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, 5 insertions, 1 deletions

diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
index 3a71ea7..971afbb 100644
--- a/hw/scsi/virtio-scsi.c
+++ b/hw/scsi/virtio-scsi.c
@@ -367,12 +367,16 @@ 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;