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

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Date:   Sat, 30 May 2020 01:07:51 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Syed Nayyar Waris <syednwaris@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.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 Sat, May 30, 2020 at 12:42 AM Andy Shevchenko
<andy.shevchenko@...il.com> wrote:
> On Fri, May 29, 2020 at 11:07 PM Syed Nayyar Waris <syednwaris@...il.com> wrote:
> > On Sat, May 30, 2020 at 12:08 AM Andy Shevchenko
> > <andriy.shevchenko@...ux.intel.com> wrote:

...

> > Explanation:
> > Actually when nbits (clump size) is 64 (BITS_PER_LONG is 64 on my computer),
> > (BIT(nbits) - 1)
> > gives a value of zero and when this zero is ANDed with any value, it
> > makes it full zero. This is unexpected and incorrect calculation happening.
> >
> > What actually happens is in the macro expansion of BIT(64), that is 1
> > << 64, the '1' overflows from leftmost bit position (most significant
> > bit) and re-enters at the rightmost bit position (least significant
> > bit), therefore 1 << 64 becomes '0x1', and when another '1' is
> > subtracted from this, the final result becomes 0.
> >
> > Since this macro is being used in both bitmap_get_value and
> > bitmap_set_value functions, it will give unexpected results when nbits or clump
> > size is BITS_PER_LONG (32 or 64 depending on arch).
>
> I see, something like
> https://elixir.bootlin.com/linux/latest/source/include/linux/dma-mapping.h#L139
> should be done.
> But yes, let's try to fix GENMASK().
>
> So, if we modify the following
>
>   #define GENMASK_INPUT_CHECK(h, l) \
>     (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
>     __builtin_constant_p((l) > (h)), (l) > (h), 0)))
>
> to be
>
>   #define GENMASK_INPUT_CHECK(h, l) \
>     (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
>     __builtin_constant_p((l) > (h)), (l) ? (l) > (h) : 0, 0)))
>
> would it work?

Actually it needs an amendment for signed types...

(l) ? (l) > (h) : !((h) > 0)

...but today is Friday night, so, mistakes are warranted :-)

-- 
With Best Regards,
Andy Shevchenko