linux-kernel - Re: static key arrays?

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Message-ID: <20150911111003.GK18489@twins.programming.kicks-ass.net>
Date:	Fri, 11 Sep 2015 13:10:03 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Johannes Berg <johannes@...solutions.net>
Cc:	Jason Baron <jbaron@...mai.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: static key arrays?

On Fri, Sep 11, 2015 at 11:45:35AM +0200, Johannes Berg wrote:
> Hi Peter, Jason, all,
> 
> Per the recent type-safe API changes, it's no longer easy to generate
> an array of static keys. I was planning to do that for a set of very
> unlikely debug options.
> 
> It sounds like you're planning to remove the previous API entirely at
> some point, so I'm wondering if you've given any thought to this
> possibility.

If possible I'd kill static_key_{true,false}() and
static_key_slow_{inc,dec}. Not sure how much more makes sense, the new
interface builds on parts of the old stuff.

> I briefly played with the idea of adding a macro for that, but the
> necessary "REPEAT(n, d)" macro for the initialisation becomes ugly
> pretty quickly and, afaict, needs to have enough macros for the maximum
> expected numbers.
> 
> For the case I was looking at it's static_key_false so a zero
> -initialized array would be sufficient, but that can't be done easily
> with a static_key_true.

As long as its all the same type it shouldn't be too hard;

struct static_key_false array[n] = { STATIC_KEY_FALSE_INIT, };

or something like that.

The scheduler has an array of these things that has different types;
which if going to be even more interesting. I'm not quite sure what to
do there, but I think it'll end up relying on the fact that both types
share the same base (struct static_key) and involve a lot of type
casting :-)

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