Re: [Xen-devel] [PATCHv2 4/6] x86: provide xadd()

[Tim Deegan <tim@xxxxxxx>]

At 15:19 +0100 on 10 Apr (1428679195), David Vrabel wrote:
> xadd() atomically adds a value and returns the previous value.  This
> is needed to implement ticket locks.
> 
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
>  xen/include/asm-x86/system.h |   29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/xen/include/asm-x86/system.h b/xen/include/asm-x86/system.h
> index 7111329..1e6c6a8 100644
> --- a/xen/include/asm-x86/system.h
> +++ b/xen/include/asm-x86/system.h
> @@ -117,6 +117,35 @@ static always_inline unsigned long __cmpxchg(
>                                     (unsigned long)__n,sizeof(*(ptr)))); \
>  })
>  
> +static always_inline unsigned long __xadd(
> +    volatile void *ptr, unsigned long v, int size)
> +{
> +    switch ( size )
> +    {
> +    case 1:
> +        asm volatile ( "lock; xaddb %b0,%1"
> +                       : "+r" (v), "+m" (*__xg((volatile void *)ptr)));
> +        return v;
> +    case 2:
> +        asm volatile ( "lock; xaddw %w0,%1"
> +                       : "+r" (v), "+m" (*__xg((volatile void *)ptr)));
> +        return v;
> +    case 4:
> +        asm volatile ( "lock; xaddl %k0,%1"
> +                       : "+r" (v), "+m" (*__xg((volatile void *)ptr)));
> +        return v;
> +    case 8:
> +        asm volatile ( "lock; xaddq %q0,%1"
> +                       : "+r" (v), "+m" (*__xg((volatile void *)ptr)));
> +        return v;
> +    }
> +    return 0;

ASSERT_UNREACHABLE(), rather?  And some appropriate BUILD_BUG_ON?

But also: AFAICS the GCC builtin __sync_fetch_and_add() does almost
exactly this (the difference being that those are also compiler
barriers where this is only a CPU barrier).  Should we be using it
instead?

Tim.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel