我写的由 GCD 代码支持的 reader writer lock 在并行测试中导致死锁

Question

我在 GCD 中实现了这个 reader/writer 锁，但它在并行测试中失败了。我可以得到失败原因的解释吗？

这是为了 iOS 开发。该代码基于 Objective C。为了数据保护，我在 GCD 中写了一个带 reader/writer 锁的 RWCache。

@interface RWCache : NSObject

- (void)setObject:(id)object forKey:(id <NSCopying>)key;

- (id)objectForKey:(id <NSCopying>)key;

@end

@interface RWCache()
@property (nonatomic, strong) NSMutableDictionary *memoryStorage;
@property (nonatomic, strong) dispatch_queue_t storageQueue;
@end

@implementation RWCache

- (instancetype)init {
    self = [super init];
    if (self) {
        _memoryStorage = [NSMutableDictionary new];
        _storageQueue = dispatch_queue_create("Storage Queue", DISPATCH_QUEUE_CONCURRENT);
    }
    return self;
}

- (void)setObject:(id)object forKey:(id <NSCopying>)key {
    dispatch_barrier_async(self.storageQueue, ^{
        self.memoryStorage[key] = object;
    });
}

- (id)objectForKey:(id <NSCopying>)key {
    __block id object = nil;
    dispatch_sync(self.storageQueue, ^{
        object = self.memoryStorage[key];
    });
    return object;
}

@end


int main(int argc, const char * argv[]) {
    @autoreleasepool {
        RWCache *cache = [RWCache new];
        dispatch_queue_t testQueue = dispatch_queue_create("Test Queue", DISPATCH_QUEUE_CONCURRENT);
        dispatch_group_t group = dispatch_group_create();
        for (int i = 0; i < 100; i++) {
            dispatch_group_async(group, testQueue, ^{
                [cache setObject:@(i) forKey:@(i)];
            });
            dispatch_group_async(group, testQueue, ^{
                [cache objectForKey:@(i)];
            });
        }
        dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    }
    return 0;
}

如果没有死锁，程序会0退出，否则程序会挂起不退出。

Answer 1

问题不在于 reader/writer 模式， 本身， 而是因为此代码中的一般线程爆炸。请参阅 WWDC 2015 视频中的“线程爆炸导致死锁”讨论 Building Responsive and Efficient Apps with GCD. The WWDC 2016 Concurrent Programming With GCD in Swift 3 也是一个不错的视频。在这两个链接中，我都会让您离开视频的相关部分，但两者都值得完整观看。

最重要的是，您正在耗尽 GCD 线程池中非常有限的工作线程。只有 64 个。但是你有 100 个带有障碍的写入，这意味着调度块不能运行直到该队列上的其他所有内容都完成。它们散布着 100 次读取，因为它们是同步的，所以会阻塞您从中调度它的工作线程，直到它 returns.

让我们将其简化为更简单的问题：

dispatch_queue_t queue1 = dispatch_queue_create("queue1", DISPATCH_QUEUE_CONCURRENT);
dispatch_queue_t queue2 = dispatch_queue_create("queue2", DISPATCH_QUEUE_CONCURRENT);
for (int i = 0; i < 100; i++) {
    dispatch_async(queue2, ^{
        dispatch_barrier_async(queue1, ^{
            NSLog(@"barrier async %d", i);
        });
        dispatch_sync(queue1, ^{
            NSLog(@"sync %d", i);
        });
    });
}
NSLog(@"done dispatching all blocks to queue1");

这会产生如下内容：

starting
done dispatching all blocks to queue1
barrier async 0
sync 0

它死锁了。

但是如果我们限制它不超过，比如 30 个项目可以同时在 queue2 上运行，那么问题就消失了：

dispatch_queue_t queue1 = dispatch_queue_create("queue1", DISPATCH_QUEUE_CONCURRENT);
dispatch_queue_t queue2 = dispatch_queue_create("queue2", DISPATCH_QUEUE_CONCURRENT);
dispatch_semaphore_t semaphore = dispatch_semaphore_create(30);

for (int i = 0; i < 100; i++) {
    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
    dispatch_async(queue2, ^{
        dispatch_barrier_async(queue1, ^{
            NSLog(@"barrier async %d", i);
        });
        dispatch_sync(queue1, ^{
            NSLog(@"sync %d", i);
        });
        dispatch_semaphore_signal(semaphore);
    });
}
NSLog(@"done dispatching all blocks to queue1");

或者，另一种方法是使用 dispatch_apply，这实际上是一个并行化的 for 循环，但在任何给定时刻将并发任务的数量限制为您机器上的核心数量（保持我们远低于耗尽工作线程的阈值):

dispatch_queue_t queue1 = dispatch_queue_create("queue1", DISPATCH_QUEUE_CONCURRENT);
dispatch_queue_t queue2 = dispatch_queue_create("queue2", DISPATCH_QUEUE_CONCURRENT);

dispatch_apply(100, queue2, ^(size_t i) {
    dispatch_barrier_async(queue1, ^{
        NSLog(@"barrier async %ld", i);
    });
    dispatch_sync(queue1, ^{
        NSLog(@"sync %ld", i);
    });
});
NSLog(@"done dispatching all blocks to queue1");

我写的由 GCD 代码支持的 reader writer lock 在并行测试中导致死锁

The reader writer lock I wrote backed by GCD code causes a deadlock in parallel test

concurrency

deadlock

readerwriterlock

grand-central-dispatch

ios