linux-kernel - Re: [RFC][PATCH 0/5] Fixes for abs() usage on 64bit values

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Date:	Mon, 14 Sep 2015 21:49:36 -0400
From:	Tejun Heo <tj@...nel.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Steven Rostedt (Red Hat)" <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Richard Cochran <richardcochran@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Dave Chinner <dchinner@...hat.com>,
	Joe Perches <joe@...ches.com>
Subject: Re: [RFC][PATCH 0/5] Fixes for abs() usage on 64bit values

Hello,

On Mon, Sep 14, 2015 at 06:05:19PM -0700, John Stultz wrote:
> As noted in include/linux/kernel.h:
>  "abs() should not be used for 64-bit types (s64, u64, long long)
>  - use abs64() for those."
> 
> Unfortunately, there are quite a number of places where abs()
> was used w/ 64bit values in the kernel, and the results are
> then silently capped to 32-bit values on 32-bit systems.

I don't get it.  Why can't we just do the following?

#define abs(x)									\
({										\
	 typeof(x) __x = (x);							\
	 __x < 0 ? -__x : __x;							\
})

The current macros are kinda broken because they'd end up converting
u32 or u64 values which are over the max values of signed counterparts
to their complements.

Thanks.

-- 
tejun
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