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

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Message-ID: <198022f201814cdc9384ef083741185e@AcuMS.aculab.com>
Date:   Tue, 1 Mar 2022 03:03:52 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Matthew Wilcox' <willy@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
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Subject: RE: [PATCH 2/6] treewide: remove using list iterator after loop body
 as a ptr

From: Matthew Wilcox
> Sent: 28 February 2022 20:16
> 
> On Mon, Feb 28, 2022 at 12:10:24PM -0800, Linus Torvalds wrote:
> > We can do
> >
> >         typeof(pos) pos
> >
> > in the 'for ()' loop, and never use __iter at all.
> >
> > That means that inside the for-loop, we use a _different_ 'pos' than outside.
> 
> Then we can never use -Wshadow ;-(  I'd love to be able to turn it on;
> it catches real bugs.
> 
> > +#define list_for_each_entry(pos, head, member)					\
> > +	for (typeof(pos) pos = list_first_entry(head, typeof(*pos), member);	\
> > +	     !list_entry_is_head(pos, head, member);	\
> >  	     pos = list_next_entry(pos, member))

Actually can't you use 'pos' to temporarily hold the address of 'member'.
Something like:
	for (pos = (void *)head; \
		pos ? ((pos = (void *)pos - offsetof(member)), 1) : 0; \
		pos = (void *)pos->next)
So that 'pos' is NULL if the loop terminates.
No pointers outside structures are generated.
Probably need to kill list_entry_is_head() - or it just checks for NULL.

	David

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