[PATCH] xen-block: Fix removal of backend instance via xenstore

[Anthony PERARD via <qemu-devel@nongnu.org>]

From: Anthony PERARD <anthony.perard@citrix.com>

Whenever a Xen block device is detach via xenstore, the image
associated with it remained open by the backend QEMU and an error is
logged:
    qemu-system-i386: failed to destroy drive: Node xvdz-qcow2 is in use

This happened since object_unparent() doesn't immediately frees the
object and thus keep a reference to the node we are trying to free.
The reference is hold by the "drive" property and the call
xen_block_drive_destroy() fails.

In order to fix that, we call drain_call_rcu() to run the callback
setup by bus_remove_child() via object_unparent().

Fixes: 2d24a6466154 ("device-core: use RCU for list of children of a bus")

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
---
CCing people whom introduced/reviewed the change to use RCU to give
them a chance to say if the change is fine.
---
 hw/block/xen-block.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/hw/block/xen-block.c b/hw/block/xen-block.c
index a3b69e27096f..fe5f828e2d25 100644
--- a/hw/block/xen-block.c
+++ b/hw/block/xen-block.c
@@ -972,6 +972,15 @@ static void xen_block_device_destroy(XenBackendInstance *backend,
 
     object_unparent(OBJECT(xendev));
 
+    /*
+     * Drall all pending RCU callbacks as object_unparent() frees `xendev'
+     * in a RCU callback.
+     * And due to the property "drive" still existing in `xendev', we
+     * cann't destroy the XenBlockDrive associated with `xendev' with
+     * xen_block_drive_destroy() below.
+     */
+    drain_call_rcu();
+
     if (iothread) {
         xen_block_iothread_destroy(iothread, errp);
         if (*errp) {
-- 
Anthony PERARD