linux-kernel - Re: [RFC][PATCH 6/7] jump_label: Add a new static

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Message-ID: <20150727092410.GQ19282@twins.programming.kicks-ass.net>
Date:	Mon, 27 Jul 2015 11:24:10 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	linux-kernel@...r.kernel.org, mingo@...nel.org
Cc:	jasonbaron0@...il.com, bp@...en8.de, luto@...capital.net,
	tglx@...utronix.de, rostedt@...dmis.org, 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: [RFC][PATCH 6/7] jump_label: Add a new static_key interface

On Fri, Jul 24, 2015 at 07:52:15PM +0200, Peter Zijlstra wrote:
> --- a/arch/sparc/include/asm/jump_label.h
> +++ b/arch/sparc/include/asm/jump_label.h
> @@ -7,16 +7,32 @@
>  
>  #define JUMP_LABEL_NOP_SIZE 4
>  
> -static __always_inline bool arch_static_branch(struct static_key *key)
> +static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
>  {
> -		asm_volatile_goto("1:\n\t"
> -			 "nop\n\t"
> -			 "nop\n\t"

It just occurred to me that this second nop is a branch delay slot, and
therefore..

> -			 ".pushsection __jump_table,  \"aw\"\n\t"
> -			 ".align 4\n\t"
> -			 ".word 1b, %l[l_yes], %c0\n\t"
> -			 ".popsection \n\t"
> -			 : :  "i" (key) : : l_yes);
> +	asm_volatile_goto("1:\n\t"
> +		 "nop\n\t"
> +		 "nop\n\t"
> +		 ".pushsection __jump_table,  \"aw\"\n\t"
> +		 ".align 4\n\t"
> +		 ".word 1b, %l[l_yes], %c0\n\t"
> +		 ".popsection \n\t"
> +		 : :  "i" (&((char *)key)[branch]) : : l_yes);
> +
> +	return false;
> +l_yes:
> +	return true;
> +}
> +
> +static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
> +{
> +	asm_volatile_goto("1:\n\t"
> +		 "b %l[l_yes]\n\t"

should very much be here too:

		"nop\n\t"

> +		 ".pushsection __jump_table,  \"aw\"\n\t"
> +		 ".align 4\n\t"
> +		 ".word 1b, %l[l_yes], %c0\n\t"
> +		 ".popsection \n\t"
> +		 : :  "i" (&((char *)key)[branch]) : : l_yes);
> +
>  	return false;
>  l_yes:
>  	return true;
--
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