linux-kernel - Re: [PATCH] sysctl.c: Change a .proc_handler = proc_dointvec to &proc

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Date:	Sun, 15 Nov 2009 09:31:39 -0800
From:	Joe Perches <joe@...ches.com>
To:	Ingo Molnar <mingo@...e.hu>, julia Lawall <julia@...u.dk>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Am??rico Wang <xiyou.wangcong@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] sysctl.c: Change a .proc_handler = proc_dointvec to
 &proc_dointvec,

On Sun, 2009-11-15 at 11:33 +0100, Ingo Molnar wrote:
> * Eric W. Biederman <ebiederm@...ssion.com> wrote:
> > Ingo Molnar <mingo@...e.hu> writes:
> > > * Joe Perches <joe@...ches.com> wrote:
> > >> On Sun, 2009-11-15 at 09:11 +0100, Ingo Molnar wrote:
> > >> > * Am??rico Wang <xiyou.wangcong@...il.com> wrote:
> > >> > > On Sat, Nov 14, 2009 at 05:52:05PM -0800, Joe Perches wrote:
> > >> > > >Seems to be a typo.
> > >> > > Acked-by: WANG Cong <xiyou.wangcong@...il.com>
> > >> > (Cc:-ed Eric who is running the sysctl tree these days)
> > >> > Almost everywhere in the kernel we use the shorter version, so all of 
> > >> > sysctl.c should eventually change to that variant.
> > >> It's closer to 50/50, but it's 1 vs 133 in that file.
> > >> $ grep -Pr --include=*.[ch] '\.proc_handler\s*=\s*&\s*\w+' * | wc -l
> > >> 339
> > >> $ grep -Pr --include=*.[ch] '\.proc_handler\s*=\s*[^&]\s*\w+' * | wc -l
> > >> 432
> > > I did not mean this specific initialization method of proc_handler, i 
> > > meant pointers to functions in general.
> > There was an argument put forward by Alexy (I think) a while ago.  
> > That argued for the form without the address of operator.
> > The reason being that without it you can do:
> > #define proc_dointvec NULL
> > in a header when sysctl support it compiled out.  Using address of
> > you wind up with stub functions in sysctl.c to handle the case when
> > sysctl is compiled out.
> > It isn't a strong case but since not using & is also shorter and as 
> > Ingo pointed out more common I think no & wins.
> I can think of another reason as well: the & operator can be dangerous 
> if code is changed from functions to function pointers.
> 
> The short form:
> 
>   val = do_my_func;
> 
> will work just fine if 'my_func' is changed to a function pointer, as it 
> will evaluate to the value of the function pointer - i.e. the address of 
> the function.
> 
> The longer form:
> 
>   val = &do_my_func;
> 
> might break in a subtle way, because it will now become the address of 
> the function pointer - not the function address.
> 
> Combined the shortness, the NULL init, the function pointer invariance, 
> plus existing in-kernel practice all suggest that the short form should 
> be used.
> 
> ( i didnt want to turn this small issue into a long argument - it's just
>   that the code was going in the wrong direction. )

That sounds like something coccinelle would do well,
so I've cc'd Julia Lawall.

http://lkml.org/lkml/2009/11/15/55

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