RE: [PATCH -mm 5/6] cpu_relax(): use in ACPI lock

["Moore, Robert" <robert.moore@intel.com>]

I may be interpreting this incorrectly, but are you busy-waiting on the
ACPI Global Lock to become free?

Bob


> -----Original Message-----
> From: linux-acpi-owner@vger.kernel.org [mailto:linux-acpi-
> owner@vger.kernel.org] On Behalf Of Andreas Mohr
> Sent: Wednesday, June 21, 2006 2:01 PM
> To: Andrew Morton
> Cc: Brown, Len; linux-acpi@vger.kernel.org;
linux-kernel@vger.kernel.org
> Subject: [PATCH -mm 5/6] cpu_relax(): use in ACPI lock
> 
> 
> Use cpu_relax() in __acpi_acquire_global_lock() etc.
> 
> 
> This could be considered overkill given the previous unlikely(),
> but it is busy-looping in case of false condition after all...
> 
> Tested on 2.6.17-mm1, i386 only (no x86_64 here).
> 
> Signed-off-by: Andreas Mohr <andi@lisas.de>
> 
> 
> diff -urN linux-2.6.17-mm1.orig/include/asm-i386/acpi.h linux-2.6.17-
> mm1.my/include/asm-i386/acpi.h
> --- linux-2.6.17-mm1.orig/include/asm-i386/acpi.h	2006-06-19
> 10:57:27.000000000 +0200
> +++ linux-2.6.17-mm1.my/include/asm-i386/acpi.h	2006-06-21
> 14:43:24.000000000 +0200
> @@ -61,11 +61,14 @@
>  __acpi_acquire_global_lock (unsigned int *lock)
>  {
>  	unsigned int old, new, val;
> -	do {
> +	while (1) {
>  		old = *lock;
>  		new = (((old & ~0x3) + 2) + ((old >> 1) & 0x1));
>  		val = cmpxchg(lock, old, new);
> -	} while (unlikely (val != old));
> +		if (likely(val == old))
> +			break;
> +		cpu_relax();
> +	}
>  	return (new < 3) ? -1 : 0;
>  }
> 
> @@ -73,11 +76,14 @@
>  __acpi_release_global_lock (unsigned int *lock)
>  {
>  	unsigned int old, new, val;
> -	do {
> +	while (1) {
>  		old = *lock;
>  		new = old & ~0x3;
>  		val = cmpxchg(lock, old, new);
> -	} while (unlikely (val != old));
> +		if (likely(val == old))
> +			break;
> +		cpu_relax();
> +	}
>  	return old & 0x1;
>  }
> 
> diff -urN linux-2.6.17-mm1.orig/include/asm-x86_64/acpi.h
linux-2.6.17-
> mm1.my/include/asm-x86_64/acpi.h
> --- linux-2.6.17-mm1.orig/include/asm-x86_64/acpi.h	2006-06-21
> 14:28:19.000000000 +0200
> +++ linux-2.6.17-mm1.my/include/asm-x86_64/acpi.h	2006-06-21
> 14:43:24.000000000 +0200
> @@ -59,11 +59,14 @@
>  __acpi_acquire_global_lock (unsigned int *lock)
>  {
>  	unsigned int old, new, val;
> -	do {
> +	while (1) {
>  		old = *lock;
>  		new = (((old & ~0x3) + 2) + ((old >> 1) & 0x1));
>  		val = cmpxchg(lock, old, new);
> -	} while (unlikely (val != old));
> +		if (likely(val == old))
> +			break;
> +		cpu_relax();
> +	}
>  	return (new < 3) ? -1 : 0;
>  }
> 
> @@ -75,7 +78,10 @@
>  		old = *lock;
>  		new = old & ~0x3;
>  		val = cmpxchg(lock, old, new);
> -	} while (unlikely (val != old));
> +		if (likely(val == old))
> +			break;
> +		cpu_relax();
> +	}
>  	return old & 0x1;
>  }
> 
> -
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