多线程 — 锁
2018-05-17 本文已影响0人
GTMYang
互斥锁 NSLock
加锁之后在解锁之前，资源独占其他人不能访问
递归锁 NSRecursiveLock
同一线程可重入的锁。
解决同一线程多次申请同一个锁造成的死锁问题。
func A(){
Lock.lock();
B();
Lock.unlock();
}
func B() {
Lock.lock();
// do sth.
Lock.unlock();
}
读写锁
读操作的时候允许其他读操作执行，但不允许写操作执行。
写操作的时候既不允许写操作也不允许读操作。
对于读数据比写数据频繁的操作，用读写锁代替互斥锁可以提高效率。
自旋锁 OSSpinLock
轮询检查锁状态，不阻塞线程。
对于预计等待锁定时间较短的多处理器系统，不阻塞线程能够提高性能，因为能减少线程切换(上下文的保存：寄存器，堆栈信息。修改内核内存中的线程数据结构)的开销。
条件锁 NSConditionLock
带条件的锁。
可以根据条件来设置线程的执行顺序。
static int index = 1;
//主线程中
NSConditionLock *lock = [[NSConditionLock alloc] initWithCondition:0];
//线程1第3个执行
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[lock lockWhenCondition:2];
NSLog(@"线程1第%i个执行", index++);
sleep(2);
[lock unlockWithCondition:3];
});
//线程2第1个执行
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[lock lockWhenCondition:0];
NSLog(@"线程2第%i个执行", index++);
sleep(2);
[lock unlockWithCondition:1];
});
//线程3第2个执行
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[lock lockWhenCondition:1];
NSLog(@"线程3第%i个执行", index++);
sleep(2);
[lock unlockWithCondition:2];
});
//线程4第4个执行
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
[lock lockWhenCondition:3];
NSLog(@"线程4第%i个执行", index++);
sleep(2);
[lock unlock];
});
条件 NSCondition
让线程可等待，可被唤醒。
使用场景：依赖关系
比Lock多了四个方法：
- (void)wait; // 等待唤醒signal，broadcast
- (BOOL)waitUntilDate:(NSDate *)limit; // 等待唤醒一段时间
- (void)signal; // 唤醒其他等待的线程
- (void)broadcast; // 唤醒其他等待的线程
同步块@synchronized
// 源代码
@synchronized(obj) {
// do work
}
// 编译器转换后
@try {
objc_sync_enter(obj);
// do work
} @finally {
objc_sync_exit(obj);
}
objc_sync_enter 和 objc_sync_exit
// Begin synchronizing on 'obj'.
// Allocates recursive mutex associated with 'obj' if needed.
// Returns OBJC_SYNC_SUCCESS once lock is acquired.
int objc_sync_enter(id obj)
{
int result = OBJC_SYNC_SUCCESS;
if (obj) {
SyncData* data = id2data(obj, ACQUIRE);
assert(data);
data->mutex.lock();
} else {
// @synchronized(nil) does nothing
if (DebugNilSync) {
_objc_inform("NIL SYNC DEBUG: @synchronized(nil); set a breakpoint on objc_sync_nil to debug");
}
objc_sync_nil();
}
return result;
}
// End synchronizing on 'obj'.
// Returns OBJC_SYNC_SUCCESS or OBJC_SYNC_NOT_OWNING_THREAD_ERROR
int objc_sync_exit(id obj)
{
int result = OBJC_SYNC_SUCCESS;
if (obj) {
SyncData* data = id2data(obj, RELEASE);
if (!data) {
result = OBJC_SYNC_NOT_OWNING_THREAD_ERROR;
} else {
bool okay = data->mutex.tryUnlock();
if (!okay) {
result = OBJC_SYNC_NOT_OWNING_THREAD_ERROR;
}
}
} else {
// @synchronized(nil) does nothing
}
return result;
}
typedef struct SyncData {
struct SyncData* nextData;
DisguisedPtr<objc_object> object;
int32_t threadCount; // number of THREADS using this block
recursive_mutex_t mutex;
} SyncData;
static SyncData* id2data(id object, enum usage why)
{
spinlock_t *lockp = &LOCK_FOR_OBJ(object);
SyncData **listp = &LIST_FOR_OBJ(object);
SyncData* result = NULL;
#if SUPPORT_DIRECT_THREAD_KEYS
// Check per-thread single-entry fast cache for matching object
bool fastCacheOccupied = NO;
SyncData *data = (SyncData *)tls_get_direct(SYNC_DATA_DIRECT_KEY);
if (data) {
fastCacheOccupied = YES;
if (data->object == object) {
// Found a match in fast cache.
