chenhao

tested gcc 9, the same problem.

build 7.10.2 + gcc 12 + `make all` get other error: `'__Y' may by use uninitialized [-Werror=maybe-uninitialized]` 

也可能[c04/a/boot/mbr.S](https://github.com/yifengyou/os-elephant/tree/master/code/c04/a/boot/mbr.S)有问题，因为连续读取多个扇区时，需要检查下一扇区的数据是否已就位， 否则会读到0.

我印象中修正了读取硬盘的实现后就解决了.

@smallnest 能帮我解读一下第2题吗? `mu.Lock()`阻塞我能理解, 为什么第二个`mu.RLock()`也会阻塞?