Messages in this thread Patch in this message | | | From | David Howells <> | Subject | [PATCH] Fix order_base_2(0) [ver #2] | Date | Thu, 15 Dec 2011 12:55:38 +0000 |
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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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