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

[Liu Ping Fan <qemulist@gmail.com>]

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