[PATCH v2 2/7] Implement drain_call_rcu and use it in hmp_device_del

[Maxim Levitsky <mlevitsk@redhat.com>]

This allows to preserve the semantics of hmp_device_del,
that the device is deleted immediatly which was changed by previos
patch that delayed this to RCU callback

Suggested-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 include/qemu/rcu.h |  1 +
 qdev-monitor.c     |  3 +++
 util/rcu.c         | 33 +++++++++++++++++++++++++++++++++
 3 files changed, 37 insertions(+)

diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
index 570aa603eb..0e375ebe13 100644
--- a/include/qemu/rcu.h
+++ b/include/qemu/rcu.h
@@ -133,6 +133,7 @@ struct rcu_head {
 };
 
 extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func);
+extern void drain_call_rcu(void);
 
 /* The operands of the minus operator must have the same type,
  * which must be the one that we specify in the cast.
diff --git a/qdev-monitor.c b/qdev-monitor.c
index 56cee1483f..70877840a2 100644
--- a/qdev-monitor.c
+++ b/qdev-monitor.c
@@ -812,6 +812,8 @@ void qmp_device_add(QDict *qdict, QObject **ret_data, Error **errp)
         return;
     }
     dev = qdev_device_add(opts, &local_err);
+    drain_call_rcu();
+
     if (!dev) {
         error_propagate(errp, local_err);
         qemu_opts_del(opts);
@@ -904,6 +906,7 @@ void qmp_device_del(const char *id, Error **errp)
         }
 
         qdev_unplug(dev, errp);
+        drain_call_rcu();
     }
 }
 
diff --git a/util/rcu.c b/util/rcu.c
index 60a37f72c3..e8b1c4d6c5 100644
--- a/util/rcu.c
+++ b/util/rcu.c
@@ -293,6 +293,39 @@ void call_rcu1(struct rcu_head *node, void (*func)(struct rcu_head *node))
     qemu_event_set(&rcu_call_ready_event);
 }
 
+
+struct rcu_drain {
+    struct rcu_head rcu;
+    QemuEvent drain_complete_event;
+};
+
+static void drain_rcu_callback(struct rcu_head *node)
+{
+    struct rcu_drain *event = (struct rcu_drain *)node;
+    qemu_event_set(&event->drain_complete_event);
+}
+
+void drain_call_rcu(void)
+{
+    struct rcu_drain rcu_drain;
+    bool locked = qemu_mutex_iothread_locked();
+
+    memset(&rcu_drain, 0, sizeof(struct rcu_drain));
+
+    if (locked) {
+        qemu_mutex_unlock_iothread();
+    }
+
+    qemu_event_init(&rcu_drain.drain_complete_event, false);
+    call_rcu1(&rcu_drain.rcu, drain_rcu_callback);
+    qemu_event_wait(&rcu_drain.drain_complete_event);
+
+    if (locked) {
+        qemu_mutex_lock_iothread();
+    }
+
+}
+
 void rcu_register_thread(void)
 {
     assert(rcu_reader.ctr == 0);
-- 
2.17.2