linux-kernel - Re: [PATCH v2 1/3] lib/find_bit: introduce FIND_FIRST

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Message-ID: <CAHp75VfrA7S033=mKkUj-SnWWyM9pwEiM_Umrrd4arpf1SNw6Q@mail.gmail.com>
Date:   Wed, 24 Aug 2022 20:45:01 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Guenter Roeck <linux@...ck-us.net>,
        Dennis Zhou <dennis@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Alexey Klimov <aklimov@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Whitcroft <apw@...onical.com>
Subject: Re: [PATCH v2 1/3] lib/find_bit: introduce FIND_FIRST_BIT() macro

On Wed, Aug 24, 2022 at 4:19 PM Yury Norov <yury.norov@...il.com> wrote:
> On Wed, Aug 24, 2022 at 12:10:02PM +0300, Andy Shevchenko wrote:
> > On Wed, Aug 24, 2022 at 4:51 AM Yury Norov <yury.norov@...il.com> wrote:

...

> > > +#define FIND_FIRST_BIT(EXPRESSION, size)                                       \
> > > +({                                                                             \
> > > +       unsigned long idx, val, sz = (size);                                    \
> > > +                                                                               \
> > > +       for (idx = 0; idx * BITS_PER_LONG < sz; idx++) {                        \
> >
> > I think we can do slightly better:
> >
> > for (unsigned long idx = 0; idx < sz; idx += BITS_PER_LONG) {
> >   unsigned long val;
>
> This will blow up the EXPRESSION. We can mitigate it on user side:

I'm not sure I understand how EXPRESSION is involved in all this. What
I proposed is to replace the for-loop one-by-one to
one-by-BITS_PER_LONG. But okay, I have re-read the above patch and now
I see what you are doing, basically you use internal variables of the
macro in the EXPRESSION. Hmm...

-- 
With Best Regards,
Andy Shevchenko