Re: [PATCH] ktime: Fix ktime_divns to do signed division

[John Stultz]

Bah. I forgot to add [RFC] to the subject. This patch isn't yet ready
for submission, I just wanted to get some initial feedback on it.

thanks
-john

On Fri, May 1, 2015 at 3:41 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
> It was noted that the 32bit implementation of ktime_divns
> was doing unsgined division adn didn't properly handle
> negative values.
>
> This patch fixes the problem by checking and preserving
> the sign bit, and then reapplying it if appropriate
> after the division.
>
> Unfortunately there is some duplication since we have
> the optimized version for constant 32bit divider. I
> was considering reworkign the __ktime_divns helper
> to simplify the sign-handling logic, but then it
> would likely just be a s64/s64 divide, and probably
> should be more generic.
>
> Thoughts?
>
> Nicolas also notes that the ktime_divns() function
> breaks if someone passes in a negative divisor as
> well. This patch doesn't yet address that issue.
>
> Cc: Nicolas Pitre <nicolas.pitre@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Josh Boyer <jwboyer@xxxxxxxxxx>
> Cc: One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx>
> Reported-by: Trevor Cordes <trevor@xxxxxxxxxxxxx>
> Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
> ---
>  include/linux/ktime.h | 12 ++++++++++--
>  kernel/time/hrtimer.c | 11 +++++++++--
>  2 files changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/ktime.h b/include/linux/ktime.h
> index 5fc3d10..d947263 100644
> --- a/include/linux/ktime.h
> +++ b/include/linux/ktime.h
> @@ -166,12 +166,20 @@ static inline bool ktime_before(const ktime_t cmp1, const ktime_t cmp2)
>  }
>
>  #if BITS_PER_LONG < 64
> -extern u64 __ktime_divns(const ktime_t kt, s64 div);
> +extern s64 __ktime_divns(const ktime_t kt, s64 div);
>  static inline u64 ktime_divns(const ktime_t kt, s64 div)
>  {
>         if (__builtin_constant_p(div) && !(div >> 32)) {
> -               u64 ns = kt.tv64;
> +               s64 ns = kt.tv64;
> +               int neg = 0;
> +
> +               if (ns < 0) {
> +                       neg = 1;
> +                       ns = -ns;
> +               }
>                 do_div(ns, div);
> +               if (neg)
> +                       ns = -ns;
>                 return ns;
>         } else {
>                 return __ktime_divns(kt, div);
> diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
> index 76d4bd9..4c1b294 100644
> --- a/kernel/time/hrtimer.c
> +++ b/kernel/time/hrtimer.c
> @@ -266,12 +266,17 @@ lock_hrtimer_base(const struct hrtimer *timer, unsigned long *flags)
>  /*
>   * Divide a ktime value by a nanosecond value
>   */
> -u64 __ktime_divns(const ktime_t kt, s64 div)
> +s64 __ktime_divns(const ktime_t kt, s64 div)
>  {
> -       u64 dclc;
> +       s64 dclc;
>         int sft = 0;
> +       int neg = 0;
>
>         dclc = ktime_to_ns(kt);
> +       if (dclc < 0) {
> +               neg = 1;
> +               dclc = -dclc;
> +       }
>         /* Make sure the divisor is less than 2^32: */
>         while (div >> 32) {
>                 sft++;
> @@ -279,6 +284,8 @@ u64 __ktime_divns(const ktime_t kt, s64 div)
>         }
>         dclc >>= sft;
>         do_div(dclc, (unsigned long) div);
> +       if (neg)
> +               dclc = -dclc;
>
>         return dclc;
>  }
> --
> 1.9.1
>
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