Re: [PATCH 2/6] treewide: remove using list iterator after loop body as a ptr

[Jakub Kicinski]

On Mon, 28 Feb 2022 16:41:04 -0800 Linus Torvalds wrote:
> So yes, initially my idea had been to just move the iterator entirely
> inside the macro. But specifying the type got so ugly that I think
> that
> 
>         typeof (pos) pos
> 
> trick inside the macro really ends up giving us the best of all worlds:
> 
>  (a) let's us keep the existing syntax and code for all the nice cases
> that did everything inside the loop anyway
> 
>  (b) gives us a nice warning for any normal use-after-loop case
> (unless you explicitly initialized it like that
> sgx_mmu_notifier_release() function did for no good reason
> 
>  (c) also guarantees that even if you don't get a warning,
> non-converted (or newly written) bad code won't actually _work_
> 
> so you end up getting the new rules without any ambiguity or mistaken

I presume the goal is that we can do this without changing existing
code? Otherwise actually moving the iterator into the loop body would
be an option, by creating a different hidden variable:

#define list_iter(head)                                         \
        for (struct list head *_l = (head)->next; _l != (head); _l = _l->next)

#define list_iter_entry(var, member)            \
        list_entry(_l, typeof(*var), member)


        list_iter(&p->a_head) {
                struct entry *e = list_iter_entry(e, a_member);

                /* use e->... */
        }


Or we can slide into soft insanity and exploit one of Kees'es tricks
to encode the type of the entries "next to" the head:

#define LIST_HEAD_MEM(name, type)                       \
        union {                                         \
                struct list_head name;                  \
                type *name ## _entry;                   \
        }

struct entry {
        struct list_head a_member;
};

struct parent {
        LIST_HEAD_MEM(a_head, struct entry);
};

#define list_for_each_magic(pos, head, member)                          \
        for (typeof(**(head ## _entry)) *pos = list_first_entry(head, 
typeof(**(head ## _entry)), member); \
             &pos->member != (head);                                    \
             pos = list_next_entry(pos, member))


        list_for_each_magic(e, &p->a_head, a_member) {
                /* use e->... */
        }


I'll show myself out...