An variable used before init

An variable used before init

[geng sun]

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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) {

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Re: An variable used before init

[Xiongwei Song]

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) {

Re: An variable used before init

[geng sun]

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Thank you for your explanation. Iunderstand. Thank you very much.

> On Nov 13, 2023, at 09:36, Xiongwei Song <sxwjean@gmail.com> wrote:
> 
> 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) {

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