linux-kernel - Re: [PATCH] jump_label: jump

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Message-ID: <20111201165009.GB2443@redhat.com>
Date:	Thu, 1 Dec 2011 11:50:09 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	rostedt@...dmis.org
Subject: Re: [PATCH] jump_label: jump_label for boot options.

On Thu, Dec 01, 2011 at 05:28:18PM +0100, Peter Zijlstra wrote:
> On Thu, 2011-12-01 at 10:40 -0500, Jason Baron wrote:
> > I should probably rename
> > static_branch() -> 'static_branch_default_false()' to make that clear.
> 
> The rename doesn't really make sense until you can also provide
> static_branch_true() also:

I think its just a matter of reversing the true and false returns.
That is, instead of:

static __always_inline bool arch_static_branch_default_false(struct jump_label_key
*key)
{
        asm goto("1:"
                JUMP_LABEL_INITIAL_NOP
                ".pushsection __jump_table,  \"aw\" \n\t"
                _ASM_ALIGN "\n\t"
                _ASM_PTR "1b, %l[l_yes], %c0 \n\t"
                ".popsection \n\t"
                : :  "i" (key) : : l_yes);
        return false;
l_yes:
        return true;
}

We just have (untested):

static __always_inline bool arch_static_branch_default_true(struct jump_label_key
*key)
{
        asm goto("1:"
                JUMP_LABEL_INITIAL_NOP
                ".pushsection __jump_table,  \"aw\" \n\t"
                _ASM_ALIGN "\n\t"
                _ASM_PTR "1b, %l[l_no], %c0 \n\t"
                ".popsection \n\t"
                : :  "i" (key) : : l_no);
        return true;
l_no:
        return false;
}

jump_label_inc/dec(), don't need to be changed, they just mean reverse
the branch on 0, 1 transitions. Although using the same key in both
static_branch_true, and static_branch_false, might be confusing. Maybe
we rename jump_label_inc/dec to static_branch_reverse_inc()/dec()?

> 
> > Maybe we need an unbiased static_branch() too, but I'm not sure excatly
> > how to implement it... 
> 
> I'd think the same way we got in this situation in the first place, go
> kick some GCC people ;-)

If you think this is a useful case, I'll try and see how to implement it.
Then we can add static_branch_default_true/false_unbiased() :)

-Jason
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