分配slab对象的kmem_cache_alloc函数

firkraag

The function tries first to retrieve a free object from the local cache. If there are free objects, the avail field contains the index in the local cache of the entry that points to the last freed object. Because the local cache array is stored right after the ac descriptor, ((void**)(ac+1))[--ac->avail] gets the address of that free object and decreases the value of ac->avail.