runtime/objc-sync.m

   1 /*
   2  * Copyright (c) 1999-2007 Apple Inc.  All Rights Reserved.
   3  *
   4  * @APPLE_LICENSE_HEADER_START@
   5  *
   6  * This file contains Original Code and/or Modifications of Original Code
   7  * as defined in and that are subject to the Apple Public Source License
   8  * Version 2.0 (the 'License'). You may not use this file except in
   9  * compliance with the License. Please obtain a copy of the License at
  10  * http://www.opensource.apple.com/apsl/ and read it before using this
  11  * file.
  12  *
  13  * The Original Code and all software distributed under the License are
  14  * distributed on an 'AS IS' basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  15  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  16  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  17  * FITNESS FOR A PARTICULAR PURPOSE, QUIET ENJOYMENT OR NON-INFRINGEMENT.
  18  * Please see the License for the specific language governing rights and
  19  * limitations under the License.
  20  *
  21  * @APPLE_LICENSE_HEADER_END@
  22  */
  23
  24 #include "objc-private.h"
  25 #include "objc-sync.h"
  26
  27 //
  28 // Allocate a lock only when needed.  Since few locks are needed at any point
  29 // in time, keep them on a single list.
  30 //
  31
  32
  33 typedef struct SyncData {
  34     struct SyncData* nextData;
  35     id               object;
  36     int              threadCount;  // number of THREADS using this block
  37     recursive_mutex_t        mutex;
  38 } SyncData;
  39
  40 typedef struct {
  41     SyncData *data;
  42     unsigned int lockCount;  // number of times THIS THREAD locked this block
  43 } SyncCacheItem;
  44
  45 typedef struct SyncCache {
  46     unsigned int allocated;
  47     unsigned int used;
  48     SyncCacheItem list[0];
  49 } SyncCache;
  50
  51 /*
  52   Fast cache: two fixed pthread keys store a single SyncCacheItem.
  53   This avoids malloc of the SyncCache for threads that only synchronize
  54   a single object at a time.
  55   SYNC_DATA_DIRECT_KEY  == SyncCacheItem.data
  56   SYNC_COUNT_DIRECT_KEY == SyncCacheItem.lockCount
  57  */
  58
  59 typedef struct {
  60     SyncData *data;
  61     OSSpinLock lock;
  62
  63     char align[64 - sizeof (OSSpinLock) - sizeof (SyncData *)];
  64 } SyncList __attribute__((aligned(64)));
  65 // aligned to put locks on separate cache lines
  66
  67 // Use multiple parallel lists to decrease contention among unrelated objects.
  68 #define COUNT 16
  69 #define HASH(obj) ((((uintptr_t)(obj)) >> 5) & (COUNT - 1))
  70 #define LOCK_FOR_OBJ(obj) sDataLists[HASH(obj)].lock
  71 #define LIST_FOR_OBJ(obj) sDataLists[HASH(obj)].data
  72 static SyncList sDataLists[COUNT];
  73
  74
  75 enum usage { ACQUIRE, RELEASE, CHECK };
  76
  77 static SyncCache *fetch_cache(BOOL create)
  78 {
  79     _objc_pthread_data *data;
  80
  81     data = _objc_fetch_pthread_data(create);
  82     if (!data) return NULL;
  83
  84     if (!data->syncCache) {
  85         if (!create) {
  86             return NULL;
  87         } else {
  88             int count = 4;
  89             data->syncCache = calloc(1, sizeof(SyncCache) +
  90                                      count*sizeof(SyncCacheItem));
  91             data->syncCache->allocated = count;
  92         }
  93     }
  94
  95     // Make sure there's at least one open slot in the list.
