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

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Date:   Tue, 20 Sep 2016 14:54:01 +0200
From:   Christoffer Dall <christoffer.dall@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Marc Zyngier <marc.zyngier@....com>, linux-kernel@...r.kernel.org,
        Jason Baron <jbaron@...mai.com>,
        Jonathan Corbet <corbet@....net>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [PATCH 1/2] jump_labels: Add API to deal with keys embedded in
 structures

On Tue, Sep 20, 2016 at 02:42:58PM +0200, Peter Zijlstra wrote:
> On Tue, Sep 20, 2016 at 02:25:14PM +0200, Christoffer Dall wrote:
> > 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.
> 
> 'struct static_key' should indeed not be used and is deprecated. 'struct
> static_key_{true,false}' however should be fine.

ah, ok, didn't realize this, especially since static_key_false() is also
listed as deprecated ;)

> 
> Part of the problem is naming, everything using 'struct static_key' has
> _insane_ names and the API is utterly confusing. The other part is that
> the new 2 type API simply has more functionality.

right, ok.  As long as you're happy with slightly increased use of
directly embeddeing struct static_key_{true,false}, we're good.

Sorry for both if I encouraged confusion here.

Thanks,
-Christoffer