urcu-bp.c

   1 /*
   2  * urcu-bp.c
   3  *
   4  * Userspace RCU library, "bulletproof" version.
   5  *
   6  * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   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 CDS_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 /* Saved fork signal mask, protected by rcu_gp_lock */
  80 static sigset_t saved_fork_signal_mask;
  81
  82 static void rcu_gc_registry(void);
  83
  84 static void mutex_lock(pthread_mutex_t *mutex)
  85 {
  86         int ret;
  87
  88 #ifndef DISTRUST_SIGNALS_EXTREME
  89         ret = pthread_mutex_lock(mutex);
  90         if (ret) {
  91                 perror("Error in pthread mutex lock");
  92                 exit(-1);
  93         }
  94 #else /* #ifndef DISTRUST_SIGNALS_EXTREME */
  95         while ((ret = pthread_mutex_trylock(mutex)) != 0) {
  96                 if (ret != EBUSY && ret != EINTR) {
  97                         printf("ret = %d, errno = %d\n", ret, errno);
  98                         perror("Error in pthread mutex lock");
  99                         exit(-1);
 100                 }
 101                 poll(NULL,0,10);
 102         }
 103 #endif /* #else #ifndef DISTRUST_SIGNALS_EXTREME */
 104 }
 105
 106 static void mutex_unlock(pthread_mutex_t *mutex)
 107 {
 108         int ret;
 109
 110         ret = pthread_mutex_unlock(mutex);
 111         if (ret) {
 112                 perror("Error in pthread mutex unlock");
 113                 exit(-1);
 114         }
 115 }
 116
 117 void update_counter_and_wait(void)
 118 {
 119         CDS_LIST_HEAD(qsreaders);
 120         int wait_loops = 0;
 121         struct rcu_reader *index, *tmp;
 122
 123         /* Switch parity: 0 -> 1, 1 -> 0 */
 124         CMM_STORE_SHARED(rcu_gp_ctr, rcu_gp_ctr ^ RCU_GP_CTR_PHASE);
 125
 126         /*
 127          * Must commit qparity update to memory before waiting for other parity
 128          * quiescent state. Failure to do so could result in the writer waiting
 129          * forever while new readers are always accessing data (no progress).
 130          * Ensured by CMM_STORE_SHARED and CMM_LOAD_SHARED.
 131          */
 132
 133         /*
 134          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 135          * model easier to understand. It does not have a big performance impact
 136          * anyway, given this is the write-side.
 137          */
 138         cmm_smp_mb();
 139
 140         /*
 141          * Wait for each thread rcu_reader.ctr count to become 0.
 142          */
 143         for (;;) {
 144                 wait_loops++;
 145                 cds_list_for_each_entry_safe(index, tmp, &registry, node) {
 146                         if (!rcu_old_gp_ongoing(&index->ctr))
 147                                 cds_list_move(&index->node, &qsreaders);
 148                 }
 149
 150                 if (cds_list_empty(&registry)) {
 151                         break;
 152                 } else {
 153                         if (wait_loops == RCU_QS_ACTIVE_ATTEMPTS)
 154                                 usleep(RCU_SLEEP_DELAY);
 155                         else
 156                                 caa_cpu_relax();
 157                 }
 158         }
 159         /* put back the reader list in the registry */
 160         cds_list_splice(&qsreaders, &registry);
 161 }
 162
 163 void synchronize_rcu(void)
 164 {
 165         sigset_t newmask, oldmask;
 166         int ret;
 167
 168         ret = sigemptyset(&newmask);
 169         assert(!ret);
 170         ret = pthread_sigmask(SIG_SETMASK, &newmask, &oldmask);
 171         assert(!ret);
 172
 173         mutex_lock(&rcu_gp_lock);
 174
 175         if (cds_list_empty(&registry))
 176                 goto out;
 177
 178         /* All threads should read qparity before accessing data structure
 179          * where new ptr points to. */
 180         /* Write new ptr before changing the qparity */
 181         cmm_smp_mb();
 182
 183         /* Remove old registry elements */
 184         rcu_gc_registry();
 185
 186         /*
 187          * Wait for previous parity to be empty of readers.
 188          */
 189         update_counter_and_wait();      /* 0 -> 1, wait readers in parity 0 */
 190
 191         /*
 192          * Adding a cmm_smp_mb() which is _not_ formally required, but makes the
 193          * model easier to understand. It does not have a big performance impact
 194          * anyway, given this is the write-side.
 195          */
 196         cmm_smp_mb();
 197
 198         /*
 199          * Wait for previous parity to be empty of readers.
 200          */
 201         update_counter_and_wait();      /* 1 -> 0, wait readers in parity 1 */
 202
 203         /*
 204          * Finish waiting for reader threads before letting the old ptr being
 205          * freed.
 206          */
 207         cmm_smp_mb();
 208 out:
 209         mutex_unlock(&rcu_gp_lock);
 210         ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
 211         assert(!ret);
 212 }
 213
 214 /*
 215  * library wrappers to be used by non-LGPL compatible source code.
 216  */
 217
 218 void rcu_read_lock(void)
 219 {
 220         _rcu_read_lock();
 221 }
 222
 223 void rcu_read_unlock(void)
 224 {
 225         _rcu_read_unlock();
 226 }
 227
 228 /*
 229  * only grow for now.
