[PATCH 04/10] device_core: use drain_call_rcu in in hmp_device_del/qmp_device_add

[Paolo Bonzini <pbonzini@redhat.com>]

From: Maxim Levitsky <mlevitsk@redhat.com>

Soon, a device removal might only happen on RCU callback execution,
but to avoid chanding HMP semantics, just drain all pending RCU
callbacks

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Suggested-by: Stefan Hajnoczi <stefanha@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200913160259.32145-4-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 qdev-monitor.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/qdev-monitor.c b/qdev-monitor.c
index e9b7228480..c66ba13780 100644
--- a/qdev-monitor.c
+++ b/qdev-monitor.c
@@ -803,6 +803,18 @@ void qmp_device_add(QDict *qdict, QObject **ret_data, Error **errp)
         return;
     }
     dev = qdev_device_add(opts, errp);
+
+    /*
+     * Drain all pending RCU callbacks. This is done because
+     * some bus related operations can delay a device removal
+     * (in this case this can happen if device is added and then
+     * removed due to a configuration error)
+     * to a RCU callback, but user might expect that this interface
+     * will finish its job completely once qmp command returns result
+     * to the user
+     */
+    drain_call_rcu();
+
     if (!dev) {
         qemu_opts_del(opts);
         return;
@@ -894,6 +906,16 @@ void qmp_device_del(const char *id, Error **errp)
         }
 
         qdev_unplug(dev, errp);
+
+        /*
+         * Drain all pending RCU callbacks. This is done because
+         * some bus related operations can delay a device removal
+         * to a RCU callback, but user might expect that this interface
+         * will finish its job completely once qmp command returns result
+         * to the user
+         */
+
+        drain_call_rcu();
     }
 }
 
-- 
2.26.2