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

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Message-ID: <Pine.LNX.4.64.0911151916100.27042@ask.diku.dk>
Date:	Sun, 15 Nov 2009 19:20:40 +0100 (CET)
From:	Julia Lawall <julia@...u.dk>
To:	Joe Perches <joe@...ches.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"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, 15 Nov 2009, Joe Perches wrote:

> 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

Searching for things that are declared as functions (either a definition 
or a prototype), and then referenced as &f gives over 2000 results in 
almost 600 files.  Here are a couple of typical examples:

arch/arm/mach-omap1/clock.c:

static const struct clkops clkops_dspck = {
       .enable         = &omap1_clk_enable_dsp_domain,
       .disable        = &omap1_clk_disable_dsp_domain,
 };


arch/arm/mach-omap1/serial.c:

       ret = request_irq(gpio_to_irq(gpio_nr), &omap_serial_wake_interrupt,
                          IRQF_TRIGGER_RISING, "serial wakeup", NULL);

Should both cases lose the initial &?

julia
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