如何锁定/解锁交叉线程

Question

对于某些代码段，我想逐个执行，例如：

self.coreDataLock = [[NSRecursiveLock alloc] init];

// Snippet One
[self.coreDataLock lock];
[childContext performBlock:^{
    // Do something
    [self.coreDataLock unlock];
}];

// Snippet Two
[self.coreDataLock lock];
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    // Do something
    [self.coreDataLock unlock];
});

但我无法锁定/解锁NSRecursiveLock交叉线程，如何弄明白？