Re: An variable used before init

[Xiongwei Song <sxwjean@gmail.com>]

Hi,

You can see the definition of struct slab, the counters uses same space
with inuse, objects and frozen because they are in a union. So when
counters gets a value, the other 3 members also get a value.

struct slab {
......snip......
                                        union {
                                                unsigned long counters;
                                                struct {
                                                        unsigned inuse:16;
                                                        unsigned objects:15;
                                                        unsigned frozen:1;
                                                };
                                        };
......snip......
 }

Regards,
Xiongwei

On Sun, Nov 12, 2023 at 5:55 AM geng sun <sun.gengeration.sun@gmail.com> wrote:
>
> Hi linux-mm
>
> I find one odd variable “new” in slub, it was used before init.
> Maybe the value in new was undefined.
> And I can not understand how it work.
> Could you explain this code for me?
>  Thanks.
>
> Best regards.
> static void __slab_free(struct kmem_cache *s, struct page *page,
> void *head, void *tail, int cnt,
> unsigned long addr)
>
> {
> void *prior;
> int was_frozen;
> struct page new;
> unsigned long counters;
> struct kmem_cache_node *n = NULL;
> unsigned long flags;
> stat(s, FREE_SLOWPATH);
> if (kmem_cache_debug(s) &&
> !free_debug_processing(s, page, head, tail, cnt, addr))
> return;
>
> do {
> if (unlikely(n)) {
> spin_unlock_irqrestore(&n->list_lock, flags);
> n = NULL;
> }
> prior = page->freelist;
> counters = page->counters;
> set_freepointer(s, tail, prior);
> new.counters = counters;
> was_frozen = new.frozen;
> new.inuse -= cnt;
> if ((!new.inuse || !prior) && !was_frozen) {