HK_INVALID
#define _NG_HOOK_NOT_VALID(hook) ((hook)->hk_flags & HK_INVALID)
#define _NG_HOOK_IS_VALID(hook) (!((hook)->hk_flags & HK_INVALID))
hook->hk_flags = HK_INVALID;
hook->hk_flags |= HK_INVALID;
hook->hk_flags &= ~HK_INVALID;
hook->hk_flags &= ~HK_INVALID; /* need both to be able to work */
HK_INVALID | HK_DEAD,
hook2->hk_flags = HK_INVALID;
hook1->hk_flags &= ~HK_INVALID;
hook2->hk_flags &= ~HK_INVALID;