From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [PATCHv3 1/4] x86: provide xadd()
Date: Tue, 21 Apr 2015 13:36:30 +0100
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To: Jan Beulich <JBeulich@suse.com>, David Vrabel <david.vrabel@citrix.com>
Cc: Keir Fraser <keir@xen.org>, Ian Campbell <ian.campbell@citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, Andrew Cooper <andrew.cooper3@citrix.com>, Tim Deegan <tim@xen.org>, Jennifer Herbert <jennifer.herbert@citrix.com>, xen-devel@lists.xenproject.org
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On 21/04/15 11:36, Jan Beulich wrote:
>>>> On 21.04.15 at 12:11, <david.vrabel@citrix.com> 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