Re: [PATCH] linux/log2.h: Fix roundup_pow_of_two(0)

From	Andrey Ryabinin <ryabinin.a.a@gmail.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] linux/log2.h: Fix roundup_pow_of_two(0)
Date	2015-12-03 17:20 +0100
Message-ID	<qBELU-7sA-9@gated-at.bofh.it> (permalink)
References	<qBE9c-6XG-27@gated-at.bofh.it> <qBE9d-6XG-49@gated-at.bofh.it> <qBECe-7oz-23@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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2015-12-03 19:04 GMT+03:00 Sasha Levin <sasha.levin@oracle.com>:
> On 12/03/2015 10:39 AM, Andrey Ryabinin wrote:
>> 2015-12-03 18:30 GMT+03:00 Sasha Levin <sasha.levin@oracle.com>:
>>> > Passing 0 to roundup_pow_of_two would lead to wrapping around and trying to
>>> > find the last set bit on (unsigned long)(-1), which is obviously wrong.
>>> >
>>> > Instead, deal with this case by rounding it up to the closest power of two
>>> > (2 ** 0).
>>> >
>>> > Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
>>> > ---
>>> >  include/linux/log2.h |    3 +++
>>> >  1 file changed, 3 insertions(+)
>>> >
>>> > diff --git a/include/linux/log2.h b/include/linux/log2.h
>>> > index fd7ff3d..b6bdf0c 100644
>>> > --- a/include/linux/log2.h
>>> > +++ b/include/linux/log2.h
>>> > @@ -60,6 +60,9 @@ bool is_power_of_2(unsigned long n)
>>> >  static inline __attribute__((const))
>>> >  unsigned long __roundup_pow_of_two(unsigned long n)
>>> >  {
>>> > +       if (n == 0)
>>> > +               return 1UL << 0;
>>> > +
>> Perhaps we should fix callers instead?
>> Comment near roundup_pow_of_two() says that result is undefined when n == 0:
>
> That's how I've started doing it, but when it showed up with 3 different callers
> I figured it's better to fix it at the source.
>
> This fix would return a valid value and is working fine with the callers.
>

In that case patch should update the comment.

> Thanks,
> Sasha
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[PATCH] linux/log2.h: Fix roundup_pow_of_two(0) Sasha Levin <sasha.levin@oracle.com> - 2015-12-03 16:40 +0100
  Re: [PATCH] linux/log2.h: Fix roundup_pow_of_two(0) Andrey Ryabinin <ryabinin.a.a@gmail.com> - 2015-12-03 16:40 +0100
    Re: [PATCH] linux/log2.h: Fix roundup_pow_of_two(0) Sasha Levin <sasha.levin@oracle.com> - 2015-12-03 17:10 +0100
      Re: [PATCH] linux/log2.h: Fix roundup_pow_of_two(0) Andrey Ryabinin <ryabinin.a.a@gmail.com> - 2015-12-03 17:20 +0100

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