linux-kernel - Re: hlist_for_each_entry && pos (Was: task_work

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Date:	Wed, 11 Apr 2012 21:12:19 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	David Smith <dsmith@...hat.com>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: hlist_for_each_entry && pos (Was: task_work_queue)

On Wed, Apr 11, 2012 at 9:00 PM, Oleg Nesterov <oleg@...hat.com> wrote:
>
> This reminds me.
>
> hlist_for_each_entry_*() do not need "pos", it can be
>
>        #define hlist_for_each_entry(pos, head, member)                                 \
>                for (pos = (void*)(head)->first;                                        \
>                pos && ({ pos = hlist_entry((void*)pos, typeof(*pos), member); 1; });   \
>                pos = (void*)(pos)->member.next)

Ugh. I'm not sure that is any better, with the extra casts to hide the
fact that you use the wrong type pointers for it.

Are there any code generation improvements?

Because quite frankly, if there aren't, I think the code churn just
isn't worth it - especially with how ugly the macro is.

This is one of those things where the C99 features would actually be
nice: one of the few features from C++ that I actually liked is the
ability to declare the induction variable. So

  #define hlist_for_each_entry(pos, head, member) \
    for (void *__pos = (head)->first; \
        __pos && ({ pos = hlist_entry(__pos, typeof(*pos), member); 1; });   \
        __pos = __pos->next)

would actually be prettier. That said, "pretty macro" isn't very high
on the list of things to worry about. Not nearly as high as the pain
changing the interface would cause for things that *should* be trivial
(like backporting patches etc).

So I'd really want to see some more tangible advantage.

                 Linus
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