linux-kernel - Re: [PATCH v7 1/4] bitops: Introduce the the for_each_set

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Date:   Fri, 29 May 2020 21:38:24 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Syed Nayyar Waris <syednwaris@...il.com>
Cc:     kbuild test robot <lkp@...el.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kbuild-all@...ts.01.org,
        William Breathitt Gray <vilhelm.gray@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v7 1/4] bitops: Introduce the the for_each_set_clump macro

On Fri, May 29, 2020 at 11:38:18PM +0530, Syed Nayyar Waris wrote:
> On Sun, May 24, 2020 at 8:15 PM kbuild test robot <lkp@...el.com> wrote:

...

> >    579  static inline unsigned long bitmap_get_value(const unsigned long *map,
> >    580                                                unsigned long start,
> >    581                                                unsigned long nbits)
> >    582  {
> >    583          const size_t index = BIT_WORD(start);
> >    584          const unsigned long offset = start % BITS_PER_LONG;
> >    585          const unsigned long ceiling = roundup(start + 1, BITS_PER_LONG);
> >    586          const unsigned long space = ceiling - start;
> >    587          unsigned long value_low, value_high;
> >    588
> >    589          if (space >= nbits)
> >  > 590                  return (map[index] >> offset) & GENMASK(nbits - 1, 0);
> >    591          else {
> >    592                  value_low = map[index] & BITMAP_FIRST_WORD_MASK(start);
> >    593                  value_high = map[index + 1] & BITMAP_LAST_WORD_MASK(start + nbits);
> >    594                  return (value_low >> offset) | (value_high << space);
> >    595          }
> >    596  }

> Regarding the above compilation warnings. All the warnings are because
> of GENMASK usage in my patch.
> The warnings are coming because of sanity checks present for 'GENMASK'
> macro in include/linux/bits.h.
> 
> Taking the example statement (in my patch) where compilation warning
> is getting reported:
> return (map[index] >> offset) & GENMASK(nbits - 1, 0);
> 
> 'nbits' is of type 'unsigned long'.
> In above, the sanity check is comparing '0' with unsigned value. And
> unsigned value can't be less than '0' ever, hence the warning.
> But this warning will occur whenever there will be '0' as one of the
> 'argument' and an unsigned variable as another 'argument' for GENMASK.
> 
> This warning is getting cleared if I cast the 'nbits' to 'long'.
> 
> Let me know if I should submit a next patch with the casts applied.
> What do you guys think?

Proper fix is to fix GENMASK(), but allowed workaround is to use
	(BIT(nbits) - 1)
instead.

-- 
With Best Regards,
Andy Shevchenko