Re: radix-tree.c:__lookup_slot() dead code removal

WU Fengguang
2006-12-04T10:37:04Z

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So, the patch is no longer valid.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