Contrived example of two GCD queue-synchronized objects deadlocking

gcd-deadlocks.m

@implementation OuterClass

- (id)init {
    self = [super init];
    if (self == nil) return nil;

    _outerQueue = dispatch_queue_create(NULL, DISPATCH_QUEUE_CONCURRENT);

    self.innerObject = [[InnerClass alloc] init];
    self.innerObject.delegate = self;

    return self;
}

- (NSString *)calculatedProperty {
    __block NSString *result;

    dispatch_barrier_sync(_outerQueue, ^{
        result = [self.innerObject.name copy];
    });

    return result;
}

- (NSString *)name {
    __block NSString *result;

    dispatch_barrier_sync(_outerQueue, ^{
        result = [_name copy];
    });

    return result;
}

- (void)setName:(NSString *)name {
    dispatch_async(_outerQueue, ^{
        _name = [name copy];
    });
}

@end

@implementation InnerClass

- (id)init {
    self = [super init];
    if (self == nil) return nil;

    _innerQueue = dispatch_queue_create(NULL, DISPATCH_QUEUE_CONCURRENT);
    
    return self;
}

- (NSString *)name {
    __block NSString *result;

    dispatch_barrier_sync(_innerQueue, ^{
        result = [NSString stringWithFormat:@"%@ wrapping %@", self.delegate.name, _name];
    });

    return result;
}

@end