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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-07-02 13:49, Paolo Bonzini wrote:
> Il 02/07/2013 13:44, Jan Kiszka ha scritto:
>> 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?).
> 
> In qemu, I made all atomic_* functions return the old value.  This is
> consistent with atomic_cmpxchg and atomic_xchg (where returning the new
> value makes no sense).

Please avoid this ambiguity by naming the functions properly. That xchg
returns old values is known, that dec and inc do, is surely not.

Jan

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