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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 21/04/15 11:36, Jan Beulich wrote:
>>>> 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?

I don't see how this helps.  Did you perhaps mean cast the result?

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

David

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