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

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Message-ID: <CALAqxLV2qfKdNyTtnGzsgj+mWUK-wYm3OGYj6xRN_-qbtaypMg@mail.gmail.com>
Date:	Mon, 14 Sep 2015 20:27:08 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Tejun Heo <tj@...nel.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

On Mon, Sep 14, 2015 at 6:49 PM, Tejun Heo <tj@...nel.org> wrote:
> 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;                                                  \
> })
>

Yea. The above make sense to me, but I suspect there's some very
subtle reason for the existing separated logic.
But I'd have to defer to akpm for hints on that.

thanks
-john
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