From 3cf5ac4ccf3388e1b227d03f116b5071498ac5f3 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Mon, 12 Dec 2011 22:06:55 -0800 Subject: [PATCH] linux/log2.h: Fix rounddown_pow_of_two(1) Exactly like roundup_pow_of_two(1), the rounddown version was buggy for the case of a compile-time constant '1' argument. Probably because it originated from the same code, sharing history with the roundup version from before the bugfix (for that one, see commit b2a20f292af7: "Fix roundup_pow_of_two(1)"). However, unlike the roundup version, the fix for rounddown is to just remove the broken special case entirely. It's simply not needed - the generic code 1UL << ilog2(n) does the right thing for the constant '1' argment too. The only reason roundup needed that special case was because rounding up does so by subtracting one from the argument (and then adding one to the result) causing the obvious problems with "ilog2(0)". But rounddown doesn't do any of that, since ilog2() naturally truncates (ie "rounds down") to the right rounded down value. And without the ilog2(0) case, there's no reason for the special case that had the wrong value. tl;dr: rounddown_pow_of_two(1) should be 1, not 0. Acked-by: Dmitry Torokhov Cc: stable@kernel.org Signed-off-by: Linus Torvalds --- include/linux/log2.h | 1 - 1 file changed, 1 deletion(-) diff --git a/include/linux/log2.h b/include/linux/log2.h index 25b808631cd92..fd7ff3d91e6a9 100644 --- a/include/linux/log2.h +++ b/include/linux/log2.h @@ -185,7 +185,6 @@ unsigned long __rounddown_pow_of_two(unsigned long n) #define rounddown_pow_of_two(n) \ ( \ __builtin_constant_p(n) ? ( \ - (n == 1) ? 0 : \ (1UL << ilog2(n))) : \ __rounddown_pow_of_two(n) \ ) -- 2.39.5