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

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Message-ID: <alpine.DEB.2.10.1406041510520.2441@hadrien>
Date:	Wed, 4 Jun 2014 15:12:23 +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>, sebott@...ux.vnet.ibm.com
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 1:00 PM, Julia Lawall <julia.lawall@...6.fr> wrote:
> > 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.
>
> "size" may have been changed between function entry and this line.
> So you have to store it in a temporary.

Sorry, after reflection it seems that indeed size + result is always the
original size, so it is actually all of the code that uses >= that is
doing something unnecessary.  == for the failure test is fine.

julia
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