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From: Anthony Liguori <anthony@codemonkey.ws>
In-Reply-To: <51D2F2D5.90801@redhat.com>
References: <51D29F27.9040706@siemens.com> <87bo6lqarq.fsf@codemonkey.ws>
	<51D2F2D5.90801@redhat.com>
Date: Tue, 02 Jul 2013 11:36:01 -0500
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Subject: Re: [Qemu-devel] [PATCH] qom: Use atomics for object refcounting
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Jan Kiszka <jan.kiszka@siemens.com>, Liu Ping Fan <pingfank@linux.vnet.ibm.com>, qemu-devel <qemu-devel@nongnu.org>

Paolo Bonzini <pbonzini@redhat.com> writes:

> Il 02/07/2013 16:47, Anthony Liguori ha scritto:
>> Jan Kiszka <jan.kiszka@siemens.com> writes:
>> 
>>> Objects can soon be referenced/dereference outside the BQL. So we need
>>> to use atomics in object_ref/unref.
>>>
>>> Based on patch by Liu Ping Fan.
>>>
>>> Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
>>> ---
>>>  qom/object.c |    5 ++---
>>>  1 files changed, 2 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/qom/object.c b/qom/object.c
>>> index 803b94b..a76a30b 100644
>>> --- a/qom/object.c
>>> +++ b/qom/object.c
>>> @@ -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);
>>>  }
>>>  
>>>  void object_unref(Object *obj)
>>>  {
>>>      g_assert(obj->ref > 0);
>>> -    obj->ref--;
>>>  
>>>      /* parent always holds a reference to its children */
>>> -    if (obj->ref == 0) {
>>> +    if (__sync_sub_and_fetch(&obj->ref, 1) == 0) {
>>>          object_finalize(obj);
>>>      }
>>>  }
>> 
>> Should we introduce something akin to kref now that referencing counting
>> has gotten fancy?
>
> I'm not a big fan of kref (it seems _too_ thin a wrapper to me, i.e. it
> doesn't really wrap enough to be useful), but I wouldn't oppose it if
> someone else does it.

I had honestly hoped Object was light enough to be used for this
purpose.  What do you think?

Regards,

Anthony Liguori

>
> Paolo