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

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Date:	Sat, 12 Dec 2015 02:00:27 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Neil Brown <neilb@...e.com>
Subject: Re: [PATCHSET v2] ->follow_link() without dropping from RCU mode

On Fri, Dec 11, 2015 at 11:16:24PM +0000, Al Viro wrote:

> #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?

Hmm...

#define set_delayed_type(call, f, arg)					\
 	(sizeof(f(arg),0),						\
	__set_delayed_type(call, ({					\
		void _(void *__) {f((typeof(arg))(unsigned long)__);}	\
		_;}), (void *)arg))

woult do nicely, but it's a gccism, and the one clang doesn't support ;-/
gcc support goes back at least to 4.1, not sure about the earlier branches
(and not at all sure how flaky it was/is, obviously).

But this thing works for e.g. f being void(char) and arg being char,
and AFAICS generates correct code - we end up with a stub like
_.1765:
        movsbl  %dil, %edi
        jmp     f
and its address gets stored in call->fn, which should DTRT on call...
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