Re: [Qemu-devel] [PATCH] qom: Use atomics for object refcounting

[Jan Kiszka]

On 2013-07-02 13:28, Paolo Bonzini wrote:
> Il 02/07/2013 13:15, Andreas Färber ha scritto:
>>>> @@ -683,16 +683,15 @@ GSList *object_class_get_list(const char 
>>>> *implements_type,
>>>>  
>>>>  void object_ref(Object *obj)
>>>>  {
>>>> -    obj->ref++;
>>>> +     __sync_fetch_and_add(&obj->ref, 1);
>> How widespread are these in GCC/clang? Is there any fallback? I remember
>> seeing some __sync_* warnings on Mac OS X around 4.2...
> 
> We are using them already in several places (vhost was the first one to
> introduce them, I think, but now they are also in migration ad in some
> tests too).  There is no fallback (asm could be a fallback, but we chose
> to require GCC 4.2 or newer).
> 
> I'll change this to atomic_inc/dec when applying.  Otherwise

But then atomic_dec_and_test or so. Letting the inc/dec return some
value leaves room for interpretations (value of before or after the
modification?).

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux