linux-kernel - Re: [PATCH 2/2] bitmap: introduce for_each_set

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Message-ID: <YPGNw4nRXsdaTHi4@alley>
Date:   Fri, 16 Jul 2021 15:46:43 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 2/2] bitmap: introduce for_each_set_bitrange

On Thu 2021-07-15 08:50:21, Yury Norov wrote:
> On Fri, Jul 09, 2021 at 09:59:50AM -0400, Steven Rostedt wrote:
> > On Thu,  8 Jul 2021 20:45:19 -0700
> > Yury Norov <yury.norov@...il.com> wrote:
> > 
> > > bitmap_list_string() is very ineffective when printing bitmaps with long
> > > ranges of set bits because it calls find_next_bit for each bit. We can do
> > > better by detecting ranges of set bits.
> > > diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> > > index 87acf66f0e4c..1ee54dace71e 100644
> > > --- a/lib/vsprintf.c
> > > +++ b/lib/vsprintf.c
> > >  
> > > -			buf = number(buf, end, rtop, default_dec_spec);
> > > -		}
> > > +	if (buf > start)
> > > +		buf--;
> > 
> > If the above is to undo the last comma, please put back the first logic.
> 
> You're asking me to move part of the logic inside the loop which generally
> should be avoided. Is there any particular reason to do this?

vsprintf() should write what is needed and keep the rest of the given
buffer intact. There is even a test for this in the test_printf module.

I think that test_printf does not complain here because only a single
character is used and it is later replaced by the trailing '\0'.

By other words, undoing the last comma does not cause visible problems
in the end. But from vsprintf() point of view, it is a hack that does
not trigger the warning only by chance. And it is better to avoid it.

Best Regards,
Petr