after unlock a NSRecursiveLock at one queue, other queue still waiting - ios

Question

I have some code not thread safe, So I tried to use NSRecursiveLock to prevent my work queue and main thread access some properties at the same time. I create two work queue(work queue and wait queue), work queue to do some heavy work. In order to prevent block main thread, I create another wait queue to wait the lock, so I can async to main thread after I get the lock.
I face two problem:
after the work queue unlock, wait queue still waiting.
code will get two kind of output
I think the output should be
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
but I get Output:
[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

if user NSRecursiveLock, should not use GCD, because lock an unlock may not on the same thread