linux-kernel - Re: [PATCH -v2 6/8] jump_label: Add a new static

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Message-ID: <20150803150359.0e76b576@gandalf.local.home>
Date:	Mon, 3 Aug 2015 15:03:59 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
	mingo@...nel.org, jasonbaron0@...il.com, bp@...en8.de,
	luto@...capital.net, tglx@...utronix.de, will.deacon@....com,
	liuj97@...il.com, rabin@....in, ralf@...ux-mips.org,
	ddaney@...iumnetworks.com, benh@...nel.crashing.org,
	michael@...erman.id.au, heiko.carstens@...ibm.com,
	davem@...emloft.net
Subject: Re: [PATCH -v2 6/8] jump_label: Add a new static_key interface

On Wed, 29 Jul 2015 10:49:06 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Jul 29, 2015 at 09:19:22AM +0200, Vlastimil Babka wrote:
> 
> > How would one define a static key that's e.g. expected to be mostly false, but
> > with initial value of true, e.g. during boot?
> 
> DEFINE_STATIC_KEY_TRUE(blah);
> 
> will get you the true at boot time.
> 
> You'll then want to use:
> 
> 	if (static_branch_unlikely(&blah)) {
> 		/* code that mostly doesn't happen */
> 	}
> 
> To indicate you expect it to be false most of the time. And you'll flip
> it to false at runtime using:
> 
> 	static_branch_disable(&blah);

I wonder if static_branch_set_false(&blah) would be a better name to
understand. What does "disable" / "enable" mean?

If we declare it "TRUE" when defining it, it only makes sense to change
it to "false" later on.

-- Steve


> 
> If GCC co-operates, the body of the branch will be placed out-of-line,
> we'll emit a jump to it by default, but once you disable it, we'll nop
> the jump and fall straight through.

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