linux-kernel - Re: [PATCH 3/4, v2] x86: enlightenment for ticket spin locks

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Date:	Tue, 29 Jun 2010 18:13:02 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jan Beulich <JBeulich@...ell.com>
CC:	mingo@...e.hu, tglx@...utronix.de, jeremy.fitzhardinge@...rix.com,
	Ky Srinivasan <KSrinivasan@...ell.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4, v2] x86: enlightenment for ticket spin locks -	 eliminate
 NOPs introduced by first patch

On 06/29/2010 07:33 AM, Jan Beulich wrote:
> Under the assumption that the nop-s added by the base ticket spinlock
> enlightenment patch might be considered undesirable (or worse), here
> is an optional patch to eliminate these nop-s again. This is done
> through extending the memory operands of the inc instructions used for
> unlocking ticket locks to the necessary size, using assembler and
> linker features.
> 
> --- 2.6.35-rc3-virt-spinlocks.orig/arch/x86/include/asm/spinlock.h
> +++ 2.6.35-rc3-virt-spinlocks/arch/x86/include/asm/spinlock.h
> @@ -10,7 +10,6 @@
>  
>  #ifdef CONFIG_ENLIGHTEN_SPINLOCKS
>  #include <asm/alternative.h>
> -#include <asm/nops.h>
>  /* Including asm/smp.h here causes a cyclic include dependency. */
>  #include <asm/percpu.h>
>  DECLARE_PER_CPU(int, cpu_number);
> @@ -156,8 +155,7 @@ static __always_inline void __ticket_spi
>  #else
>  	unsigned int token;
>  
> -	alternative_io(UNLOCK_LOCK_PREFIX "incb %[lock]\n\t"
> -		ASM_NOP3,
> +	alternative_io(UNLOCK_LOCK_PREFIX "unary incb %[lock]\n\t",
>  		ALTERNATIVE_TICKET_UNLOCK_HEAD
>  		UNLOCK_LOCK_PREFIX "incb %[lock]\n\t"
>  		"movzwl %[lock], %[token]\n\t"
> @@ -228,8 +226,7 @@ static __always_inline void __ticket_spi
>  #else
>  	unsigned int token, tmp;
>  
> -	alternative_io(UNLOCK_LOCK_PREFIX "incw %[lock]\n\t"
> -		ASM_NOP2,
> +	alternative_io(UNLOCK_LOCK_PREFIX "unary incw %[lock]\n\t",
>  		ALTERNATIVE_TICKET_UNLOCK_HEAD
>  		UNLOCK_LOCK_PREFIX "incw %[lock]\n\t"
>  		"movl %[lock], %[token]\n\t"

If you're stretching (bloating) them anyway, perhaps we should be using
"add" instructions instead, with their better EFLAGS behavior?

	-hpa
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