关于FsRtlEnterFileSystem

看ddk的解释，是阻止普通APC的执行。
我想是出于同步的需要吧？ddk的解释好深奥！ :(

请大虾给解释一下这个函数的作用？

Every file system driver entry point routine must call FsRtlEnterFileSystem immediately before performing a file I/O request and FsRtlExitFileSystem immediately afterward. This ensures that the routine cannot be suspended while running and thus block other file I/O requests.

这是我从ddk拷贝的说明。言简意赅，深入深出。

根据《Windows NT File System Internals》：
The FsRtlEnterFileSystemf) macro is simply defined to KeEnterCriticalRegion(),
while the FsRtlExitFileSystem()
macro is defined to KeLeaveCriticalRegion ( ) .
所以肯定是用做同步，再往深处追究我也不理解。