linux-kernel - Re: radix-tree.c:__lookup

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Message-ID: <20061204103704.GA7792@mail.ustc.edu.cn>
Date:	Mon, 4 Dec 2006 18:37:04 +0800
From:	WU Fengguang <wfg@...l.ustc.edu.cn>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Frank van Maarseveen <frankvm@...nkvm.com>,
	linux-kernel@...r.kernel.org, Andrew Morton <akpm@...gle.com>
Subject: Re: radix-tree.c:__lookup_slot() dead code removal

On Mon, Dec 04, 2006 at 09:18:40PM +1100, Nick Piggin wrote:
> Frank van Maarseveen wrote:
> >Most of the code suggests that it is valid to insert a NULL item,
> >possibly a zero item with pointer cast. However, in __lookup_slot()
> >whether or not the slot is found seems to depend on the actual value
> >of the item in one special case. But further on it doesn't make any
> >difference so to remove some dead code:
> >
> >--- a/lib/radix-tree.c	2006-12-03 13:23:00.000000000 +0100
> >+++ b/lib/radix-tree.c	2006-12-03 17:57:03.000000000 +0100
> >@@ -319,9 +319,6 @@ static inline void **__lookup_slot(struc
> > 	if (index > radix_tree_maxindex(height))
> > 		return NULL;
> >
> >-	if (height == 0 && root->rnode)
> >-		return (void **)&root->rnode;
> >-
> > 	shift = (height-1) * RADIX_TREE_MAP_SHIFT;
> > 	slot = &root->rnode;
>
> I would say it is not valid to insert a NULL item (because NULL
> means an unsuccessful lookup, you may as well just delete the
> item).
>
> Also, I don't see how this is dead code anyway. height == 0
> radix-trees are a special case and do not have a radix_tree_node
> at ->rnode.

Sorry.

For linux-2.6.19 it was redundant code:

        if (height == 0 && root->rnode)
                return (void **)&root->rnode;

        shift = (height-1) * RADIX_TREE_MAP_SHIFT;
        slot = &root->rnode;

        while (height > 0) {
                ......
        }

        return (void **)slot;

I just find out it is no longer the case for -mm: the function has been heavily
reworked.

So, the patch is no longer valid.

Fengguang Wu
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