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Subject[PATCH] Fix order_base_2(0) [ver #2]
Date
The order_base_2() function is either wrongly documented or wrongly
implemented. In the preceding comment, it says that:

ob2(0) = 0

but this is not valid as roundup_pow_of_two()'s documentation asserts that:

* - the result is undefined when n == 0

Remove the bit of comment that says order_base_2(0) returns 0.

Add comments to order_base_2() and other functions in the same header file to
indicate that the result is undefined if n <= 0. Note that it is possible to
give ilog2() a negative number if passing a constant value and this will result
in a compilation failure (it'll try and reference ____ilog2_NaN because n < 1).

Also, whilst we're at it, stick brackets around the body of order_base_2().

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Robert P. J. Day <rpjday@crashcourse.ca>
cc: stable@kernel.org
---

include/linux/log2.h | 12 +++++++-----
1 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/include/linux/log2.h b/include/linux/log2.h
index fd7ff3d..358620d 100644
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -81,6 +81,8 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
* the massive ternary operator construction
*
* selects the appropriately-sized optimised version depending on sizeof(n)
+ *
+ * The result is undefined if n <= 0.
*/
#define ilog2(n) \
( \
@@ -162,7 +164,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
* @n - parameter
*
* round the given value up to the nearest power of two
- * - the result is undefined when n == 0
+ * - the result is undefined when n <= 0
* - this can be used to initialise global variables from constant data
*/
#define roundup_pow_of_two(n) \
@@ -179,7 +181,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
* @n - parameter
*
* round the given value down to the nearest power of two
- * - the result is undefined when n == 0
+ * - the result is undefined when n <= 0
* - this can be used to initialise global variables from constant data
*/
#define rounddown_pow_of_two(n) \
@@ -194,15 +196,15 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
* @n: parameter
*
* The first few values calculated by this routine:
- * ob2(0) = 0
* ob2(1) = 0
* ob2(2) = 1
* ob2(3) = 2
* ob2(4) = 2
* ob2(5) = 3
* ... and so on.
+ *
+ * The result is undefined if n <= 0.
*/
-
-#define order_base_2(n) ilog2(roundup_pow_of_two(n))
+#define order_base_2(n) (ilog2(roundup_pow_of_two(n)))

#endif /* _LINUX_LOG2_H */


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