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Date: Mon, 15 Jul 2013 13:38:35 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/2] object: introduce Qref to abstract the
	refcnt interface
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To: Liu Ping Fan <qemulist@gmail.com>
Cc: qemu-devel@nongnu.org, Richard Henderson <rth@twiddle.net>

Il 15/07/2013 04:49, Liu Ping Fan ha scritto:
> Qref is similar to kref. It hides the refcnt detail and provides
> a common interface. And this patch is based on the idiom of refcnt,
> and adopts some optimization about memory model, which finally
> falls back on gcc implementation.
> 
> Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
> ---
>  include/qom/object.h | 34 ++++++++++++++++++++++++++++++++++
>  1 file changed, 34 insertions(+)
> 
> diff --git a/include/qom/object.h b/include/qom/object.h
> index 23fc048..2e165fb 100644
> --- a/include/qom/object.h
> +++ b/include/qom/object.h
> @@ -18,6 +18,7 @@
>  #include <stdint.h>
>  #include <stdbool.h>
>  #include "qemu/queue.h"
> +#include "qemu/osdep.h"
>  
>  struct Visitor;
>  struct Error;
> @@ -363,6 +364,10 @@ struct ObjectClass
>      ObjectUnparent *unparent;
>  };
>  
> +typedef struct Qref {
> +    int32_t count;
> +} Qref;
> +
>  /**
>   * Object:
>   *
> @@ -1133,5 +1138,34 @@ int object_child_foreach(Object *obj, int (*fn)(Object *child, void *opaque),
>   */
>  Object *container_get(Object *root, const char *path);
>  
> +static inline void qref_get(Qref *qref)
> +{
> +#ifdef __ATOMIC_RELAXED
> +    __atomic_fetch_add(&qref->count, 1, __ATOMIC_RELAXED);
> +#else
> +    __sync_fetch_and_add(&qref->count, 1);
> +#endif
> +}
> +
> +typedef void (*ReleaseFn)(Qref *);
>  
> +static inline void qref_put(Qref *qref, ReleaseFn release)
> +{
> +    int32_t i;
> +
> +#ifdef __ATOMIC_RELAXED
> +    i = __atomic_fetch_add(&qref->count, -1, __ATOMIC_RELAXED);
> +    g_assert(i > 0);
> +    if (unlikely(i == 1)) {
> +        __atomic_thread_fence(__ATOMIC_SEQ_CST);
> +        release(qref);
> +    }
> +#else
> +    i = __sync_fetch_and_add(&qref->count, -1);
> +    g_assert(i > 0);
> +    if (unlikely(i == 1)) {
> +        release(qref);
> +    }
> +#endif
> +}
>  #endif
> 

I don't think this should depend on QOM, so include/qom/object.h is not
the right place to put it.  I suggested a different way in the other thread.

Also, get/put is not used (except in a couple places for historical
reasons) in QEMU.

Paolo