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

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Date:	Mon, 14 Apr 2008 09:21:22 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jan Beulich <jbeulich@...ell.com>
CC:	Ingo Molnar <mingo@...e.hu>, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [RFC] x86: bitops asm constraint fixes

Jan Beulich wrote:
>  
> +struct __bits { int _[1UL << (32 - 3 - sizeof(int))]; };
>   

I don't understand what you're doing here.  The array can be 1<<(32 - 1) 
bytes (assuming we never allow a 64-bit bit offset).  The int array 
makes that 1<<(32 - 1 - log2(sizeof(int))) ints.  But I don't see what 
the sizeof(int) is doing in there.

> +
>  #if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 1)
>  /* 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])
> +#define ADDR "=m" (*(volatile long *) addr)
> +#define BIT_ADDR "=m" (((volatile int *) addr)[nr >> 5])
> +#define FULL_ADDR "=m" (*(volatile struct __bits *) addr)
>  #else
>  #define ADDR "+m" (*(volatile long *) addr)
> -#define BIT_ADDR "+m" (((volatile int *)addr)[nr >> 5])
> +#define BIT_ADDR "+m" (((volatile int *) addr)[nr >> 5])
> +#define FULL_ADDR "+m" (*(volatile struct __bits *) addr)
>  #endif
> -#define BASE_ADDR "m" (*(volatile int *)addr)
> +#define BASE_ADDR "m" (*(volatile int *) addr)
>   

Shouldn't BASE_ADDR also use __bits?  Otherwise it won't get write-read 
dependencies right (a read could move before a write).

    J
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