linux-kernel - Re: [PATCH 1/1] scripts/coccinelle/free: add conditional kfree test

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Date:	Tue, 17 Jun 2014 14:58:19 -0700
From:	Joe Perches <joe@...ches.com>
To:	Julia Lawall <julia.lawall@...6.fr>, Jesper Juhl <jj@...osbits.net>
Cc:	Fabian Frederick <fabf@...net.be>, linux-kernel@...r.kernel.org,
	Gilles Muller <Gilles.Muller@...6.fr>
Subject: Re: [PATCH 1/1] scripts/coccinelle/free: add conditional kfree test

(adding Jesper Juhl)

On Tue, 2014-06-17 at 23:33 +0200, Julia Lawall wrote:
> On Tue, 17 Jun 2014, Joe Perches wrote:
> > On Tue, 2014-06-17 at 21:43 +0200, Fabian Frederick wrote:
> > > This patch adds a trivial script warning on
> > > 
> > > if(foo)
> > > 	kfree(foo)
> > > 
> > > (based on checkpatch)
> > []
> > > diff --git a/scripts/coccinelle/free/cond_kfree.cocci b/scripts/coccinelle/free/cond_kfree.cocci
> > []
> > > +* if (E)
> > > +*	kfree@p(E);
> > 
> > You should probably add all of the unnecessary
> > conditional tests that checkpatch uses:
> > 
> > kfree
> > usb_free_urb
> > debugfs_remove
> > debugfs_remove_recursive
> 
> Personally, I would prefer that the message encourage the user to consider 
> whether it is necessary to call these functions with NULL as an argument 
> in any case.

Jesper quite awhile ago wrote:

https://lkml.org/lkml/2005/10/13/81

- Since kfree always checks for a NULL argument there's no reason to have an
additional check prior to calling kfree. It's redundant.
- In many cases gcc produce significantly smaller code without the redundant
check before the call.
- It's been shown in the past (in discussions on LKML) that it's generally a
win performance wise to avoid the extra NULL check even though it might save
a function call. Only when the NULL check avoids the function call in the vast
majority of cases and the code is in a hot path does it make sense to have it.
- This patch removes quite a few more source lines than it adds, cutting down
on the overall number of source lines is generally a good thing.
- This patch reduces the indentation level, which is nice when the kfree call
is inside some deeply nested construct.



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