linux-kernel - Re: [PATCH 2/5] x86/jump-label: Do not bother updating nops if they are correct

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Message-Id: <20120127123437.481791ba.akpm@linux-foundation.org>
Date:	Fri, 27 Jan 2012 12:34:37 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Jason Baron <jbaron@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 2/5] x86/jump-label: Do not bother updating nops if they
 are correct

On Fri, 27 Jan 2012 15:14:44 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> --- a/arch/x86/kernel/jump_label.c
> +++ b/arch/x86/kernel/jump_label.c
> @@ -53,7 +53,25 @@ void arch_jump_label_transform(struct jump_entry *entry,
>  void arch_jump_label_transform_static(struct jump_entry *entry,
>  				      enum jump_label_type type)
>  {
> -	__jump_label_transform(entry, type, text_poke_early);
> +	static int once;
> +	static int update;
> +
> +	/*
> +	 * This function is called at boot up and when modules are
> +	 * first loaded. Check if the default nop, the one that is
> +	 * inserted at compile time, is the ideal nop. If it is, then
> +	 * we do not need to update the nop, and we can leave it as is.
> +	 * If it is not, then we need to update the nop to the ideal nop.
> +	 */
> +	if (!once) {
> +		unsigned char default_nop[] = { JUMP_LABEL_INIT_NOP };

const.  It generates less code.  With my compiler version, at least. 
That was a bit dumb of it, given that memcmp() is declared to take
const args.

> +		const unsigned char *ideal_nop = ideal_nops[NOP_ATOMIC5];
> +		once++;
> +		if (memcmp(ideal_nop, default_nop, 5) != 0)
> +			update = 1;
> +	}
> +	if (update)
> +		__jump_label_transform(entry, type, text_poke_early);
>  }
--
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