linux-kernel - Re: [RFC] timers, pointers to functions and type safety

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Date:	Sat, 2 Dec 2006 11:47:38 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Al Viro <viro@....linux.org.uk>
Cc:	Linus Torvalds <torvalds@...l.org>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, gcc@....gnu.org
Subject: Re: [RFC] timers, pointers to functions and type safety

On Friday 01 December 2006 18:21, Al Viro wrote:
>  There is a tempting
> possibility to do that gradually, with all intermediates still in working
> state, provided that on all platforms currently supported by the kernel
> unsigned long and void * are indeed passed the same way (again, only for
> current kernel targets and existing gcc versions).  Namely, we could
>         * introduce SETUP_TIMER variant with cast to void (*)(unsigned long)
>         * switch to its use, converting callback instances to take pointers
> at the same time.  That would be done in reasonably sized groups.
>         * once it's over, flip ->function to void (*)(void *), ->data to
> void * and update SETUP_TIMER accordingly; deal with remaining few instances
> of ->function.

Another alternative might be to define an intermediate version of
timer_list that avoids adding new macros, like

struct timer_list {
	struct list_head entry;
	unsigned long expires;

	void (*function)(union { unsigned long l; void *p; }
				 __attribute__((transparent_union)));
	union {
		unsigned long data __deprecated;
		void *obj;
	};

	struct tvec_t_base_s *base;
};

Unfortunately, this relies more on gcc specific behavior than we
may want, and it makes it possible to abuse in new ways, like defining
a function to take an unsigned long argument, but passing the data
as void *obj.

	Arnd <><
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