Re: [Qemu-devel] testandset asm fix

[Fabrice Bellard <fabrice@bellard.org>]

OK. Anyway, the locking in QEMU is mostly boggus. If SMP is implemented 
someday with host threads, then it will be the right time to correct it !

Fabrice.

Paul Brook wrote:
> The inline assembly used for the x86/x86-64 host testandset routine is bogus.
> The operand constraints are wrong (Fails to compile at -O0).
> Also the return value is incorrect. It should return 0 if the lock was 
> successfully acquired. 
> 
> Patch below fixes it. The additional sete test is unnecessary, we can just use 
> the comparison/writeback value.
> 
> Paul
> 
> Index: exec-all.h
> ===================================================================
> RCS file: /cvsroot/qemu/qemu/exec-all.h,v
> retrieving revision 1.26
> diff -u -p -r1.26 exec-all.h
> --- exec-all.h 10 Jan 2005 23:23:48 -0000 1.26
> +++ exec-all.h 6 Feb 2005 14:35:43 -0000
> @@ -392,28 +392,24 @@ static inline int testandset (int *p)
>  #ifdef __i386__
>  static inline int testandset (int *p)
>  {
> -    char ret;
> -    long int readval;
> -    
> -    __asm__ __volatile__ ("lock; cmpxchgl %3, %1; sete %0"
> -                          : "=q" (ret), "=m" (*p), "=a" (readval)
> -                          : "r" (1), "m" (*p), "a" (0)
> -                          : "memory");
> -    return ret;
> +    long int readval = 0;
> +
> +    __asm__ __volatile__ ("lock; cmpxchgl %2, %0"
> +                          : "+m" (*p), "+a" (readval)
> +                          : "r" (1));
> +    return readval;
>  }
>  #endif
>  
>  #ifdef __x86_64__
>  static inline int testandset (int *p)
>  {
> -    char ret;
> -    int readval;
> -    
> -    __asm__ __volatile__ ("lock; cmpxchgl %3, %1; sete %0"
> -                          : "=q" (ret), "=m" (*p), "=a" (readval)
> -                          : "r" (1), "m" (*p), "a" (0)
> -                          : "memory");
> -    return ret;
> +    long int readval = 0;
> +
> +    __asm__ __volatile__ ("lock; cmpxchgl %2, %0"
> +                          : "+m" (*p), "+a" (readval)
> +                          : "r" (1));
> +    return readval;
>  }
>  #endif
>  
> 
> 
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