From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758767Ab1LOMz6 (ORCPT ); Thu, 15 Dec 2011 07:55:58 -0500 Received: from mx1.redhat.com ([209.132.183.28]:55709 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751245Ab1LOMz5 (ORCPT ); Thu, 15 Dec 2011 07:55:57 -0500 Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells Subject: [PATCH] Fix order_base_2(0) [ver #2] To: torvalds@linux-foundation.org, akpm@linux-foundation.org Cc: linux-kernel@vger.kernel.org, David Howells , "Robert P. J. Day" , stable@kernel.org Date: Thu, 15 Dec 2011 12:55:38 +0000 Message-ID: <20111215125538.2838.41548.stgit@warthog.procyon.org.uk> User-Agent: StGIT/0.14.3 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 cc: Robert P. J. Day 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 */