linux-kernel - Re: gcc7 log2 compile issues in kernel/time/timekeeping.c

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Date:   Fri, 24 Feb 2017 13:25:08 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     Laura Abbott <labbott@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>
Subject: Re: gcc7 log2 compile issues in kernel/time/timekeeping.c

On Thu, Feb 23, 2017 at 10:43 AM, Laura Abbott <labbott@...hat.com> wrote:
> Hi,
>
> Fedora was previously carrying a workaround for a gcc7 issue reported
> on arm64 http://lists.infradead.org/pipermail/linux-arm-kernel/2016-October/461597.html.
> The workaround got rid of __ilog2_NaN. I dropped the patch this morning
> because a proper fix (29905b52fad0 ("log2: make order_base_2() behave
> correctly on const input value zero")) was merged. This fixed the arm64
> problem linked in the thread but there seems to be another issue in
> timekeeping.c:
>
> /kernel/time/timekeeping.c:2051: undefined reference to `____ilog2_NaN'
>
> Fedora enables CONFIG_CLOCKSOURCE_VALIDATE_LAST_CYCLE so I think the
> compiler is calculating a possible constant of 0 once again.
>
> Any ideas about a proper fix?

Huh. So if I understand this, its because we don't explicit checks for
offsec or cycle_interval being zero in:

        shift = ilog2(offset) - ilog2(tk->cycle_interval);

Right?

Clearly that case isn't expected to happen, but if it did we'd want
the result of ilog2 to return zero.  So I'm not sure if that
order_base_2() function is maybe the right function to use as it has
an explict zero check?

thanks
-john