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

["Andreas Färber" <afaerber@suse.de>]

Am 02.07.2013 11:36, schrieb Jan Kiszka:
> 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);

How widespread are these in GCC/clang? Is there any fallback? I remember
seeing some __sync_* warnings on Mac OS X around 4.2...

Andreas

>  }
>  
>  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);
>      }
>  }

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