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

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Date:   Mon, 28 Feb 2022 22:32:28 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jakob Koschel <jakobkoschel@...il.com>,
        Christian König <christian.koenig@....com>,
        alsa-devel@...a-project.org, linux-aspeed@...ts.ozlabs.org,
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Subject: Re: [PATCH 2/6] treewide: remove using list iterator after loop
 body as a ptr

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...