linux-kernel - Re: [x86-64] remove locked instruction from switch

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Message-ID: <20090110223616.GB16719@elte.hu>
Date:	Sat, 10 Jan 2009 23:36:16 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Benjamin LaHaise <bcrl@...ck.org>
Cc:	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Alexander van Heukelum <heukelum@...lshack.com>
Subject: Re: [x86-64] remove locked instruction from switch_to()


* Benjamin LaHaise <bcrl@...ck.org> wrote:

> The patch below removes an unnecessary locked instruction from 
> switch_to().  TIF_FORK is only ever set in copy_thread() on initial 
> process creation, and gets cleared during the first scheduling of the 
> process.  As such, it is safe to use an unlocked test for the flag 
> within switch_to()./
> 
> Signed-off-by: Benjamin LaHaise <bcrl@...ck.org
> 
> diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h
> index 2ed3f0f..ee2e589 100644
> --- a/arch/x86/include/asm/system.h
> +++ b/arch/x86/include/asm/system.h
> @@ -96,15 +96,15 @@ do {									\
>  	     "thread_return:\n\t"					  \
>  	     "movq %%gs:%P[pda_pcurrent],%%rsi\n\t"			  \
>  	     "movq %P[thread_info](%%rsi),%%r8\n\t"			  \
> -	     LOCK_PREFIX "btr  %[tif_fork],%P[ti_flags](%%r8)\n\t"	  \
>  	     "movq %%rax,%%rdi\n\t" 					  \
> -	     "jc   ret_from_fork\n\t"					  \
> +	     "testl  %[_tif_fork],%P[ti_flags](%%r8)\n\t"	  \
> +	     "jnz   ret_from_fork\n\t"					  \
>  	     RESTORE_CONTEXT						  \
>  	     : "=a" (last)					  	  \
>  	     : [next] "S" (next), [prev] "D" (prev),			  \
>  	       [threadrsp] "i" (offsetof(struct task_struct, thread.sp)), \
>  	       [ti_flags] "i" (offsetof(struct thread_info, flags)),	  \
> -	       [tif_fork] "i" (TIF_FORK),			  	  \
> +	       [_tif_fork] "i" (_TIF_FORK),			  	  \
>  	       [thread_info] "i" (offsetof(struct task_struct, stack)),   \
>  	       [pda_pcurrent] "i" (offsetof(struct x8664_pda, pcurrent))  \
>  	     : "memory", "cc" __EXTRA_CLOBBER)
> diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S
> index b86f332..16b4587 100644
> --- a/arch/x86/kernel/entry_64.S
> +++ b/arch/x86/kernel/entry_64.S
> @@ -247,6 +247,7 @@ ENTRY(native_usergs_sysret64)
>   */ 	
>  /* rdi:	prev */	
>  ENTRY(ret_from_fork)
> +	LOCK ; btr $TIF_FORK,TI_flags(%r8)
>  	CFI_DEFAULT_STACK

nice!

Small detail, it does not apply to latest -git cleanly:

 hunk #1 FAILED at 247.
 1 out of 1 hunk FAILED -- rejects in file arch/x86/kernel/entry_64.S

Probably trivial - would you mind to repost?

	Ingo
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