linux-kernel - Re: [PATCH v3 1/5] lib/bitmap: add bitmap_{set,get}

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Message-ID: <ZLbJ3yz1yGtR9pCG@smile.fi.intel.com>
Date:   Tue, 18 Jul 2023 20:20:31 +0300
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Alexander Potapenko <glider@...gle.com>, catalin.marinas@....com,
        will@...nel.org, pcc@...gle.com, andreyknvl@...il.com,
        linux@...musvillemoes.dk, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, eugenis@...gle.com,
        syednwaris@...il.com, william.gray@...aro.org,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v3 1/5] lib/bitmap: add bitmap_{set,get}_value()

On Tue, Jul 18, 2023 at 10:03:25AM -0700, Yury Norov wrote:
> On Tue, Jul 18, 2023 at 05:01:28PM +0300, Andy Shevchenko wrote:
> > On Tue, Jul 18, 2023 at 11:30:00AM +0200, Alexander Potapenko wrote:

...

> > The idea behind is to eliminate the code completely for the cases nbits != 0.
> > In your case the dynamic check will be there. That's what we want to avoid.
> 
> Alexander is right - we can't avoid testing against 0 if we need to
> test for 0... In case of other functions we have inline and outline
> implementations, controlled by small_const_nbits().
> 
> As you can see, the small_const_nbits() tests against 0 explicitly,
> although it's free at compile time. But if nbits == 0, we pick
> outline version of a function regardless.
> 
> On their turn, outline versions again do their test against nbits == 0,
> but most of the time implicitly.
> 
> In case of bitmap_set_val, we are touching at max 2 words, and there's
> no reason for outline version, so we have to test nbits against 0
> inside inline code. 
> 
> Having all that in mind, and because nbits == 0 is most likely an
> error we'd follow the following rules:
>  - no memory must be touched as we're potentially in error condition,
>    and pointer may be corrupted;
>  - the cost of the check must be as minimal as possible.
> 
> So I suggest:
> 
>         if (unlikely(nbits == 0))
>                 return;
> 
> For readers that would literally mean: we don't expect that, and we find
> it suspicious, but we'll handle that as correct as we can.

Okay, thank you for elaborated answer.

-- 
With Best Regards,
Andy Shevchenko