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

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Date:	Sun, 15 Nov 2009 11:33:07 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Joe Perches <joe@...ches.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,


* 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. )

	Ingo
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