linux-kernel - Re: [PATCH v3] ktime: Fix ktime

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Date:	Tue, 12 May 2015 08:14:10 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Ingo Molnar <mingo@...nel.org>,
	Josh Boyer <jwboyer@...hat.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Trevor Cordes <trevor@...nopolis.ca>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH v3] ktime: Fix ktime_divns to do signed division

On Tue, May 12, 2015 at 7:14 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Fri, 8 May 2015, John Stultz wrote:
>> +static inline s64 ktime_divns(const ktime_t kt, s64 div)
>>  {
>> +     /*
>> +      * Negative divisors could cause an inf loop,
>> +      * so bug out here.
>> +      */
>> +     BUG_ON(div < 0);
>>       if (__builtin_constant_p(div) && !(div >> 32)) {
>> -             u64 ns = kt.tv64;
>> +             s64 ns = kt.tv64;
>> +             int neg = (ns < 0);
>> +
>> +             if (neg)
>> +                     ns = -ns;
>>               do_div(ns, div);
>
> This triggers the typecheck for u64 in do_div(). The delta patch below
> should do the trick.
>

I started looking into this, and this landed in my inbox. Thanks Thomas! :)

I reproduced the issue w/ the kbuildbot instructions and the patch
below resolves it.

Acked-by: John Stultz <john.stultz@...aro.org>


> diff --git a/include/linux/ktime.h b/include/linux/ktime.h
> index ab2de1c7932c..b9620ebe356f 100644
> --- a/include/linux/ktime.h
> +++ b/include/linux/ktime.h
> @@ -177,13 +177,13 @@ static inline s64 ktime_divns(const ktime_t kt, s64 div)
>         if (__builtin_constant_p(div) && !(div >> 32)) {
>                 s64 ns = kt.tv64;
>                 int neg = (ns < 0);
> +               u64 tmp;
>
>                 if (neg)
>                         ns = -ns;
> -               do_div(ns, div);
> -               if (neg)
> -                       ns = -ns;
> -               return ns;
> +               tmp = ns;
> +               do_div(tmp, div);
> +               return neg ? -tmp : tmp;
>         } else {
>                 return __ktime_divns(kt, div);
>         }
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index c98ce4d9f613..bac65f810afd 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -268,8 +268,9 @@ lock_hrtimer_base(const struct hrtimer *timer, unsigned long *flags)
>   */
>  s64 __ktime_divns(const ktime_t kt, s64 div)
>  {
> -       s64 dclc;
>         int neg, sft = 0;
> +       s64 dclc;
> +       u64 tmp;
>
>         dclc = ktime_to_ns(kt);
>         neg = (dclc < 0);
> @@ -280,12 +281,9 @@ s64 __ktime_divns(const ktime_t kt, s64 div)
>                 sft++;
>                 div >>= 1;
>         }
> -       dclc >>= sft;
> -       do_div(dclc, (unsigned long) div);
> -       if (neg)
> -               dclc = -dclc;
> -
> -       return dclc;
> +       tmp = dclc >> sft;
> +       do_div(tmp, (unsigned long) div);
> +       return neg ? -tmp : tmp;
>  }
>  EXPORT_SYMBOL_GPL(__ktime_divns);
>  #endif /* BITS_PER_LONG >= 64 */
--
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