请教各位关于address creation mutex 和APC的问题~~~~~~？！

win2k souce 里有这么一段：
//
        // Get the address creation mutex to block multiple threads from
        // creating or deleting address space at the same time and
        // get the working set mutex so virtual address descriptors can
        // be inserted and walked.  Block APCs so an APC which takes a page
        // fault does not corrupt various structures.
        //

它的实现是：
ExAcquireFastMutex ( &((CurrentProcess)->AddressCreationLock));
ASSERT (MI_NOT_WS_OWNER(CurrentProcess));                      
ASSERT (KeGetCurrentIrql() == APC_LEVEL);              
ExAcquireFastMutexUnsafe( &((CurrentProcess)->WorkingSetLock));
(CurrentProcess)->WorkingSetLock.OldIrql = MI_MUTEX_ACQUIRED_UNSAFE;

1）但是当我调用时，在第一个函数就阻塞了，不知道为何？
2）而当我把第一句去掉时，又在KeGetCurrentIrql（）返回时！=APC_LEVEL
那么又如何使得APC禁用呢？
这里有两个疑惑：为什么要禁用APC,如何禁用APC呢？