From: Maxim Levitsky <mlevitsk@redhat.com>
Date: Tue, 6 Oct 2020 15:38:59 +0300
Subject: device-core: use RCU for list of children of a bus

Git-commit: 2d24a64661549732fc77f632928318dd52f5bce5
References: bsc#1184574

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>
Signed-off-by: Lin Ma <lma@suse.com>
---
 hw/core/bus.c          | 28 ++++++++++++++++------------
 hw/core/qdev.c         | 37 +++++++++++++++++++++++--------------
 hw/scsi/scsi-bus.c     | 12 +++++++++---
 hw/scsi/virtio-scsi.c  |  6 +++++-
 include/hw/qdev-core.h |  9 +++++++++
 5 files changed, 62 insertions(+), 30 deletions(-)

diff --git a/hw/core/bus.c b/hw/core/bus.c
index 7f3d2a3dbda72fe0a5dfea3ff1f1..85bc9436e603c43813936f24aba9 100644
--- a/hw/core/bus.c
+++ b/hw/core/bus.c
@@ -49,12 +49,14 @@ int qbus_walk_children(BusState *bus,
         }
     }
 
-    QTAILQ_FOREACH(kid, &bus->children, sibling) {
-        err = qdev_walk_children(kid->child,
-                                 pre_devfn, pre_busfn,
-                                 post_devfn, post_busfn, opaque);
-        if (err < 0) {
-            return err;
+    WITH_RCU_READ_LOCK_GUARD() {
+        QTAILQ_FOREACH_RCU(kid, &bus->children, sibling) {
+            err = qdev_walk_children(kid->child,
+                                     pre_devfn, pre_busfn,
+                                     post_devfn, post_busfn, opaque);
+            if (err < 0) {
+                return err;
+            }
         }
     }
 
@@ -158,12 +160,14 @@ static void bus_set_realized(Object *obj, bool value, Error **errp)
 
         /* TODO: recursive realization */
     } else if (!value && bus->realized) {
-        QTAILQ_FOREACH(kid, &bus->children, sibling) {
-            DeviceState *dev = kid->child;
-            object_property_set_bool(OBJECT(dev), false, "realized",
-                                     &local_err);
-            if (local_err != NULL) {
-                break;
+        WITH_RCU_READ_LOCK_GUARD() {
+            QTAILQ_FOREACH_RCU(kid, &bus->children, sibling) {
+                DeviceState *dev = kid->child;
+                object_property_set_bool(OBJECT(dev), false, "realized",
+                                         &local_err);
+                if (local_err != NULL) {
+                    break;
+                }
             }
         }
         if (bc->unrealize && local_err == NULL) {
diff --git a/hw/core/qdev.c b/hw/core/qdev.c
index 342ea8a3feb955c3318616252ead..917f3f6ae2efbcf01c8ed65a3d34 100644
--- a/hw/core/qdev.c
+++ b/hw/core/qdev.c
@@ -49,6 +49,12 @@ const VMStateDescription *qdev_get_vmsd(DeviceState *dev)
     return dc->vmsd;
 }
 
+static void bus_free_bus_child(BusChild *kid)
+{
+    object_unref(OBJECT(kid->child));
+    g_free(kid);
+}
+
 static void bus_remove_child(BusState *bus, DeviceState *child)
 {
     BusChild *kid;
@@ -58,15 +64,16 @@ static void bus_remove_child(BusState *bus, DeviceState *child)
             char name[32];
 
             snprintf(name, sizeof(name), "child[%d]", kid->index);
-            QTAILQ_REMOVE(&bus->children, kid, sibling);
+            QTAILQ_REMOVE_RCU(&bus->children, kid, sibling);
 
             bus->num_children--;
 
             /* This gives back ownership of kid->child back to us.  */
             object_property_del(OBJECT(bus), name, NULL);
-            object_unref(OBJECT(kid->child));
-            g_free(kid);
-            return;
+
+            /* free the bus kid, when it is safe to do so*/
+            call_rcu(kid, bus_free_bus_child, rcu);
+            break;
         }
     }
 }
@@ -81,7 +88,7 @@ static void bus_add_child(BusState *bus, DeviceState *child)
     kid->child = child;
     object_ref(OBJECT(kid->child));
 
-    QTAILQ_INSERT_HEAD(&bus->children, kid, sibling);
+    QTAILQ_INSERT_HEAD_RCU(&bus->children, kid, sibling);
 
     /* This transfers ownership of kid->child to the property.  */
     snprintf(name, sizeof(name), "child[%d]", kid->index);
@@ -640,17 +647,19 @@ DeviceState *qdev_find_recursive(BusState *bus, const char *id)
     DeviceState *ret;
     BusState *child;
 
