[Qemu-devel] [PATCH 1/2] object: introduce Qref to abstract the refcnt interface

[Qemu-devel] [PATCH 1/2] object: introduce Qref to abstract the refcnt interface

[Liu Ping Fan]

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
-- 
1.8.1.4

[Qemu-devel] [PATCH 2/2] object: Object apply the refcnt interface by Qref

[Liu Ping Fan]

Signed-off-by: Liu Ping Fan <pingfank@linux.vnet.ibm.com>
---
 include/qom/object.h |  2 +-
 qom/object.c         | 15 +++++----------
 2 files changed, 6 insertions(+), 11 deletions(-)

diff --git a/include/qom/object.h b/include/qom/object.h
index 2e165fb..8426dd1 100644
--- a/include/qom/object.h
+++ b/include/qom/object.h
@@ -389,7 +389,7 @@ struct Object
     ObjectClass *class;
     ObjectFree *free;
     QTAILQ_HEAD(, ObjectProperty) properties;
-    uint32_t ref;
+    Qref ref;
     Object *parent;
 };
 
diff --git a/qom/object.c b/qom/object.c
index b2479d1..736d4ba 100644
--- a/qom/object.c
+++ b/qom/object.c
@@ -388,15 +388,15 @@ static void object_deinit(Object *obj, TypeImpl *type)
     }
 }
 
-static void object_finalize(void *data)
+static void object_finalize(Qref *qref)
 {
-    Object *obj = data;
+    Object *obj = container_of(qref, Object, ref);
     TypeImpl *ti = obj->class->type;
 
     object_deinit(obj, ti);
     object_property_del_all(obj);
 
-    g_assert(obj->ref == 0);
+    g_assert(qref->count == 0);
     if (obj->free) {
         obj->free(obj);
     }
@@ -683,17 +683,12 @@ GSList *object_class_get_list(const char *implements_type,
 
 void object_ref(Object *obj)
 {
-     atomic_inc(&obj->ref);
+     qref_get(&obj->ref);
 }
 
 void object_unref(Object *obj)
 {
-    g_assert(obj->ref > 0);
-
-    /* parent always holds a reference to its children */
-    if (atomic_fetch_dec(&obj->ref) == 1) {
-        object_finalize(obj);
-    }
+    qref_put(&obj->ref, object_finalize);
 }
 
 void object_property_add(Object *obj, const char *name, const char *type,
-- 
1.8.1.4

Re: [Qemu-devel] [PATCH 1/2] object: introduce Qref to abstract the refcnt interface

[Paolo Bonzini]

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