Re: [PATCH 1/2] jump_labels: Add API to deal with keys embedded in structures

[Christoffer Dall]

On Tue, Sep 20, 2016 at 11:42:23AM +0200, Peter Zijlstra wrote:
> On Mon, Sep 19, 2016 at 06:21:27PM +0100, Marc Zyngier wrote:
> > It is desirable to allow static keys to be integrated in structures,
> > as it can lead do slightly more readable code. But the current API
> > only provides DEFINE_STATIC_KEY_TRUE/FALSE, which is not exactly
> > nice and leads to the following idiom:
> > 
> > 	static struct {
> > 		int			foo;
> > 		struct static_key_false	key;
> > 	} bar = {
> > 		.key	= STATIC_KEY_FALSE_INIT,
> > 	};
> > 
> > 	[...]
> > 
> > 	if (static_branch_unlikely(&bar.key))
> > 		foo = -1;
> > 
> > which doesn't follow the recommended API, and uses the internals
> > of the static key implementation.
> > 
> > This patch introduces DECLARE_STATIC_KEY_TRUE/FALSE, as well as
> > INIT_STATIC_KEY_TRUE/FALSE, which abstract such construct and
> > allow the internals to evolve without having to fix everything else:
> > 
> > 	static struct {
> > 		int			 foo;
> > 		DECLARE_STATIC_KEY_FALSE(key);
> > 	} bar = {
> > 		INIT_STATIC_KEY_FALSE(.key),
> > 	};
> 
> Hurm..
> 
> I think I like the first better, it looks more like actual C. Either way
> around you need to now manually match up the type and initializer.
> 

It may have been one of my review comments the prompted these patches,
because from reading Documentation/static-keys.txt, it seems that
referencing 'struct static_key' directly should be deprecated, and
instead developers should use the update API replacements.

I wonder if it's worth slightly updating the documentation then?

Thanks,
-Christoffer