src/common/RWLock.h
RWLock使用了C++11新特性Deleted函数，禁用其拷贝构造函数与赋值操作符。
public:
RWLock(const RWLock& other) = delete;
const RWLock& operator=(const RWLock& other) = delete;
使用两个原子变量对读和写锁进行计数：
mutable atomic_t nrlock, nwlock;
为了防止写进程被饿死，可使用PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP对读写锁进行初始化：
#if defined(PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP)
if (prioritize_write) {
pthread_rwlockattr_t attr;
pthread_rwlockattr_init(&attr);
// PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP
// Setting the lock kind to this avoids writer starvation as long as
// long as any read locking is not done in a recursive fashion.
pthread_rwlockattr_setkind_np(&attr,
PTHREAD_RWLOCK_PREFER_WRITER_NONRECURSIVE_NP);
pthread_rwlock_init(&L, &attr);
} else
#endif