 230  */
 231 static void resize_arena(struct registry_arena *arena, size_t len)
 232 {
 233         void *new_arena;
 234
 235         if (!arena->p)
 236                 new_arena = mmap(arena->p, len,
 237                                  PROT_READ | PROT_WRITE,
 238                                  MAP_ANONYMOUS | MAP_PRIVATE,
 239                                  -1, 0);
 240         else
 241                 new_arena = mremap(arena->p, arena->len,
 242                                    len, MREMAP_MAYMOVE);
 243         assert(new_arena != MAP_FAILED);
 244
 245         /*
 246          * re-used the same region ?
 247          */
 248         if (new_arena == arena->p)
 249                 return;
 250
 251         memcpy(new_arena, arena->p, arena->len);
 252         bzero(new_arena + arena->len, len - arena->len);
 253         arena->p = new_arena;
 254 }
 255
 256 /* Called with signals off and mutex locked */
 257 static void add_thread(void)
 258 {
 259         struct rcu_reader *rcu_reader_reg;
 260
 261         if (registry_arena.len
 262             < registry_arena.used + sizeof(struct rcu_reader))
 263                 resize_arena(&registry_arena,
 264                 max(registry_arena.len << 1, ARENA_INIT_ALLOC));
 265         /*
 266          * Find a free spot.
 267          */
 268         for (rcu_reader_reg = registry_arena.p;
 269              (void *)rcu_reader_reg < registry_arena.p + registry_arena.len;
 270              rcu_reader_reg++) {
 271                 if (!rcu_reader_reg->alloc)
 272                         break;
 273         }
 274         rcu_reader_reg->alloc = 1;
 275         registry_arena.used += sizeof(struct rcu_reader);
 276
 277         /* Add to registry */
 278         rcu_reader_reg->tid = pthread_self();
 279         assert(rcu_reader_reg->ctr == 0);
 280         cds_list_add(&rcu_reader_reg->node, &registry);
 281         rcu_reader = rcu_reader_reg;
 282 }
 283
 284 /* Called with signals off and mutex locked */
 285 static void rcu_gc_registry(void)
 286 {
 287         struct rcu_reader *rcu_reader_reg;
 288         pthread_t tid;
 289         int ret;
 290
 291         for (rcu_reader_reg = registry_arena.p;
 292              (void *)rcu_reader_reg < registry_arena.p + registry_arena.len;
 293              rcu_reader_reg++) {
 294                 if (!rcu_reader_reg->alloc)
 295                         continue;
 296                 tid = rcu_reader_reg->tid;
 297                 ret = pthread_kill(tid, 0);
 298                 assert(ret != EINVAL);
 299                 if (ret == ESRCH) {
 300                         cds_list_del(&rcu_reader_reg->node);
 301                         rcu_reader_reg->ctr = 0;
 302                         rcu_reader_reg->alloc = 0;
 303                         registry_arena.used -= sizeof(struct rcu_reader);
 304                 }
 305         }
 306 }
 307
 308 /* Disable signals, take mutex, add to registry */
 309 void rcu_bp_register(void)
 310 {
 311         sigset_t newmask, oldmask;
 312         int ret;
 313
 314         ret = sigemptyset(&newmask);
 315         assert(!ret);
 316         ret = pthread_sigmask(SIG_SETMASK, &newmask, &oldmask);
 317         assert(!ret);
 318
 319         /*
 320          * Check if a signal concurrently registered our thread since
 321          * the check in rcu_read_lock(). */
 322         if (rcu_reader)
 323                 goto end;
 324
 325         mutex_lock(&rcu_gp_lock);
 326         add_thread();
 327         mutex_unlock(&rcu_gp_lock);
 328 end:
 329         ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
 330         assert(!ret);
 331 }
 332
 333 void rcu_bp_exit()
 334 {
 335         munmap(registry_arena.p, registry_arena.len);
 336 }
 337
 338 /*
 339  * Holding the rcu_gp_lock across fork will make sure we fork() don't race with
 340  * a concurrent thread executing with this same lock held. This ensures that the
 341  * registry is in a coherent state in the child.
 342  */
 343 void rcu_bp_before_fork(void)
 344 {
 345         sigset_t newmask, oldmask;
 346         int ret;
 347
 348         ret = sigemptyset(&newmask);
 349         assert(!ret);
 350         ret = pthread_sigmask(SIG_SETMASK, &newmask, &oldmask);
 351         assert(!ret);
 352         mutex_lock(&rcu_gp_lock);
 353         saved_fork_signal_mask = oldmask;
 354 }
 355
 356 void rcu_bp_after_fork_parent(void)
 357 {
 358         sigset_t oldmask;
 359         int ret;
 360
 361         oldmask = saved_fork_signal_mask;
 362         mutex_unlock(&rcu_gp_lock);
 363         ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
 364         assert(!ret);
 365 }
 366
 367 void rcu_bp_after_fork_child(void)
 368 {
 369         sigset_t oldmask;
 370         int ret;
 371
 372         rcu_gc_registry();
 373         oldmask = saved_fork_signal_mask;
 374         mutex_unlock(&rcu_gp_lock);
 375         ret = pthread_sigmask(SIG_SETMASK, &oldmask, NULL);
 376         assert(!ret);
 377 }