From: Jeff Epler <jepler@unpythonic.net>
To: Tejun Heo <tj@kernel.org>
Cc: John Stultz <john.stultz@linaro.org>,
LKML <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@kernel.org>,
"Steven Rostedt (Red Hat)" <rostedt@goodmis.org>,
Peter Zijlstra <peterz@infradead.org>,
Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
Michal Nazarewicz <mina86@mina86.com>,
Prarit Bhargava <prarit@redhat.com>,
Richard Cochran <richardcochran@gmail.com>,
Thomas Gleixner <tglx@linutronix.de>,
"Theodore Ts'o" <tytso@mit.edu>,
Andreas Dilger <adilger.kernel@dilger.ca>,
Dave Chinner <dchinner@redhat.com>, Joe Perches <joe@perches.com>
Subject: Re: [RFC][PATCH 0/5] Fixes for abs() usage on 64bit values
Date: Tue, 15 Sep 2015 07:09:42 -0500 [thread overview]
Message-ID: <20150915120941.GA81750@unpythonic.net> (raw)
In-Reply-To: <20150915034632.GB25658@htj.duckdns.org>
On Mon, Sep 14, 2015 at 11:46:32PM -0400, Tejun Heo wrote:
> Hello,
>
> On Mon, Sep 14, 2015 at 08:27:08PM -0700, John Stultz wrote:
> > 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.
>
> Hmmm... people could be using it for calculating the distance between
> two unsigned values and in that case the original behavior would be
> the correct one. e.g.
>
> unsigned diff, a, b;
> diff = abs(a - b);
This kind of construct can overflow whether a and b are signed or
unsigned. Only a two-argument function can correctly return the
absolute difference of two integers.
At my day job, we arranged for
abs(unsigned type) to be a compile-time error and supplied a two args
function absdiff for use in these situations -- absdiff(a,b) is the same
as abs(a-b) except it avoids overflow and returns an unsigned type.
(though I have no doubt that some instances of abs(a - b) still exist
where a and b are signed and the intermediate could still overflow...)
Jeff
next prev parent reply other threads:[~2015-09-15 12:48 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-09-15 1:05 [RFC][PATCH 0/5] Fixes for abs() usage on 64bit values John Stultz
2015-09-15 1:05 ` [RFC][PATCH 1/5] clocksource: Fix abs() usage w/ " John Stultz
2015-10-02 20:57 ` [tip:timers/urgent] " tip-bot for John Stultz
2015-09-15 1:05 ` [RFC][PATCH 2/5] time: Fix abs() usage with 64-bit values John Stultz
2015-09-15 1:05 ` [RFC][PATCH 3/5] ext4: Fix abs() usage in ext4_mb_check_group_pa John Stultz
2015-09-15 1:05 ` [RFC][PATCH 4/5] percpu: Fix abs() usage in percpu_counter_compare() John Stultz
2015-09-15 1:05 ` [RFC][PATCH 5/5] abs(): Provide build error on passing 64bit value to abs() John Stultz
2015-09-15 5:22 ` Ingo Molnar
2015-09-15 23:52 ` Linus Torvalds
2015-09-16 12:57 ` [PATCH] kernel.h: make abs() work with 64-bit types Michal Nazarewicz
2015-09-18 3:12 ` John Stultz
2015-09-15 1:49 ` [RFC][PATCH 0/5] Fixes for abs() usage on 64bit values Tejun Heo
2015-09-15 3:27 ` John Stultz
2015-09-15 3:46 ` Tejun Heo
2015-09-15 12:09 ` Jeff Epler [this message]
2015-09-15 21:21 ` Andrew Morton
2015-09-15 22:54 ` Michal Nazarewicz
2015-09-15 5:20 ` Ingo Molnar
2015-09-15 23:43 ` Linus Torvalds
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