([email protected] only bcc'ed because it's subscribers only,
Lameter addressed because I think he touched the code last, Velikov and
Hellwig because they touched the code first.)
The current code in __max_index() will shift by a negative amount first
and only then fix the situation by ignoring the result if the shift
amount would have been negative. This happens to work on almost any
architecture despite not being valid C.
Chapter and verse from the (draft) ISO C99 standard, "6.5.7 Bitwise
shift operators", paragraph 3:
"The integer promotions are performed on each of the operands. The
type of the result is that of the promoted left operand. If the
value of the right operand is negative or is greater than or equal
to the width of the promoted left operand, the behavior is
undefined."
Right-shifting by a negative amount causes a "reserved operand fault" on
the VAX with some gcc versions.
Applies to 2.6.22.
Boot tested lightly on an emulated VAX.
(The function is called 7 times on booth with the values 0..6 -- I
checked that the return values were the same + that it returns ~0UL for
height==6 as was clearly the intention.)
-Peter
--- lib/radix-tree-old.c 2007-08-25 15:36:40.000000000 +0200
+++ lib/radix-tree.c 2007-08-25 15:36:51.000000000 +0200
@@ -980,12 +980,14 @@ radix_tree_node_ctor(void *node, struct
static __init unsigned long __maxindex(unsigned int height)
{
- unsigned int tmp = height * RADIX_TREE_MAP_SHIFT;
- unsigned long index = (~0UL >> (RADIX_TREE_INDEX_BITS - tmp - 1)) >> 1;
-
- if (tmp >= RADIX_TREE_INDEX_BITS)
- index = ~0UL;
- return index;
+ unsigned int tmp = height * RADIX_TREE_MAP_SHIFT;
+ int shift = RADIX_TREE_INDEX_BITS - tmp;
+ unsigned long index;
+
+ if (shift < 0)
+ return ~0UL;
+ else
+ return ~0UL >> shift;
}
static __init void radix_tree_init_maxindex(void)
-
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