在一个队列解锁 NSRecursiveLock 后，其他队列仍在等待

Question

我有一些代码不是线程安全的，所以我尝试使用 NSRecursiveLock 来防止我的工作队列和主线程同时访问某些属性。我创建了两个工作队列（工作队列和等待队列），工作队列来做一些繁重的工作。为了防止阻塞主线程，我创建了另一个等待队列来等待锁，这样我就可以在获得锁后异步到主线程。

我面临两个问题：

1. 工作队列解锁后，等待队列仍在等待。
2. 代码将得到两种输出

我认为输出应该是

1. work queue get lock
2. wait queue wait lock
3. post notification from work queue
noti called
unlock
get lock
4. main thread do work

但我得到输出：

[Output 1]
1. work queue get lock
2. wait queue wait lock
3. post notification from work queue
noti called
unlock

[Output 2]
1. work queue get lock
2. wait queue wait lock
get lock
4. main thread do work
3. post notification from work queue
noti called
unlock

@interface ViewController ()
@property (nonatomic) NSLock *lock;
@property (nonatomic) NSRecursiveLock *recursiveLock;
@property (nonatomic) dispatch_queue_t queue;
@property (nonatomic) dispatch_queue_t lockWaitQueue;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    _queue = dispatch_queue_create("serial", DISPATCH_QUEUE_SERIAL);
    _lockWaitQueue = dispatch_queue_create("wait_queue", DISPATCH_QUEUE_SERIAL);

    [[NSNotificationCenter defaultCenter] addObserverForName:@"noti" object:nil queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification * _Nonnull note) {
        NSLog(@"noti called");
    }];
    _lock = [NSLock new];
    _recursiveLock = [NSRecursiveLock new];
    [self tryFixDeadLockCace];
}

- (void)tryFixDeadLockCace {
    NSRecursiveLock *lock = _recursiveLock;
    dispatch_queue_t queue = _queue;
    dispatch_queue_t lockWaitQueue = _lockWaitQueue;
    NSLog(@"1. work queue get lock");
    dispatch_async(queue, ^{
        [lock lock];
        dispatch_async(dispatch_get_main_queue(), ^{
            dispatch_async(lockWaitQueue, ^{
                NSLog(@"2. wait queue wait lock");
                [lock lock];
                NSLog(@"get lock");
                dispatch_async(dispatch_get_main_queue(), ^{
                    NSLog(@"4. main thread do work");
                    [lock unlock];
                });
            });
        });
    });
    
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), queue, ^{
        NSLog(@"3. post notification from work queue");
        [[NSNotificationCenter defaultCenter] postNotificationName:@"noti" object:nil];
        NSLog(@"unlock");
        [lock unlock];
    });
}

- (void)noti {
    
}
@end

Answer 1

如果用户使用NSRecursiveLock，不应该使用GCD，因为锁定和解锁可能不在同一个线程上