  96     if (data->syncCache->allocated == data->syncCache->used) {
  97         data->syncCache->allocated *= 2;
  98         data->syncCache =
  99             realloc(data->syncCache, sizeof(SyncCache)
 100                     + data->syncCache->allocated * sizeof(SyncCacheItem));
 101     }
 102
 103     return data->syncCache;
 104 }
 105
 106
 107 PRIVATE_EXTERN void _destroySyncCache(struct SyncCache *cache)
 108 {
 109     if (cache) free(cache);
 110 }
 111
 112
 113 static SyncData* id2data(id object, enum usage why)
 114 {
 115     OSSpinLock *lockp = &LOCK_FOR_OBJ(object);
 116     SyncData **listp = &LIST_FOR_OBJ(object);
 117     SyncData* result = NULL;
 118
 119 #if SUPPORT_DIRECT_THREAD_KEYS
 120     // Check per-thread single-entry fast cache for matching object
 121     BOOL fastCacheOccupied = NO;
 122     SyncData *data = tls_get_direct(SYNC_DATA_DIRECT_KEY);
 123     if (data) {
 124         fastCacheOccupied = YES;
 125
 126         if (data->object == object) {
 127             // Found a match in fast cache.
 128             uintptr_t lockCount;
 129
 130             result = data;
 131             lockCount = (uintptr_t)tls_get_direct(SYNC_COUNT_DIRECT_KEY);
 132             require_action_string(result->threadCount > 0, fastcache_done,
 133                                   result = NULL, "id2data fastcache is buggy");
 134             require_action_string(lockCount > 0, fastcache_done,
 135                                   result = NULL, "id2data fastcache is buggy");
 136
 137             switch(why) {
 138             case ACQUIRE: {
 139                 lockCount++;
 140                 tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)lockCount);
 141                 break;
 142             }
 143             case RELEASE:
 144                 lockCount--;
 145                 tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)lockCount);
 146                 if (lockCount == 0) {
 147                     // remove from fast cache
 148                     tls_set_direct(SYNC_DATA_DIRECT_KEY, NULL);
 149                     // atomic because may collide with concurrent ACQUIRE
 150                     OSAtomicDecrement32Barrier(&result->threadCount);
 151                 }
 152                 break;
 153             case CHECK:
 154                 // do nothing
 155                 break;
 156             }
 157
 158         fastcache_done:
 159             return result;
 160         }
 161     }
 162 #endif
 163
 164     // Check per-thread cache of already-owned locks for matching object
 165     SyncCache *cache = fetch_cache(NO);
 166     if (cache) {
 167         unsigned int i;
 168         for (i = 0; i < cache->used; i++) {
 169             SyncCacheItem *item = &cache->list[i];
 170             if (item->data->object != object) continue;
 171
 172             // Found a match.
 173             result = item->data;
 174             require_action_string(result->threadCount > 0, cache_done,
 175                                   result = NULL, "id2data cache is buggy");
 176             require_action_string(item->lockCount > 0, cache_done,
 177                                   result = NULL, "id2data cache is buggy");
 178
 179             switch(why) {
 180             case ACQUIRE:
 181                 item->lockCount++;
 182                 break;
 183             case RELEASE:
 184                 item->lockCount--;
 185                 if (item->lockCount == 0) {
 186                     // remove from per-thread cache
 187                     cache->list[i] = cache->list[--cache->used];
 188                     // atomic because may collide with concurrent ACQUIRE
 189                     OSAtomicDecrement32Barrier(&result->threadCount);
 190                 }
 191                 break;
 192             case CHECK:
 193                 // do nothing
 194                 break;
 195             }
 196
 197         cache_done:
 198             return result;
 199         }
 200     }
 201
 202     // Thread cache didn't find anything.
 203     // Walk in-use list looking for matching object
 204     // Spinlock prevents multiple threads from creating multiple
 205     // locks for the same new object.
 206     // We could keep the nodes in some hash table if we find that there are
 207     // more than 20 or so distinct locks active, but we don't do that now.
 208
 209     OSSpinLockLock(lockp);
 210
 211     {
 212         SyncData* p;
 213         SyncData* firstUnused = NULL;
 214         for (p = *listp; p != NULL; p = p->nextData) {
 215             if ( p->object == object ) {
 216                 result = p;
 217                 // atomic because may collide with concurrent RELEASE
 218                 OSAtomicIncrement32Barrier(&result->threadCount);
 219                 goto done;
 220             }
 221             if ( (firstUnused == NULL) && (p->threadCount == 0) )
 222                 firstUnused = p;
 223         }
 224
 225         // no SyncData currently associated with object
 226         if ( (why == RELEASE) || (why == CHECK) )
 227             goto done;
 228
 229         // an unused one was found, use it
 230         if ( firstUnused != NULL ) {
 231             result = firstUnused;
 232             result->object = object;
 233             result->threadCount = 1;
 234             goto done;
 235         }
 236     }
 237
 238     // malloc a new SyncData and add to list.
