linux-kernel - Re: kfree(0)

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Date:	Fri, 17 Aug 2007 11:31:54 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Satyam Sharma <satyam@...radead.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, 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....


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