linux-kernel - Re: [PATCH 56/62] watchdog: tangox_wdt: Convert to use device managed functions

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Message-ID: <0f68aa11-161b-0435-ea0f-851432464bc1@sigmadesigns.com>
Date:   Thu, 12 Jan 2017 10:44:07 +0100
From:   Marc Gonzalez <marc_gonzalez@...madesigns.com>
To:     Guenter Roeck <linux@...ck-us.net>
CC:     Wim Van Sebroeck <wim@...ana.be>, <linux-watchdog@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        "Mans Rullgard" <mans@...sr.com>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>,
        Mark Rutland <mark.rutland@....com>,
        Uwe Kleine-Konig <u.kleine-koenig@...gutronix.de>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 56/62] watchdog: tangox_wdt: Convert to use device managed
 functions

On 11/01/2017 18:51, Guenter Roeck wrote:

> However, some other unrelated undefined behavior does not mean that this
> specific behavior is undefined.

True :-)

Let me just give two additional examples of UB that /have/ bitten
Linux kernel devs.

int i;
for (i = 1; i > 0; ++i)
	/* do_something(); */

=> optimized into an infinite loop

and

void func(struct foo *p) {
	int n = p->field;
	if (!p) return;

=> null-pointer check optimized away

> So far we have a claim that a cast to a void * may somehow be different
> to a cast to a different pointer, if used as function argument, and that
> the behavior with such a cast may be undefined. In other words, you claim
> that a function implemented as, say,
> 
>    void func(int *var) {}
> 
> might result in undefined behavior if some header file declares it as
> 
>     void func(void *);
> 
> and it is called as
> 
>     int var;
> 
>     func(&var);
> 
> That seems really far fetched to me.

Thanks for giving me an opportunity to play the language lawyer :-)

C99 6.3.2.3 sub-clause 8 states:

"A pointer to a function of one type may be converted to a pointer to a function of another
type and back again; the result shall compare equal to the original pointer. If a converted
pointer is used to call a function whose type is not compatible with the pointed-to type,
the behavior is undefined."

So, the behavior is undefined, not when you cast clk_disable_unprepare,
but when clk_disable_unprepare is later called through the devres->action
function pointer.

However, I agree that it will work as expected on typical platforms
(where all pointers are the same size, and the calling convention
treats all pointers the same).

> I do get the message that you do not like this kind of cast. But that doesn't
> mean it is not correct.

If it's already widely used in the kernel, it seems there is no point
fighting it ;-)

Regards.