Re: [PATCHSET v2] ->follow_link() without dropping from RCU mode

[Al Viro <viro@ZenIV.linux.org.uk>]

On Fri, Dec 11, 2015 at 08:49:24AM +0100, Rasmus Villemoes wrote:

> union delayed_call_fn {
> 	void (*fn)(void *);
> 	void (*kfree_like)(const void *);
> } __attribute__((__transparent_union__));
>
> void
> set_delayed_call(struct delayed_call *call, union delayed_call_fn u, void *arg)
> {
> 	call->fn = u.fn;
> 	call->arg = arg;
> }

	Yecchhhh...  If we are into that kind of gccisms, I'd rather use
__builtin_choose_expr/__builtin_types_compatible_p.  At least that is
used elsewhere in the kernel; __transparent_union__ kludge isn't.  Sure, it
means having set_delayed_call() a macro, but IMO it's less nasty that way...

	FWIW, another possibility is to have
#define CLOSURE_CALLBACK(f,type) \
	static inline void __closure_##f(void *p) {return f((type)p);}

#define set_delayed_type(call, f, arg) \
	sizeof(f(arg),0), \
	__set_delayed_type(call, __closure_##f, (void *)arg)

That could be reused for timers with typechecking - we have a lot of timer
callbacks that start with casting the argument (unsigned long, not void *,
but that's not a big deal) to whatever it is that callback really wants,
with setup_timer() callers explicitly casting that whatever the callback
really wants to unsigned long.  Which, of course, defeats the typechecking
by both cc(1) and sparse(1)...

I still hope for better solution, though...  Comments?