linux-kernel - Re: kfree(0)

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Message-ID: <alpine.LFD.0.999.0708180019470.3666@enigma.security.iitk.ac.in>
Date:	Sat, 18 Aug 2007 00:20:09 +0530 (IST)
From:	Satyam Sharma <satyam@...radead.org>
To:	Arjan van de Ven <arjan@...radead.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tim Bird <tim.bird@...sony.com>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Christoph Lameter <clameter@....com>
Subject: Re: kfree(0) - ok?



On Fri, 17 Aug 2007, Arjan van de Ven wrote:

> 
> On Fri, 2007-08-17 at 11:22 -0700, Andrew Morton wrote:
> > On Wed, 15 Aug 2007 05:12:41 +0530 (IST)
> > Satyam Sharma <satyam@...radead.org> wrote:
> > 
> > > [PATCH] {slub, slob}: use unlikely() for kfree(ZERO_OR_NULL_PTR) check
> > > 
> > > Considering kfree(NULL) would normally occur only in error paths and
> > > kfree(ZERO_SIZE_PTR) is uncommon as well, so let's use unlikely() for
> > > the condition check in SLUB's and SLOB's kfree() to optimize for the
> > > common case. SLAB has this already.
> > 
> > I went through my current versions of slab/slub/slub and came up with this:
> > 
> > diff -puN mm/slob.c~slub-slob-use-unlikely-for-kfreezero_or_null_ptr-check mm/slob.c
> > --- a/mm/slob.c~slub-slob-use-unlikely-for-kfreezero_or_null_ptr-check
> > +++ a/mm/slob.c
> > @@ -360,7 +360,7 @@ static void slob_free(void *block, int s
> >  	slobidx_t units;
> >  	unsigned long flags;
> >  
> > -	if (ZERO_OR_NULL_PTR(block))
> > +	if (unlikely(ZERO_OR_NULL_PTR(block)))
> 
> 
> 
> btw this makes NO sense at all; gcc already defaults to assuming
> unlikely if you check a pointer for NULL....

ZERO_OR_NULL_PTR() is not a check for NULL.
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