *new_arena;
 236
 237         if (!arena->p)
 238                 new_arena = mmap(arena->p, len,
 239                                  PROT_READ | PROT_WRITE,
 240                                  MAP_ANONYMOUS | MAP_PRIVATE,
 241                                  -1, 0);
 242         else
 243                 new_arena = mremap(arena->p, arena->len,
 244                                    len, MREMAP_MAYMOVE);
 245         assert(new_arena != MAP_FAILED);
 246
 247         /*
 248          * re-used the same region ?
 249          */
 250         if (new_arena == arena->p)
 251                 return;
 252
 253         memcpy(new_arena, arena->p, arena->len);
 254         bzero(new_arena + arena->len, len - arena->len);
 255         arena->p = new_arena;
 256 }
 257
 258 /* Called with signals off and mutex locked */
 259 static void add_thread(void)
 260 {
 261         struct rcu_reader *rcu_reader_reg;
 262
 263         if (registry_arena.len
 264             < registry_arena.used + sizeof(struct rcu_reader))
 265                 resize_arena(&registry_arena,
 266                 max(registry_arena.len << 1, ARENA_INIT_ALLOC));
 267         /*
 268          * Find a free spot.
 269          */
 270         for (rcu_reader_reg = registry_arena.p;
 271              (void *)rcu_reader_reg < registry_arena.p + registry_arena.len;
 272              rcu_reader_reg++) {
 273                 if (!rcu_reader_reg->alloc)
 274                         break;
 275         }
 276         rcu_reader_reg->alloc = 1;
 277         registry_arena.used += sizeof(struct rcu_reader);
 278
 279         /* Add to registry */
 280         rcu_reader_reg->tid = pthread_self();
 281         assert(rcu_reader_reg->ctr == 0);
 282         list_add(&rcu_reader_reg->head, &registry);
 283         rcu_reader = rcu_reader_reg;
 284 }
 285
 286 /* Called with signals off and mutex locked */
 287 static void rcu_gc_registry(void)
 288 {
 289         struct rcu_reader *rcu_reader_reg;
 290         pthread_t tid;
 291         int ret;
 292
 293         for (rcu_reader_reg = registry_arena.p;
 294              (void *)rcu_reader_reg < registry_arena.p + registry_arena.len;
 295              rcu_reader_reg++) {
 296                 if (!rcu_reader_reg->alloc)
 297                         continue;
 298                 tid = rcu_reader_reg->tid;
 299                 ret = pthread_kill(tid, 0);
 300                 assert(ret != EINVAL);
 301                 if (ret == ESRCH) {
 302                         list_del(&rcu_reader_reg->head);
 303                         rcu_reader_reg->alloc = 0;
 304                         registry_arena.used -= sizeof(struct rcu_reader);
 305                 }
 306         }
 307 }
 308
 309 /* Disable signals, take mutex, add to registry */
 310 void rcu_bp_register(void)
 311 {
 312         sigset_t newmask, oldmask;
 313         int ret;
 314
 315         ret = sigemptyset(&newmask);
 316         assert(!ret);
 317         ret = pthread_sigmask(SIG_SETMASK, &newmask, &oldmask);
 318         assert(!ret);
 319
 320         /*
 321          * Check if a signal concurrently registered our thread since
 322          * the check in rcu_read_lock(). */
 323         if (rcu_reader)
 324                 goto end;
 325
 326         mutex_lock(&rcu_gp_lock);
 327         add_thread();
 328         mutex_unlock(&rcu_gp_lock);
 329 end:
 330         ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
 331         assert(!ret);
 332 }
 333
 334 void rcu_bp_exit()
 335 {
 336         munmap(registry_arena.p, registry_arena.len);
 337 }