[Qemu-devel] [PATCH v2 1/2] refcnt: introduce Qref to abstract the refcnt interface

[Qemu-devel] [PATCH v2 1/2] refcnt: 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>
---
v2:
    put Qref to a dedicated file.
    rename qref_get/_put as qref_inc/_dec
---
 include/qemu/refcnt.h | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 57 insertions(+)
 create mode 100644 include/qemu/refcnt.h

diff --git a/include/qemu/refcnt.h b/include/qemu/refcnt.h
new file mode 100644
index 0000000..815534a
--- /dev/null
+++ b/include/qemu/refcnt.h
@@ -0,0 +1,57 @@
+/*
+ * refcount management routine
+ *
+ * Copyright IBM, Corp. 2013
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.  See
+ * the COPYING file in the top-level directory.
+ *
+ */
+
+#ifndef QEMU_REFCNT_H
+#define QEMU_REFCNT_H
+
+#include "qemu/osdep.h"
+
+/*
+ * Qref defines the interface for thread-safe refcount management.
+ * As to atomic ops, it falls back on gcc implementation, and do some
+ * optimization if gcc supports C11 relaxed memory model, otherwise
+ * just using seq_cst memory model.
+ */
+typedef struct Qref {
+    int32_t count;
+} Qref;
+
+static inline void qref_inc(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 *);
+
+/* When refcnt comes to zero, @release will be called */
+static inline void qref_dec(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 v2 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 |  3 ++-
 qom/object.c         | 15 +++++----------
 2 files changed, 7 insertions(+), 11 deletions(-)

diff --git a/include/qom/object.h b/include/qom/object.h
index 23fc048..6331972 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/refcnt.h"
 
 struct Visitor;
 struct Error;
@@ -384,7 +385,7 @@ 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..e7e5c26 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_inc(&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_dec(&obj->ref, object_finalize);
 }
 
 void object_property_add(Object *obj, const char *name, const char *type,
-- 
1.8.1.4