linux-kernel - Re: [RFC][PATCH v2] ktime: Fix ktime

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Date:	Wed, 6 May 2015 14:43:07 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Josh Boyer <jwboyer@...hat.com>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Trevor Cordes <trevor@...nopolis.ca>
Subject: Re: [RFC][PATCH v2] ktime: Fix ktime_divns to do signed division

On Wed, May 6, 2015 at 1:11 PM, Nicolas Pitre <nicolas.pitre@...aro.org> wrote:
> On Wed, 6 May 2015, John Stultz wrote:
>
>> It was noted that the 32bit implementation of ktime_divns()
>> was doing unsigned division and 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, it also changes the return type to a s64
>> to make it more obvious this is expected.
>>
> [...]
>>
>> -# define ktime_divns(kt, div)          (u64)((kt).tv64 / (div))
>> +static inline s64 ktime_divns(const ktime_t kt, s64 div)
>> +{
>> +     /*
>> +      * 32-bit implementation cannot handle negative divisors,
>> +      * so catch them on 64bit as well.
>> +      */
>> +     BUG_ON(div < 0);
>> +     return (u64)((kt).tv64 / (div))
>
> You forgot to remove the u64 cast?

Thanks for catching that!

> Is there a reason why it was there originally?

I assume because nanoseconds are usually though of as unsigned. It was
introduced way back in the original hrtimer core code:
c0a3132963db68f1fbbd0e316b73de100fee3f08

thanks
-john
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