linux-kernel - Re: [PATCH] timekeeping: Change type of nsec variable to unsigned in its calculation.

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Message-ID: <CALAqxLVAy5mST2ChwQ4hJUXcz8e6wep7y9hYs=fo6ZPtMiGv9Q@mail.gmail.com>
Date:   Sun, 25 Sep 2016 23:02:04 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Liav Rehana <liavr@...lanox.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        lkml <linux-kernel@...r.kernel.org>,
        Elad Kanfi <eladkan@...lanox.com>,
        Noam Camus <noamca@...lanox.com>
Subject: Re: [PATCH] timekeeping: Change type of nsec variable to unsigned in
 its calculation.

On Sun, Sep 25, 2016 at 10:45 PM, Liav Rehana <liavr@...lanox.com> wrote:
> From: Liav Rehana <liavr@...lanox.com>
>
> During the calculation of the nsec variable in the inline function
> timekeeping_delta_to_ns, it may undergo a sign extension if its msb
> is set just before the shift. The sign extension may, in some cases,
> gain it a value near the maximum value of the 64-bit range. This is
> bad when it is later used in a division function, such as
> __iter_div_u64_rem, where the amount of loops it will go through to
> calculate the division will be too large.
> The following commit fixes that chance of sign extension, while
> maintaining the type of the nsec variable as signed for other
> functions that use this variable, for possible legit negative
> time intervals.

Apologies for my foggy memory here (just got off a plane).  But remind
me, is this something that you actually ran into, or is it more of
just a theoretical concern?

If it is something that you did trip on, please describe your case in
the changelog so that the proper urgency is applied when weighing if
such a commit should go into -stable.

thanks
-john