[PATCH 06/10] device-core: use atomic_set on .realized property

[Paolo Bonzini <pbonzini@redhat.com>]

From: Maxim Levitsky <mlevitsk@redhat.com>

Some code might race with placement of new devices on a bus.
We currently first place a (unrealized) device on the bus
and then realize it.

As a workaround, users that scan the child device list, can
check the realized property to see if it is safe to access such a device.
Use an atomic write here too to aid with this.

A separate discussion is what to do with devices that are unrealized:
It looks like for this case we only call the hotplug handler's unplug
callback and its up to it to unrealize the device.
An atomic operation doesn't cause harm for this code path though.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200913160259.32145-6-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/core/qdev.c         | 19 ++++++++++++++++++-
 include/hw/qdev-core.h |  3 +++
 2 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/hw/core/qdev.c b/hw/core/qdev.c
index 59e5e710b7..fc4daa36fa 100644
--- a/hw/core/qdev.c
+++ b/hw/core/qdev.c
@@ -946,7 +946,25 @@ static void device_set_realized(Object *obj, bool value, Error **errp)
             }
        }
 
+       qatomic_store_release(&dev->realized, value);
+
     } else if (!value && dev->realized) {
+
+        /*
+         * Change the value so that any concurrent users are aware
+         * that the device is going to be unrealized
+         *
+         * TODO: change .realized property to enum that states
+         * each phase of the device realization/unrealization
+         */
+
+        qatomic_set(&dev->realized, value);
+        /*
+         * Ensure that concurrent users see this update prior to
+         * any other changes done by unrealize.
+         */
+        smp_wmb();
+
         QLIST_FOREACH(bus, &dev->child_bus, sibling) {
             qbus_unrealize(bus);
         }
@@ -961,7 +979,6 @@ static void device_set_realized(Object *obj, bool value, Error **errp)
     }
 
     assert(local_err == NULL);
-    dev->realized = value;
     return;
 
 child_realize_fail:
diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index 8067497074..39490e76ee 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -163,6 +163,9 @@ struct NamedClockList {
 /**
  * DeviceState:
  * @realized: Indicates whether the device has been fully constructed.
+ *            When accessed without the iothread mutex, consider using
+ *            qatomic_load_acquire() before accessing any other field in
+ *            the device.
  * @reset: ResettableState for the device; handled by Resettable interface.
  *
  * This structure should not be accessed directly.  We declare it here
-- 
2.26.2