device-core: use RCU for list of children of a bus

This fixes the race between device emulation code that tries to find
a child device to dispatch the request to (e.g a scsi disk),
and hotplug of a new device to that bus.

Note that this doesn't convert all the readers of the list
but only these that might go over that list without BQL held.

This is a very small first step to make this code thread safe.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200913160259.32145-5-mlevitsk@redhat.com>
[Use RCU_READ_LOCK_GUARD in more places, adjust testcase now that
 the delay in DEVICE_DELETED due to RCU is more consistent. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20201006123904.610658-9-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index 14d476c..2c6307e 100644
--- a/include/hw/qdev-core.h
+++ b/include/hw/qdev-core.h
@@ -3,6 +3,8 @@
 
 #include "qemu/queue.h"
 #include "qemu/bitmap.h"
+#include "qemu/rcu.h"
+#include "qemu/rcu_queue.h"
 #include "qom/object.h"
 #include "hw/hotplug.h"
 #include "hw/resettable.h"
@@ -238,6 +240,7 @@ struct BusClass {
 };
 
 typedef struct BusChild {
+    struct rcu_head rcu;
     DeviceState *child;
     int index;
     QTAILQ_ENTRY(BusChild) sibling;
@@ -258,6 +261,12 @@ struct BusState {
     int max_index;
     bool realized;
     int num_children;
+
+    /*
+     * children is a RCU QTAILQ, thus readers must use RCU to access it,
+     * and writers must hold the big qemu lock
+     */
+
     QTAILQ_HEAD(, BusChild) children;
     QLIST_ENTRY(BusState) sibling;
     ResettableState reset;