linux-kernel - Re: [PATCH 0/10] use safer test on the result of find_first_zero

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Date:	Wed, 4 Jun 2014 13:00:07 +0200 (CEST)
From:	Julia Lawall <julia.lawall@...6.fr>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
cc:	Julia Lawall <julia.lawall@...6.fr>,
	David Laight <David.Laight@...lab.com>,
	linux-rdma <linux-rdma@...r.kernel.org>,
	"kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>,
	Linux Fbdev development list <linux-fbdev@...r.kernel.org>,
	Linux-sh list <linux-sh@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"ath10k@...ts.infradead.org" <ath10k@...ts.infradead.org>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	driverdevel <devel@...verdev.osuosl.org>,
	"iss_storagedev@...com" <iss_storagedev@...com>,
	scsi <linux-scsi@...r.kernel.org>,
	linux-s390 <linux-s390@...r.kernel.org>,
	"adi-buildroot-devel@...ts.sourceforge.net" 
	<adi-buildroot-devel@...ts.sourceforge.net>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 0/10] use safer test on the result of
 find_first_zero_bit



On Wed, 4 Jun 2014, Geert Uytterhoeven wrote:

> Hi Julia,
>
> On Wed, Jun 4, 2014 at 11:52 AM, Julia Lawall <julia.lawall@...6.fr> wrote:
> >> Maybe the documented return code should be changed to allow for the
> >> existing behaviour.
> >
> > Sorry, I'm not sure to understand what you suggest here.
>
> include/asm-generic/bitops/find.h:
>
> | /**
> |  * find_first_zero_bit - find the first cleared bit in a memory region
> |  * @addr: The address to start the search at
> |  * @size: The maximum number of bits to search
> |  *
> |  * Returns the bit number of the first cleared bit.
> |  * If no bits are zero, returns @size.
>
> "If no bits are zero, returns @size or a number larger than @size."

OK, thanks.  I was only looking at the C code.

But the C code contains a loop that is followed by:

        if (!size)
                return result;
        tmp = *p;

found_first:
        tmp |= ~0UL << size;
        if (tmp == ~0UL)        /* Are any bits zero? */
                return result + size;   /* Nope. */

In the first return, it would seem that result == size.  Could the second
one be changed to just return size?  It should not hurt performance.

julia

>
> |  */
> | extern unsigned long find_first_zero_bit(const unsigned long *addr,
> |                                          unsigned long size);
>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds
>
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