linux-kernel - Re: [PATCH 5/7] lib: rework bitmap

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Message-Id: <20190523195110.f98e60898c8c29884d231a6e@linux-foundation.org>
Date:   Thu, 23 May 2019 19:51:10 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Dmitry Torokhov <dmitry.torokhov@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Amritha Nambiar <amritha.nambiar@...el.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Matthew Wilcox <willy@...radead.org>,
        "Tobin C . Harding" <tobin@...nel.org>,
        Will Deacon <will.deacon@....com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Vineet Gupta <vineet.gupta1@...opsys.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        linux-kernel@...r.kernel.org, Yury Norov <ynorov@...vell.com>,
        Jens Axboe <axboe@...nel.dk>,
        Steffen Klassert <steffen.klassert@...unet.com>
Subject: Re: [PATCH 5/7] lib: rework bitmap_parse()

On Thu, 9 May 2019 19:26:33 -0700 Yury Norov <yury.norov@...il.com> wrote:

> Hi Andy,
> 
> Thanks for thorough review.
> 
> On Wed, May 08, 2019 at 11:46:32AM +0300, Andy Shevchenko wrote:
> > On Tue, Apr 30, 2019 at 06:06:34PM -0700, Yury Norov wrote:
> > > bitmap_parse() is ineffective and full of opaque variables and opencoded
> > > parts. It leads to hard understanding and usage of it. This rework
> > > includes:
> > >  - remove bitmap_shift_left() call from the cycle. Now it makes the
> > >    complexity of the algorithm as O(nbits^2). In the suggested approach
> > >    the input string is parsed in reverse direction, so no shifts needed;
> > >  - relax requirement on a single comma and no white spaces between chunks.
> > >    It is considered useful in scripting, and it aligns with
> > >    bitmap_parselist();
> > >  - split bitmap_parse() to small readable helpers;
> > >  - make an explicit calculation of the end of input line at the
> > >    beginning, so users of the bitmap_parse() won't bother doing this.
> > 
> > > +static inline bool in_str(const char *start, const char *ptr)
> > > +{
> > > +	return start <= ptr;
> > > +}
> > > +
> > 
> > The explicit use of the conditional is better.
> > 
> > -- 
> > With Best Regards,
> > Andy Shevchenko
> 
> I still think that is_str() is more verbose, but it's minor issue
> anyways, so I obey. Below is the patch that removes the function.
> It's up to Andrew finally, either apply it or not.

I agree with Andy - open-coding the comparisons makes it easier to
understand the varoius in_str() callsites, IMO.

> @@ -653,7 +648,7 @@ int bitmap_parse(const char *start, unsigned int buflen,
>  	u32 *bitmap = (u32 *)maskp;
>  	int unset_bit;
>  
> -	while (in_str(start, (end = bitmap_find_region_reverse(start, end)))) {
> +	while (start <= (end = bitmap_find_region_reverse(start, end))) {

This statement hurts my little brain.  Can it be broken into easier to digest
chunks?

>  		if (!chunks--)
>  			return -EOVERFLOW;
>