linux-kernel - Re: [RFC] resource: Avoid unnecessary resource tree walking in __region

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Message-ID: <671bb5d165_10e592941e@dwillia2-xfh.jf.intel.com.notmuch>
Date: Fri, 25 Oct 2024 08:14:25 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>, Dan Williams
	<dan.j.williams@...el.com>
CC: "Huang, Ying" <ying.huang@...el.com>, David Hildenbrand
	<david@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>,
	<linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<linux-cxl@...r.kernel.org>, Davidlohr Bueso <dave@...olabs.net>, "Jonathan
 Cameron" <jonathan.cameron@...wei.com>, Alistair Popple <apopple@...dia.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>, Baoquan He <bhe@...hat.com>, Dave Jiang
	<dave.jiang@...el.com>, Alison Schofield <alison.schofield@...el.com>
Subject: Re: [RFC] resource: Avoid unnecessary resource tree walking in
 __region_intersects()

Andy Shevchenko wrote:
[..]
> > > but if you want to stick with your variant some improvements can be done:
> > > 
> > > #define for_each_resource_XXX(_root, _p)				\
> > > 	for (typeof(_root) __root = (_root), __p = _p = __root->child;	\
> > > 	     __p && _p; _p = next_resource_XXX(__root, _p))
> > > 
> > > 
> > > 1) no need to have local variable in parentheses;
> > > 2) no need to have iterator in parentheses, otherwise it would be crazy code
> > > that has put something really wrong there and still expect the thing to work.
> > 
> > Why not:
> > 
> > #define for_each_resource_XXX(_root, _p)				\
> > 	for (typeof(_root) __root = (_root), __p = _p = __root->child;	\
> > 	     _p; _p = next_resource_XXX(__root, _p))
> > 
> > The __p is only to allow for _p to be initialized in the first statement
> > without causing a new "_p" shadow to be declared.
> 
> If people think this would be better than the existing patterns, okay. fine.

I think this case is different than the existing patterns in that the
iterator variable needs to be initiatlized from a declared variable, and
as Ying said, my proposal is busted.

To your point though, lets add a comment on why this macro is a bit
different to avoid people like me making bad cleanup suggestions.