From: Ingo Molnar <mingo@kernel.org>
To: John Stultz <john.stultz@linaro.org>,
Linus Torvalds <torvalds@linux-foundation.org>
Cc: Tejun Heo <tj@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.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>,
Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [RFC][PATCH 0/5] Fixes for abs() usage on 64bit values
Date: Tue, 15 Sep 2015 07:20:49 +0200 [thread overview]
Message-ID: <20150915052049.GA14215@gmail.com> (raw)
In-Reply-To: <CALAqxLV2qfKdNyTtnGzsgj+mWUK-wYm3OGYj6xRN_-qbtaypMg@mail.gmail.com>
* John Stultz <john.stultz@linaro.org> wrote:
> On Mon, Sep 14, 2015 at 6:49 PM, Tejun Heo <tj@kernel.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.
On one hand there's a real cost from abs() bugs: the fact that abs() trims the
high bits silently led to a (serious) timekeeping bug on 32-bit kernels, that was
not found for almost 2 years:
2619d7e9c92d time: Fix timekeeping_freqadjust()'s incorrect use of abs() instead of abs64()
On the other hand, there's literally hundreds of abs() usages in the kernel - I
think it would be a lot safer to just introduce a build time warning and migrate
the few affected ones over to abs64() (i.e. what John has done), than to silently
change semantics in an all-or-nothing fashion, even if arguably many (most?) of
the 64-bit values passed to abs() are probably bugs.
This has another advantage: we'll see all the bugs that occured so far, and can
judge their effect on a case by case basis. There's value in that kind of gradual
approach as well.
Once we've gone through that fixing process (for 1-2 kernel releases) we could
perhaps do the change and unify abs() and abs64(): users who really want 32-bit
trimming in the future can do the cast explicitly.
Linus, any preferences?
Thanks,
Ingo
next prev parent reply other threads:[~2015-09-15 5:20 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
2015-09-15 21:21 ` Andrew Morton
2015-09-15 22:54 ` Michal Nazarewicz
2015-09-15 5:20 ` Ingo Molnar [this message]
2015-09-15 23:43 ` Linus Torvalds
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