 239     // XXX calling malloc with a global lock held is bad practice,
 240     // might be worth releasing the lock, mallocing, and searching again.
 241     // But since we never free these guys we won't be stuck in malloc very often.
 242     result = (SyncData*)calloc(sizeof(SyncData), 1);
 243     result->object = object;
 244     result->threadCount = 1;
 245     recursive_mutex_init(&result->mutex);
 246     result->nextData = *listp;
 247     *listp = result;
 248
 249  done:
 250     OSSpinLockUnlock(lockp);
 251     if (result) {
 252         // Only new ACQUIRE should get here.
 253         // All RELEASE and CHECK and recursive ACQUIRE are
 254         // handled by the per-thread caches above.
 255
 256         require_string(result != NULL, really_done, "id2data is buggy");
 257         require_action_string(why == ACQUIRE, really_done,
 258                               result = NULL, "id2data is buggy");
 259         require_action_string(result->object == object, really_done,
 260                               result = NULL, "id2data is buggy");
 261
 262 #if SUPPORT_DIRECT_THREAD_KEYS
 263         if (!fastCacheOccupied) {
 264             // Save in fast thread cache
 265             tls_set_direct(SYNC_DATA_DIRECT_KEY, result);
 266             tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)1);
 267         } else
 268 #endif
 269         {
 270             // Save in thread cache
 271             if (!cache) cache = fetch_cache(YES);
 272             cache->list[cache->used].data = result;
 273             cache->list[cache->used].lockCount = 1;
 274             cache->used++;
 275         }
 276     }
 277
 278  really_done:
 279     return result;
 280 }
 281
 282
 283 BREAKPOINT_FUNCTION(
 284     void objc_sync_nil(void)
 285 );
 286
 287
 288 // Begin synchronizing on 'obj'.
 289 // Allocates recursive mutex associated with 'obj' if needed.
 290 // Returns OBJC_SYNC_SUCCESS once lock is acquired.
 291 int objc_sync_enter(id obj)
 292 {
 293     int result = OBJC_SYNC_SUCCESS;
 294
 295     if (obj) {
 296         SyncData* data = id2data(obj, ACQUIRE);
 297         require_action_string(data != NULL, done, result = OBJC_SYNC_NOT_INITIALIZED, "id2data failed");
 298
 299         result = recursive_mutex_lock(&data->mutex);
 300         require_noerr_string(result, done, "mutex_lock failed");
 301     } else {
 302         // @synchronized(nil) does nothing
 303         if (DebugNilSync) {
 304             _objc_inform("NIL SYNC DEBUG: @synchronized(nil); set a breakpoint on objc_sync_nil to debug");
 305         }
 306         objc_sync_nil();
 307     }
 308
 309 done:
 310     return result;
 311 }
 312
 313
 314 // End synchronizing on 'obj'.
 315 // Returns OBJC_SYNC_SUCCESS or OBJC_SYNC_NOT_OWNING_THREAD_ERROR
 316 int objc_sync_exit(id obj)
 317 {
 318     int result = OBJC_SYNC_SUCCESS;
 319
 320     if (obj) {
 321         SyncData* data = id2data(obj, RELEASE);
 322         require_action_string(data != NULL, done, result = OBJC_SYNC_NOT_OWNING_THREAD_ERROR, "id2data failed");
 323
 324         result = recursive_mutex_unlock(&data->mutex);
 325         require_noerr_string(result, done, "mutex_unlock failed");
 326     } else {
 327         // @synchronized(nil) does nothing
 328     }
 329
 330 done:
 331     if ( result == RECURSIVE_MUTEX_NOT_LOCKED )
 332          result = OBJC_SYNC_NOT_OWNING_THREAD_ERROR;
 333
 334     return result;
 335 }
 336