-    QTAILQ_FOREACH(kid, &bus->children, sibling) {
-        DeviceState *dev = kid->child;
+    WITH_RCU_READ_LOCK_GUARD() {
+        QTAILQ_FOREACH_RCU(kid, &bus->children, sibling) {
+            DeviceState *dev = kid->child;
 
-        if (dev->id && strcmp(dev->id, id) == 0) {
-            return dev;
-        }
+            if (dev->id && strcmp(dev->id, id) == 0) {
+                return dev;
+            }
 
-        QLIST_FOREACH(child, &dev->child_bus, sibling) {
-            ret = qdev_find_recursive(child, id);
-            if (ret) {
-                return ret;
+            QLIST_FOREACH(child, &dev->child_bus, sibling) {
+                ret = qdev_find_recursive(child, id);
+                if (ret) {
+                    return ret;
+                }
             }
         }
     }
diff --git a/hw/scsi/scsi-bus.c b/hw/scsi/scsi-bus.c
index 4f277985f64be532c8151a0ac09b..3c604bfe22e02a4e7b7f11f80769 100644
--- a/hw/scsi/scsi-bus.c
+++ b/hw/scsi/scsi-bus.c
@@ -412,7 +412,10 @@ static bool scsi_target_emulate_report_luns(SCSITargetReq *r)
     id = r->req.dev->id;
     found_lun0 = false;
     n = 0;
-    QTAILQ_FOREACH(kid, &r->req.bus->qbus.children, sibling) {
+
+    RCU_READ_LOCK_GUARD();
+
+    QTAILQ_FOREACH_RCU(kid, &r->req.bus->qbus.children, sibling) {
         DeviceState *qdev = kid->child;
         SCSIDevice *dev = SCSI_DEVICE(qdev);
 
@@ -433,7 +436,7 @@ static bool scsi_target_emulate_report_luns(SCSITargetReq *r)
     memset(r->buf, 0, len);
     stl_be_p(&r->buf[0], n);
     i = found_lun0 ? 8 : 16;
-    QTAILQ_FOREACH(kid, &r->req.bus->qbus.children, sibling) {
+    QTAILQ_FOREACH_RCU(kid, &r->req.bus->qbus.children, sibling) {
         DeviceState *qdev = kid->child;
         SCSIDevice *dev = SCSI_DEVICE(qdev);
 
@@ -442,6 +445,7 @@ static bool scsi_target_emulate_report_luns(SCSITargetReq *r)
             i += 8;
         }
     }
+
     assert(i == n + 8);
     r->len = len;
     return true;
@@ -1584,7 +1588,8 @@ SCSIDevice *scsi_device_find(SCSIBus *bus, int channel, int id, int lun)
     BusChild *kid;
     SCSIDevice *target_dev = NULL;
 
-    QTAILQ_FOREACH(kid, &bus->qbus.children, sibling) {
+    RCU_READ_LOCK_GUARD();
+    QTAILQ_FOREACH_RCU(kid, &bus->qbus.children, sibling) {
         DeviceState *qdev = kid->child;
         SCSIDevice *dev = SCSI_DEVICE(qdev);
 
@@ -1603,6 +1608,7 @@ SCSIDevice *scsi_device_find(SCSIBus *bus, int channel, int id, int lun)
             }
         }
     }
+
     return target_dev;
 }
 
diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 2e5bcf442384905d8d80fd487eea..52c3a964ecb112a9d1c00bfbe57d 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -374,12 +374,16 @@ static int virtio_scsi_do_tmf(VirtIOSCSI *s, VirtIOSCSIReq *req)
     case VIRTIO_SCSI_T_TMF_I_T_NEXUS_RESET:
         target = req->req.tmf.lun[1];
         s->resetting++;
-        QTAILQ_FOREACH(kid, &s->bus.qbus.children, sibling) {
+
+        rcu_read_lock();
+        QTAILQ_FOREACH_RCU(kid, &s->bus.qbus.children, sibling) {
              d = SCSI_DEVICE(kid->child);
              if (d->channel == 0 && d->id == target) {
                 qdev_reset_all(&d->qdev);
              }
         }
+        rcu_read_unlock();
+
         s->resetting--;
         break;
 
diff --git a/include/hw/qdev-core.h b/include/hw/qdev-core.h
index 2b0186f0af593deee82a02693589..bcc0c572c5a4ed431219fd902ece 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"
 
@@ -216,6 +218,7 @@ struct BusClass {
 };
 
 typedef struct BusChild {
+    struct rcu_head rcu;
     DeviceState *child;
     int index;
     QTAILQ_ENTRY(BusChild) sibling;
@@ -235,6 +238,12 @@ 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;
 };
