linux-kernel - Re: [PATCH v3 2/9] rbtree: add __rb_change

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Date:	Wed, 26 Sep 2012 17:34:11 -0500
From:	Daniel Santos <danielfsantos@....net>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Michel Lespinasse <walken@...gle.com>, riel@...hat.com,
	peterz@...radead.org, daniel.santos@...ox.com, aarcange@...hat.com,
	dwmw2@...radead.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org
Subject: Re: [PATCH v3 2/9] rbtree: add __rb_change_child() helper function

Sorry to resurrect the dead here, but I'm playing catch-up and this
looks important.

On 08/20/2012 05:17 PM, Andrew Morton wrote:
> I'm inclined to agree with Peter here - "inline" is now a vague,
> pathetic and useless thing.  The problem is that the reader just
> doesn't *know* whether or not the writer really wanted it to be
> inlined.
>
> If we have carefully made a decision to inline a function, we should
> (now) use __always_inline.
Are we all aware here that __always_inline (a.k.a.
"__attribute__((always_inline))") just means "inline even when not
optimizing"?  This appears to be a very common misunderstanding (unless
the gcc docs are wrong, see
http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html#index-g_t_0040code_007bflatten_007d-function-attribute-2512).

If you want to *force* gcc to inline a function (when inlining is
enabled), you can currently only do it from the calling function by
adding the |flatten attribute to it, which I have proposed adding here:
https://lkml.org/lkml/2012/9/25/643.

Thus, all of the __always_inline markings we have in the kernel only
affect unoptimized builds (and maybe -O1?).  If we need this feature
(and I think it would be darned handy!) we'll have to work on gcc to get it.

Daniel
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