Re: [Xen-devel] [PATCHv3 1/4] x86: provide xadd()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.04.15 at 12:11, <david.vrabel@xxxxxxxxxx> wrote:
> +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))
> +                       :: "memory");
> +        return v;

This doesn't seem to guarantee to return the old value: When the
passed in v has more than 8 significant bits (which will get ignored
as input), nothing will zap those bits from the register. Same for
the 16-bit case obviously.

> +#define xadd(ptr, v) ({                                         \
> +            __xadd((ptr), (unsigned long)(v), sizeof(*(ptr)));  \
> +        })

Assuming only xadd() is supposed to be used directly, perhaps
the easiest would be to cast v to typeof(*(ptr)) (instead of
unsigned long) here?

Also note that there's still a superfluous pair of parentheses
left here.

Jan


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