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

Jakub Kicinski
2022-03-01T06:32:38Z

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...