uintptr_t lockCount;
result = data;
lockCount = (uintptr_t)tls_get_direct(SYNC_COUNT_DIRECT_KEY);
if (result->threadCount <= 0 || lockCount <= 0) {
_objc_fatal("id2data fastcache is buggy");
}
switch(why) {
case ACQUIRE: {
lockCount++;
tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)lockCount);
break;
}
case RELEASE:
lockCount--;
tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)lockCount);
if (lockCount == 0) {
// remove from fast cache
tls_set_direct(SYNC_DATA_DIRECT_KEY, NULL);
// atomic because may collide with concurrent ACQUIRE
OSAtomicDecrement32Barrier(&result->threadCount);
}
break;
case CHECK:
// do nothing
break;
}
return result;
}
}
#endif
// Check per-thread cache of already-owned locks for matching object
SyncCache *cache = fetch_cache(NO);
if (cache) {
unsigned int i;
for (i = 0; i < cache->used; i++) {
SyncCacheItem *item = &cache->list[i];
if (item->data->object != object) continue;
// Found a match.
result = item->data;
if (result->threadCount <= 0 || item->lockCount <= 0) {
_objc_fatal("id2data cache is buggy");
}
switch(why) {
case ACQUIRE:
item->lockCount++;
break;
case RELEASE:
item->lockCount--;
if (item->lockCount == 0) {
// remove from per-thread cache
cache->list[i] = cache->list[--cache->used];
// atomic because may collide with concurrent ACQUIRE
OSAtomicDecrement32Barrier(&result->threadCount);
}
break;
case CHECK:
// do nothing
break;
}
return result;
}
}
// Thread cache didn't find anything.
// Walk in-use list looking for matching object
// Spinlock prevents multiple threads from creating multiple
// locks for the same new object.
// We could keep the nodes in some hash table if we find that there are
// more than 20 or so distinct locks active, but we don't do that now.
lockp->lock();
{
SyncData* p;
SyncData* firstUnused = NULL;
for (p = *listp; p != NULL; p = p->nextData) {
if ( p->object == object ) {
result = p;
// atomic because may collide with concurrent RELEASE
OSAtomicIncrement32Barrier(&result->threadCount);
goto done;
}
if ( (firstUnused == NULL) && (p->threadCount == 0) )
firstUnused = p;
}
// no SyncData currently associated with object
if ( (why == RELEASE) || (why == CHECK) )
goto done;
// an unused one was found, use it
if ( firstUnused != NULL ) {
result = firstUnused;
result->object = (objc_object *)object;
result->threadCount = 1;
goto done;
}
}
// malloc a new SyncData and add to list.
// XXX calling malloc with a global lock held is bad practice,
// might be worth releasing the lock, mallocing, and searching again.
// But since we never free these guys we won't be stuck in malloc very often.
result = (SyncData*)calloc(sizeof(SyncData), 1);
result->object = (objc_object *)object;
result->threadCount = 1;
new (&result->mutex) recursive_mutex_t();
result->nextData = *listp;
*listp = result;
done:
lockp->unlock();
if (result) {
// Only new ACQUIRE should get here.
// All RELEASE and CHECK and recursive ACQUIRE are
// handled by the per-thread caches above.
if (why == RELEASE) {
// Probably some thread is incorrectly exiting
// while the object is held by another thread.
return nil;
}
if (why != ACQUIRE) _objc_fatal("id2data is buggy");
if (result->object != object) _objc_fatal("id2data is buggy");
#if SUPPORT_DIRECT_THREAD_KEYS
if (!fastCacheOccupied) {
// Save in fast thread cache
tls_set_direct(SYNC_DATA_DIRECT_KEY, result);
tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)1);
} else
#endif
{
// Save in thread cache
if (!cache) cache = fetch_cache(YES);
cache->list[cache->used].data = result;
cache->list[cache->used].lockCount = 1;
cache->used++;
}
}
return result;
}
有一点不明白：为什么object对应的是持有锁的结构体的链表，为什么是一对多的关系？？ 求高手解答
其他
同步块@synchronized
互斥锁NSLock,
互斥锁pthread_mutex,
GCD信号量dispatch_semaphore
