rcu: add g_free_rcu

This simplifies calling g_free from an RCU callback.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

docs/rcu.txt

@@ -120,12 +120,15 @@ The core RCU API is small:
      void call_rcu(T *p,
                    void (*func)(T *p),
                    field-name);
+     void g_free_rcu(T *p,
+                     field-name);
 
-        call_rcu1 is typically used through this macro, in the common case
-        where the "struct rcu_head" is the first field in the struct.  In
-        the above case, one could have written simply:
+        call_rcu1 is typically used through these macro, in the common case
+        where the "struct rcu_head" is the first field in the struct.  If
+        the callback function is g_free, in particular, g_free_rcu can be
+        used.  In the above case, one could have written simply:
 
-            call_rcu(foo_reclaim, g_free, rcu);
+            g_free_rcu(foo_reclaim, rcu);
 
      typeof(*p) atomic_rcu_read(p);
 

include/qemu/rcu.h

@@ -140,6 +140,14 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func);
       }),                                                                \
       (RCUCBFunc *)(func))
 
+#define g_free_rcu(obj, field) \
+    call_rcu1(({                                                         \
+        char __attribute__((unused))                                     \
+            offset_must_be_zero[-offsetof(typeof(*(obj)), field)];       \
+        &(obj)->field;                                                   \
+      }),                                                                \
+      (RCUCBFunc *)g_free);
+
 #ifdef __cplusplus
 }
 #endif