linux-kernel - [PATCH] Fix order_base

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Message-ID: <20111215125538.2838.41548.stgit@warthog.procyon.org.uk>
Date:	Thu, 15 Dec 2011 12:55:38 +0000
From:	David Howells <dhowells@...hat.com>
To:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org
Cc:	linux-kernel@...r.kernel.org, David Howells <dhowells@...hat.com>,
	"Robert P. J. Day" <rpjday@...shcourse.ca>, stable@...nel.org
Subject: [PATCH] Fix order_base_2(0) [ver #2]

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@...hat.com>
cc: Robert P. J. Day <rpjday@...shcourse.ca>
cc: stable@...nel.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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