与 `dispatch_barrier` 的死锁

Question

随着学习dispatch_barrier，我写了一个例子如下：

static dispatch_queue_t queue;
    static dispatch_queue_t readWriteLockQueue;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        queue = dispatch_queue_create("com.test.testasync", DISPATCH_QUEUE_CONCURRENT);
        readWriteLockQueue = dispatch_queue_create("com.test.readWriteLockQueueOfMeta", DISPATCH_QUEUE_CONCURRENT);
    });

    for (NSInteger i=0; i<100; i++) {
        dispatch_async(queue, ^{
            dispatch_sync(readWriteLockQueue, ^{
                NSLog(@"read");
            });

            dispatch_barrier_sync(readWriteLockQueue, ^{
                NSLog(@"write");
            });
        });
    }
    dispatch_barrier_async(queue, ^{
        NSLog(@"finished!");
    });

但是输出是：

2016-05-20 16:23:14.066 Test[23867:781553] read
2016-05-20 16:23:14.066 Test[23867:781555] read
2016-05-20 16:23:14.066 Test[23867:781554] read
2016-05-20 16:23:14.066 Test[23867:781545] read
2016-05-20 16:23:14.066 Test[23867:781559] read
2016-05-20 16:23:14.067 Test[23867:781564] read
2016-05-20 16:23:14.066 Test[23867:781560] read
2016-05-20 16:23:14.066 Test[23867:781561] read
2016-05-20 16:23:14.067 Test[23867:781562] read
2016-05-20 16:23:14.067 Test[23867:781565] read

没有write出现。它超出了我的预期。所以我需要一些帮助。

dispatch_barrier_sync或dispatch_sync里面没有调用sync。为什么会出现死锁？

更新： 感谢@originaluser2 的回答。我在下面用读写锁更新了我的工具以避免虚拟线程限制。

#define THREAD_ASSERT_ON_ERROR(x_) do { \
_Pragma("clang diagnostic push"); \
_Pragma("clang diagnostic ignored \"-Wunused-variable\""); \
volatile int res = (x_); \
assert(res == 0); \
_Pragma("clang diagnostic pop"); \
} while (0)

    static dispatch_queue_t queue;
    static dispatch_once_t onceToken;
    static pthread_rwlock_t rwlock;
    dispatch_once(&onceToken, ^{
        queue = dispatch_queue_create("com.test.testasync", DISPATCH_QUEUE_CONCURRENT);
        THREAD_ASSERT_ON_ERROR(pthread_rwlock_init(&rwlock, NULL));
    });

    NSMutableDictionary *dict = [NSMutableDictionary dictionary];

    for (NSInteger i=0; i<200; i++) {
        dispatch_async(queue, ^{
            NSString *result = nil;
            THREAD_ASSERT_ON_ERROR(pthread_rwlock_rdlock(&rwlock));
            result = dict[@"test"];
            THREAD_ASSERT_ON_ERROR(pthread_rwlock_unlock(&rwlock));

            if (!result) {
                THREAD_ASSERT_ON_ERROR(pthread_rwlock_wrlock(&rwlock));
                NSLog(@"%ld:try to write",i);
                result = dict[@"test"];
                if (!result) {
                    NSLog(@"%ld:write",i);
                    dict[@"test"] = @"result";
                    result = dict[@"test"];
                }
                THREAD_ASSERT_ON_ERROR(pthread_rwlock_unlock(&rwlock));
            }
            NSLog(@"%ld:%@",i,result);
        });
    }

    dispatch_barrier_sync(queue, ^{});
    NSLog(@"completed");

Answer 1

问题是您要安排 100 个任务同时执行。这将超过虚拟线程限制（通常为 64）——因此您将有 64 个任务坐在那里等待它们的读取或写入完成，但它们无法完成，因为没有更多的线程可以执行它们。

如果您将循环减少到 64 个，或者使 queue 成为串行队列以限制任务，代码将再次工作。虽然，这是一个非常人为的例子。实际上，你 永远不会 同时发生这么多有争议的读写（这表明你的逻辑中存在更基本的问题）——即使你这样做了，你的写入也应该最有可能与 dispatch_barrier_async.

异步发生

与 `dispatch_barrier` 的死锁

Dead Lock With `dispatch_barrier`

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