非递归上锁情况下，同一个锁，已经被锁过一次，那么另外一个线程尝试上锁，会被挡在外面，但是会被挡多久呢？挡一分钟会被崩溃，还十分钟，还是永久等待而不崩溃（希望如此）？还是会立刻崩溃？因为不可以重复上锁。
enum QReadWriteLock::RecursionMode
| Constant | Value | Description |
|---|---|---|
QReadWriteLock::Recursive |
1 |
In this mode, a thread can lock the same QReadWriteLock multiple times. TheQReadWriteLock won't be unlocked until a corresponding number of unlock() calls have been made. |
QReadWriteLock::NonRecursive |
0 |
In this mode, a thread may only lock a QReadWriteLock once. |
This enum was introduced or modified in Qt 4.4.
See also QReadWriteLock().