linux-kernel - Re: [RFC] x86: bitops asm constraint fixes

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Date:	Fri, 14 Mar 2008 17:07:21 -0400
From:	Chuck Ebbert <cebbert@...hat.com>
To:	Jan Beulich <jbeulich@...ell.com>
CC:	mingo@...e.hu, tglx@...utronix.de, hpa@...or.com,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] x86: bitops asm constraint fixes

On 03/13/2008 05:08 AM, Jan Beulich wrote:
> This (simplified) piece of code didn't behave as expected due to
> incorrect constraints in some of the bitops functions, when
> X86_FEATURE_xxx is referring to other than the first long:
> 
> int test(struct cpuinfo_x86 *c) {
> 	if (cpu_has(c, X86_FEATURE_xxx))
> 		clear_cpu_cap(c, X86_FEATURE_xxx);
> 	return cpu_has(c, X86_FEATURE_xxx);
> }
> 

This is a long-standing bug and your patch appears to fix it.

> --- linux-2.6.25-rc5/include/asm-x86/bitops.h	2008-03-10 13:24:33.000000000 +0100
> +++ 2.6.25-rc5-x86-clear-bit/include/asm-x86/bitops.h	2008-03-13 08:45:40.000000000 +0100
> @@ -24,9 +24,12 @@
>  /* Technically wrong, but this avoids compilation errors on some gcc
>     versions. */
>  #define ADDR "=m" (*(volatile long *) addr)
> +#define BIT_ADDR "=m" (((volatile int *) addr)[nr >> 5])
>  #else
>  #define ADDR "+m" (*(volatile long *) addr)
> +#define BIT_ADDR "+m" (((volatile int *) addr)[nr >> 5])
>  #endif
> +#define BASE_ADDR "m" (*(volatile int *) addr)

Can't you just do everything with unsigned longs, like this?

In include/asm-x86/types.h:

 ifdef CONFIG_X86_32
 # define BITS_PER_LONG 32
+# define BITMAP_ORDER 5
 #else
 # define BITS_PER_LONG 64
+# define BITMAP_ORDER 6
 #endif

Then:

>  #define ADDR "=m" (*(volatile long *) addr)
> +#define BIT_ADDR "=m" (((volatile long *) addr)[nr >> BITMAP_ORDER])
>  #else
>  #define ADDR "+m" (*(volatile long *) addr)
> +#define BIT_ADDR "+m" (((volatile long *) addr)[nr >> BITMAP_ORDER])
>  #endif

No need for BASE_ADDR that way (or ADDR could be renamed to that.)
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